THE MICBOSCOPE FRONTISPIECE X510 X510 X6 X490 X77 X1860 X140 Xi 38 THE MICROSCOPE AND ITS REVELATIONS JBY THE LATE WILLIAM B. CARPENTER, C.B., M.D., LL.D., F.R.S, SEVENTH EDITION IN WHICH THE FIRST SEVEN CHAPTERS HAVE BEEN ENTIRELY REWRITTEN AND THE TEXT THROUGHOUT RECONSTRUCTED, ENLARGED, AND REVISED BY THE REV. W. H. DALLINGER, LL.D., F.R.S., &c. WITH TWENTY-ONE PLATES AND EIGHT HUNDRED WOOD ENGRA VINGS PHILADELPHIA P. BLAKISTON, SON, & CO. 1012 WALNUT STREET 1891 PREFACE The use of the Microscope, both as an instrument of scientific research and as a means of affording pleasure and recreative instruction, has become so widespread, and the instrument is now so frequently found in an expensive form capable of yielding in skilled hands good optical results, that it is eminently desirable that a treatise should be within the reach of the student and the tiro alike, which would provide both with the elements of the theory and principles involved in the construction of the instrument itself, the nature of its latest appliances, and the proper conditions on which they can be em- ployed with the best results. Beyond this it should provide an outline of the latest and best modes of preparing, examining, and mounting objects, and glance, with this purpose in view, at what is oasily accessible for the requirements of the amateur in the entire organic and inorganic kingdoms. This need has been for many years met by this book, and its six preceding editions have been an extremely gratifying evidence of the industry and erudition of its Author. From the beginning it opened the right path, and afforded excellent aid to the earnest amateur and the careful student. But the Microscope in its very highest form has become—so far at least as objectives of the most perfect construction and greatest useful magnifying power are concerned—so common that a much more accurate account of the theoretical basis of the instrument itself and of the optical apparatus employed with it to obtain the best results with ‘ high powers 5 is a want very widely felt. The advances in the mathematical optics involved in the con- struction of the most perfect form of the present Microscope have been very rapid during the last twenty years ; and the progress in the principles of practical construction and the application of theory have, even since the last edition of this book was published, been so marked as to produce a revolution in the instrument itself and in its vi PREFACE application. The new dispensation was dimly indicated in the last edition ; but it has effected so radical a change in all that apper- tains to Microscopy that a thorough revision of the treatment of this treatise was required. The great principles involved in the use of the new objectives and the interpretation of the images pre- sented by their means, are distinct and unique ; and unless these be clearly understood the intelligent use of the finest optical appliances now produced by mathematical and practical optics cannot be brought about. They have not rendered the use of the instrument more difficult—they have rather simplified its employment, provided the operator understands the general nature and conditions on which his Microscope should be used. If the modern Microscope be, as a mechanical instrument with its accompanying optical apparatus, as good as it can be, a critical image—a picture of the object having the most delicately beautiful character—is attainable with ‘ low powers’ and ‘high powers’ alike. Microscopists are no longer divisible into those who work with ‘ high powers ’ and those who work with ‘low powers.’ No one can work properly with either if he does not understand the theory of their construction and the principles upon which to interpret the results of their employment. If he is familiar with these the employment of any range of magni- fying power is simply a question of care, experiment, and practice; the principles applicable to the one are involved in the other. Thus, for example, a proper understanding of the nature and mode of optical action of a ‘ sub-stage condenser ’ is as essential for the very finest results in the use of a 1-inch object-glass as in the use of a 2 mm. with N.A. 1-40 or the 2'5 mm. with N.A. 1‘60, while it gives advantages not otherwise realisable if the right class of con- denser used in the right way be employed with the older dnth inch or -gbth inch achromatic objectives, and especially the TVth inch and o3„th inch objectives of Powell and Lealand, of N.A. 1*50. Without comparing the value of the respective lenses, the best possible results in every case will depend upon a knowledge of the nature of the instrument, the quality of the condenser required by it, and its employment upon right principles. This is but one instance out of the whole range of manipulation in Microscopy to which the same principles apply. In its present form, therefore, a treatise of this sort, preserving the original idea of its Author and ranging from the theory and construction of the Microscope and its essential apparatus, embracing a discussion of all their principal forms, and the right use of each, and passing to a consideration of the best methods of preparation and mounting of objects, and a review of the whole Animal, Vegetable, and Inorganic Kingdoms specially suited for microscopic purposes, PREFACE vii must be essentially a cyclopa?dic work. This was far more possible to one man when'TDr. Carpenter began his work than it was eA’en when he issued his last edition. But it is practically impossible now. It is with Microscopy as with every department of scientific work—we must depend upon the specialist for accurate knowledge. In the following pages I have been most generously aided. In no department, not even that in which for twenty years I have been specially at work, have I acted without the cordial interest, suggestion, and enlightenment afforded by kindred or similar workers. In every section experts have given me their unstinted help. To preserve the character of the book, however, and give it homo- geneity, it was essential that all should pass through one mind and be so presented. My work for many years has familiarised me, more or less, with every department of Microscopy, and with the great majority of branches to which it is applied. I have therefore given a common form, for which I take the sole responsibility, to the entire treatise. The subject might have been carried over ten such volumes as this ; but we were of necessity limited as to space, and the specific aim has been to give such a condensed view of the whole range of subjects as would make this treatise at once a practical and a suggestive one. The first five chapters of the last edition are represented in this edition by seven chapters ; the whole matter of these seven chapters has been re-written, and two of them are on subjects not treated in any former edition. These seven chapters represent the experience of a lifetime, confirmed and aided by the advice and practical help of some of the most experienced men in the world, and they may be read by anyone familiar with the use of algebraic symbols and the practice of the rule of three. They are not in any sense abstruse, and they are everywhere practical. In the second chapter, on The Principles and Theory of Vision with the Compound Microscope, so much has been done during the past twenty years by Dr. Abbe, of Jena, that my first desire was to induce him to summarise, for this treatise, the results of his twenty years of unremitting and marvellously productive labour. But the state of his health and his many obligations forbade this ; and at length it became apparent that if this most desirable end were to be secured, I must re-study with this object all the monographs of this author, I summarised them, not without anxiety ; but that was speedily removed, for Dr. Abbe, with great generosity, consented to examine my results, and has been good enough to write that he has ‘ read [my] clear expositions with the greatest interest; ’ and, after words which show his cordial friendliness, he says : £I find the whole . . . much more adequate to the purposes of the book than I should PREFACE have been able to write it. ... I feel the greatest satisfaction in seeing my views represented in the book so extensively and inten- sively.’ These words are more than generous ; but I quote them here in order that the reader may be assured of the accuracy and efficiency of the account given in the following pages of the invalu- able demonstrations, theories, and explanations presented by Dr. Abbe on the optical principles and practice upon which the recent improvement in the construction of microscopical lens systems has so much depended. It will not be supposed that I implicitly coincide with every detail. Dr. Abbe is too sincere a lover of independent judgment to even desire this. But it was important that his views as such should be found in an accessible English form ; in that form I have endeavoured to present them ; and in the main there can be no doubt whatever that these teachings are absolutely incident with fact and experience. In details, as may appear here and there in these pages, especially where it becomes a question of practice, I may differ as to method, and even interpretation, from this distinguished master in Mathematical Optics. But our differences in no way affect the great principles he has enunciated or the comprehensive theory of microscopical vision he has with such keen insight laid down. In preparing the I’emainder of the seven new chapters of this book I have sought and, without hesitancy, obtained advice and the advantage of the support of my own judgment and experience from many competent men of science, who have shown a sincere interest in my work and have aided me in my endeavours. But, first on the list, I must place my friend Mr. E. M. Nelson. Our lines of experience with the Microscope have run parallel for many years, although the subjects of our study have been wholly different ; but the advantages of his suggestion, confirmation, and help have been of constant and inestimable value to me. He know- ledge, instruments, and experience at my disposal, fully and without limit or condition ; and his exceptional skill in Photo-micrography has enabled me to add much to the value of this book. To Count Castracane I am indebted for valuable suggestions regarding the Diatomacese, to be used at my discretion ; to Dr. van Heurck I am also under much obligation for his courtesy in preparing Plate XI. of this book, giving some of his photo-micro- graphic work with the new object-glass of 2‘5 mm. N.A. 1'60. The full description of this plate is given, with some critical remarks, in the General Description of Plates. To the late and deeply lamented Dr. H. B. Brady, F.R.S., I am under obligation for valuable suggestions regarding the Foraminifera. PREFACE From Dr. Hudson I have received cordial aid in dealing with his special subject, the llotifera ; and to Mr. Albert Michael I am under equal obligation for his assistance in regard to the Acarina. Mr. W. T. Suffolk gave me his most welcome judgment and advice regarding my chapter on Mounting, and I received also the suggestions of Mr. A. Cole with much pleasure and advantage. I have received help from Dr. A. Hill, of Downing College, Cambridge, and from Professor J. N. Langley, of Trinity College, Cambridge—from both of whom special processes of preparation for histological work were sent. Mr. Frank Crisp, with characteristic generosity, aided me much by suggestions of special and practical value ; and Mr. John Mayall, Jun., the present Secretary of the Royal Microscopical Society, has been untiring in his willingness to furnish the aid which his influence was able to secure. To Professor W. Hicks, F.R.S., Principal of Firth College, Sheffield, I am indebted for the revision of special sheets; so also I owe acknowledgments to Dr. Henry Clifton Sorby, F.R.S., and to Dr. Groves, as well as to others, whose suggestions, advice, or con- tinuation of my judgments have been much esteemed ; and prominent amongst these are Professor Alfred W. Bennett, B.Sc., and Professor F. Jeffrey Bell, M.A., wdiose constant advice in their departments of Biology I have received throughout ; while in Micro-geological subjects 1 have been aided by the suggestions and experience of Professor J. Siiearson Hyland, D.Sc. It will be observed that every endeavour has been made to bring each of the many subjects discussed in this book into conformity with the most recent knowledge of experts. Many of the sections, in fact, have been wholly rewritten and illustrated from new and original sources ; this may be seen in the sections on the History as well as the Construction and Use of the Microscope and its appli- ances, as also in those on Diatomacea?, Desmids, Saprophytes, Bacteria, Botifera, Acarina, and in the chapters on Microscopic Geology and Mineralogy. To the same end nineteen new plates have been prepared and 300 additional woodcuts, many of which are also new ; and for the use of the majority of those which are not so, I am indebted to the Editors and Secretary of the Royal Microscopical Society. There certainly never was a time when the Microscope was so generally used as it now is. With many, as already stated, it is simply an instrument employed for elegant and instructive relaxation and amusement. For this there can be nothing but commendation, but it is desirable that even this end should be sought intelligently. The social influence of the Microscope as an instrument employed for recreation PREFACE and pleasure will be greater in proportion as a knowledge of the general principles on which the instrument is constructed are known, and as the principles of visual interpretation are understood. The interests of these have been specially considered in the following pages ; but such an employment of the Microscope, if intelligently pursued, often leads to more or less of steady endeavour on the part of amateurs to understand the instrument and use it to a purpose in some special work, however modest. This is the reason of the great increase of ‘ Clubs ’ and Societies of various kinds, not only in London, and in the provinces, but throughout America ; and these are doing most valuable work. Their value consists not merely in the constant accumulation of new details concerning minute vegetable and animal life, and the minute details of larger forms, but in the constant improvement of the quality of the entire Microscope on its optical and mechanical sides. It is largely to Amateur Microscopy that the desire and motive for the great improvements in object-glasses and eye-pieces for the last twenty years are due. The men who have compared the qualities of respective lenses, and have had specific ideas as to how these could become possessed of still higher qualities, have been comparatively rarely those who have employed the Microscope for professional and educational purposes. They have the rather simply used— employed in the execution of their professional work —the best with which the practical optician could supply them. It has been by amateur microscopists that the opticians have been incited to the production of new and improved objectives. But it is the men who work in our biological and medical schools that ultimately reap the immense advantage—not only of greatly im- proved, but in the end of greatly cheapened, object-glasses. It is on this account to the advantage of all that the amateur micro- scopist should have within his reach a handbook dealing with the principles of Lis instrument and his subject. To the medical student, and even to the histologist and patho- logist, a treatise which deals specifically with the Microscope, its principles, and their application in practice, cannot fail, one may venture to hope, to be of service. This book is a practical attempt—the result of large experience and study—to meet this want in its latest form ; and I sincerely desire that it may prove useful to many. W. H. DALLINGER. London: 1891. EXPLANATION OF PLATES FRONTISPIECE Fig. 1. x 6 diameters. Horizontal and transverse section of an orbitolite. Fig. 2. An imperfect or uncritical image of the minute hairs on the lining membrane of the extremity of the proboscis of the blow-fly x 510 diams., taken with a Zeiss apochromatic j-inch objective of -95 N.A. x 3 projection eye-piece ; but it was illuminated by a cone of small angle, viz. of 0T N.A., and illustrates the unadvisability of small cones for illumination. The first obvious feature in the picture is the doubling of the hairs which are out of focus; but the important difference lies in the bright line with a dark edge round the hairs which are precisely in focus. This is a diffraction effect which is always present round the outlines of every object illuminated by a cone of insufficient angle. Experiment shows that this diffraction line always ceases to be visible when the aperture of the illuminating cone is equal to about two-thirds the aperture of the objective used; but it will become again distinctly apparent when the aperture of the cone is reduced less than half that of the objective. Fig. 3. x 510 diams. A correct or critical image of the minute hairs on the lining membrane of the extremity of the blow-fly’s proboscis. In this picture the focus has been adjusted for the long central hair. It will be observed that this hair is very fine and spinous ; it has not the ring socket which is common to many hairs on insects, but grows from a very delicate membrane, which in the balsam mount is transparent. This photograph was taken with a Zeiss apochromatic J of -95 N.A. x 3 projection eye-piece. The illumination was that of a large solid axial cone of -65 N.A. from an achromatic condenser, the source of light being focussed on the object. Fig. 4. Section of cerebellum of a lamb, x 77 diams., by apochromatic 1-inch .3 N.A. This preparation was courteously supplied to the present Editor by Dr. Hill, whose imbedding and staining processes for these tissues it beautifully illustrates. Fig. 5. Amphipleura pellucida x 1860 diams., by apochromatic £ P4 N.A. illuminated by a very oblique pencil in one azimuth along the valve. Fig. 6. A hair of Polyxenus lagurus, a well-known and excellent test object for medium powers x 490 diams. by apochromatic '95 N.A. Fig. 7. A small vessel in the bladder of a frog, prepared with nitrate of silver stain, showing endothelium-cells, x 40 diams., by Zeiss A. -2 N.A. This object has been photographed for the purpose of exposing the fallacy which underlies the generally accepted statement that ‘low-angled’ glasses are the most suitable for histological purposes. The supposition that it is so has been founded on the fact that the penetration of a lens varies inversely as its aperture; therefore, it is said, a ‘ low-angled ’ glass is to be preferred to a wide-angled one, because ‘ depth of focus,’ which is supposed to enable one to see into tissues, is the end in view. On carefully examining this figure it will be noticed that it is almost xii EXPLANATION OF PLATES impossible to trace the outline of any particular endothelium-cell because its image is confused with that of the lower side of the pipe. In a monocular microscopical image a perspective view does not exist; it is better, therefore, to use a wide-angled lens, and so obtain a clear view of a thin plane at one time, and educate the mind to appreciate solidity by means of focal adjustment. It will be admitted that unless one approaches fig. 7 with a preconceived idea of what an endothelium-cell is like, the knowledge gained of it will be small indeed. Fie. 8 represents the same structure, x 138 diams., by an apochromatic | -65 N.A. Here only the upper surface of the pipe is seen, so that the out- line of the endothelium-cells can be clearly traced. The circular elastic tissue is also displayed. There is, moreover, an increased sharpness over the whole picture, due to the greater aperture of the objective. PLATE I Fig. 1. The inside of a valve of Pleurosigma angulatum, showing a ‘ postage stamp ’ fracture, x 1750 diams., with an apochromatic 1-4 N.A. by Mr. T. P. Smith, and illustrating his view of the nature of the Pleurosigma valve. Fig. 2. The outside of a valve of Pleurosigma angulatum, showing a dif- ferent form of structure, x 1750 diams., with an apochromatic ~ P4 N.A., by Mr. T. F. Smith. These two photo-micrographs demonstrate the existence of at least Javo layers in the angulatum. Fig. 3. Coscinodiscus asteromphalus, x 110 diams., with an apochromatic 1-inch 3 N.A. Fig. 4. A portion of the preceding, x 2000 diams. to show the lacework inside the areolations. This lacework is believed to be a perforated structure, as a fracture passes through the markings. In the central areolation there are forty-six smaller perforations surrounded by a crown of fifteen larger ones.1 Photographed with an apochromatic £ P4 N.A. Fig. 5. Aulacodiscus Kittonii, x 270, by an apochromatic 1-inch '3 N.A. Fig. 6. A small portion in the centre of an Aulacodiscus Sturtii, x 2000, by an apochromatic 1*4 N.A. Broadly speaking, the difference between the Coscinodisci and the Aulacodisci lies in the fact that in the former the secondary structure is inside the primary, while in the latter it is exterior to it. This definition, however, is not strictly accurate, as it is believed that the fine perforated structure covers the entire valve, it being only hidden by the primary structure. The whole of these demonstrations were photographed for the present Editor by his friend E. M. Nelson, Esq., and have been reproduced from the negatives by a process of photo-printing. PLATE II ARRANGEMENT OF THE MICROSCOPE WITH A STAND FOR THE MICROMETER EYE-PIECE, TO SECURE STEADINESS AND ACCURACY IN MEASURE- MENT PLATE III ARRANGEMENT OF THE MICROSCOPE AND ACCESSORIES FOR THE EMPLOY- MENT OF THE CAMERA LUCIDA PLATE IV THE METHOD OF USING THE SILVER SIDE REFLECTOR OR PARAEOLOID 1 A section of this diatom will be found in the Transactions of the Comity of Middlesex Natural History Society for 1 !S89, Plate I. fig. 2. EXPLANATION OF PLATES PLATE V METHOD OF USING DIRECT TRANSMITTED LIGHT WITHOUT THE EMPLOYMENT OF THE MIRROR Plates II. to V. are engraved from photographs, taken at the request of the Editor by Mr. E. M. Nelson, from the arranged instruments. PLATE VI SEXUAL GENERATION OF YOLVOX GLOBATOR. (After Cohn) Fig. 1. Sphere of Volvox globator at the epoch of sexual generation : a,. sperm-cell containing cluster of antherozoids; a1, sperm-cell showing side- view of discoidal cluster of antherozoids; a3, sperm-cell whose cluster has broken up into its component antherozoids; a4, sperm-cell partly emptied by the escape of its antherozoids; b b, flask-shaped germ-cells showing great increase in size without subdivision ; b-, fr, germ-cells with large vacuoles in their interior; b3, germ-cell whose shape has changed to the globular. Fig. 2. Sexual cell, a, distinguishable from sterile cells, b, by its larger size. Fig. 3. Germ-cell, with antheroids swarming over its endocbrome. Fig. 4. Fertilised germ-cell, or oosphere, with dense envelope. Fig. 5. Sperm-cell, with its contained cluster of antherozoids, more enlarged. Figs. 6, 7. Liberated antherozoids, -with their flagella. PLATE VII Fig. 1. Lyngbya eestuarii, Lieb. x 160. Fig. 2. Spirulina Jennrri, Ktz. x 400. Fig. 3. Tolypothrix cirrhosa, Carm. x 400. Fig. 4. Oscillaria insignis, Thw. x 400. Fig. 6. O. Frolichii, Ktz. x 400. Fig. 6. O. teuerrima, Ktz. x 400. These figures are after Cooke. OSCIELARIACEiE AND SCYTONEMACE^E PLATE VIII DESM1DIACEJE, KlVUEARlAUEAi:, AND SCITONEMACEiE Fig. 1. Zygosperm of Micrasterias dentieulata, Breb. (After Ralfs.) Fig. 2. Cosmarium Rrebissonii, Men. (Affer Cooke.) Fig. 3. Euastrum pectinatwm, Breb. (After Ralfs.) Fig. 4. Zygosperm of Staurastrum hirsvtum, Breb. (Aft3r Balfs.) Fig. 5. S. gracile, Ralfs. (After Cooke.) Fig. 6. Xanthidium aeuleatum, Ehrb. (After Ralfs.'i Fig. 7. Rivularia dura, Ktz. (After Cooke.) Fig. 8. R. dura, Ktz x 400. (After Cooke.) Fig. 9. Scytonema natans, Breb. x 400. (After Cooke.) Fig. 10. Staurastrum hirsutum, Breb. (After Cooke.) EXPLANATION OF PLATES PLATE IX DESMIDIACE.E Fig. 1. Micrasterias crux-melitemis, Ehrb. (After Cooke.) Fig. 2. Clostc'rium setaoenm,, Ehrb. (After Cooke.) Fig. 3. Desmidium Snsartzii, Ag. (After Cooke.) Fig. 4. Penium digitus, Ehrb. (After Cooke.) Fig. 5. P. digitus, Ehrb. (transverse view). Fig. 6. Spirotcsnia condensata, Breb. (After Cooke.) Fig. 7. Docidium baoulum, Breb. (After Cooke.) Fig. 8. Gonatozggon Brebissonii, De Bary, conjugating. (After Cooke.) PLATE X PLEUROSIGMA ANGULATUM This is a direct photo-micrograph, taKen by Dr. R. Zeiss, as magnified 4900 diameters. We direct attention specially to it as giving evidence of the pre- sence (however originated) of the intercostal markings, which may be seen with considerable clearness on the right-hand side of the midrib and in the middle of the valve. PLATE XI This plate has a twofold purpose. It is designed, first, to justify the opinions held by Dr. Henry van Heurck upon the structure of the valves of diatoms, and also to show how the usual microscopical tests present them- selves when examined with the new objective with N.A. 1-60, lately constructed by the Firm of Zeiss. This objective is believed by Dr. van Heurck to realise what he considers the highest results of photographic optics, which in his judgment could only be surpassed by finding a new immersion liquid of still higher refractive index presenting all the necessary qualities, and which at the same time would not affect the very delicate flint of which it is necessary to make the frontlens of this objective. This mediumhe hopes may be some day realised. Unfortunately, up to this time, no indication permits us to foresee the discovery of the liquid desired. The following is the way in which Dr. Henry van Heurck summarises his ideas upon the structure of the valve :— 1. The valve of diatoms 1 is formed by two membranes or thin plates and by an intermediate septum. By this he understands a plate pierced with openings. The superior membrane, often very delicate, may be destroyed in the treatment by acids in the washings, by rubbing, &c. It, is possible also that it sometimes only exists in a very rudimentary state. The majority of the students of diatoms agree in believing that these membranes may be suf- ficiently permeable to permit of exchange by endosmose between the contents of the valve and the surrounding outer water, but that these membranes have no real openings so long as the diatom is living and intact. 2. When the openings of the septum are disposed in alternate rows then they take an hexagonal form. When in perpendicular rows then the openings are square or elongated. The hexagonal form, which is besides so frequent in nature, seems to be the typical form of the openings of the septum, and it is found most frequently when the valve is large, destitute of consolidated sides, and must offer resistance to outside agents. Even in the forms of the square openings we see very frequently deviations and returns to the hexagonal type upon certain parts of the valve. It is possible that the septa may be sometimes composed of many layers, placed one above another, formed suc- cessively and closely united; but up to this time we have no proof of it, neither have we met with any form presenting layers placed one above another. 1 ‘ The structure of the Valve of Diatoms ’ in Records of the Belgian Society, v. xiii. 1890. EXPLANATION OF PLATES Such, in brief, is the view held, by Dr. van Heurck as an interpretation of our present knowledge of the structure of the valve of the diatoms. We give now a description of the objects represented on the plate. Figs. 1, 2, 3. Amphipleura pellucida, Kiitz, 1 and 2, valve resolved into- pearls. Fig. 2 x 2000 diams. Fig. 1 x 3000 diams. Fig. 3. Valve resolved- in striae at about 2300 diams. Fig. 4. Amphipleura Lindheimeri, Gr., x 2500 diams. Fig. 5. Pleurosigma angulatum, in hexagons, x (about) 10,000 diams. Fig. 6. Idem x 2000 diams., illusory pearls which are formed by the angles of the hexagonal cells when the focussing is not perfect. Fig. 7. The nineteenth band of Nobert’s test plate. This photo-micro- graph has been made exceptionally with the apochromatic ~ of 1-4 N.A, The lines being traced upon a cover in crown-glass, the objective of N.A. P6 cannot be used here. Fig. 8. Surirella gemma, Ehrb. x (about) 1000 diams. Fig. 9. Van Heurcldu crassinervis, Breb. (Frustulia saxonica, Rabh) x 2000 diams. All the photo-micrographs (except fig. 7) have been done with the new inch N.A. P60 of MM. Zeiss. These micro-photographs have been produced by sunlight in a monochro- matic form, the special compensating eye-piece 12, and the Abbe condenser of N.A. 1-6 Covers and slides in flint of 1'72 ; diatoms in a medium 2-4. We are bound, however, to note that the condenser used is not corrected in any way ; its aberrations are enormous. Although the highest admiration must be expressed for the skill exercised by Dr. van Heurck in these remarkable photo-micrographs, and the highest esteem for his courtesy to the present Editor in supplying them, it must not be forgotten that Dr. van Heurck was obliged to employ an imperfect condenser—a condenser absolutely uncorrected—and although we can testify to the high quality and tine corrections of at least one of the lenses of N.A. 1*6, we are convinced that much of its real perfection in image-forming is destroyed by uncorrected sub-stage illumination. Upon the corrections and large aplanatic area presented by the condenser and its careful and efficient employment depends entirely the nature of the image presented by the finest objective ever constructed; and as the perfection of the- objective, with a high amplification and a great aperture, is more nearly approached, the more dependent are we upon perfect corrections in the con- denser to bring out the perfect image-forming power of the objective. No image formed by such an objective as that possessing N.A. D60 can be consi- dered reliable until a condenser corrected for all abberrations like the objective itself is produced; and so convinced are we of the possible value of this objec- tive that we trust its distinguished devisor and maker may be soon induced to produce the condenser referred to. If, then, by the aid of the chemist we can discover media which will be of sufficiently high refractive index, and still tolerant of or non-injurious to organic tissues immersed in it, a new line of investigation may be open to- histology and pathology.—W. H. D. XV PLATE XII arachnoidiscus japonicus. (After R. Beck) The specimens attached to the surface of a sea-weed are represented as- seen under a |4h objective, with Lieberkiihn illumination: A, internal, surface ; B, external surface; C, front view, showing incipient subdivision. PLATE XIII COMPLETE LIFE-HISTORIES OP TWO SAPROPHYTES (Drawn from nature by Dr. Dallinger) EXPLANATION OF PLATES PLATE XIV The various stages of the development of the nucleus in two saprophytic organisms, as studied with recent homogeneous and apochromatic objectives, both in the several stages of fission and genetic fusion, indicating ltaryold- nesis, and proving, as established in detail by the text, that all the steps in the cyclic changes of these unicellular forms are initiated in the nucleus befce being participated in by the whole body of the organism. (Drawn from nature by Dr. Dallinger.) PLATE XV EOTIFEEiE Fig. 1. Floscularia campanulata. Fig. 2. Stephanoceros Eichhornii. Fig. 3. Melicerta ringens. Fig. 4. Pedalion mirum (side view). Fig. 5. P. mirum (dorsal view, showing muscles). Fig. 6. Copeus cerberus (side view). Fig. 7. Philodina aculeata (side view, corona expanded). Fig. 8. Male of Pedalion mirum. All these figures, save fig. 2, are reduced to scale from the beautiful plates in Hudson and Goss’s Rotifera. FORAMINIFERA PLATE XVI Fig. 1. Milinlina seminnlum («■ and b, lateral aspects). Fig. 2. Alreolina Boscii (a, lateral aspect; b, longitudinal section). Fig. 3. A.strorhiza limicola (a, lateral aspect; b, portion of the test more highly magnified, showing structure). Fig. 4. Haliphysema Tumanon-iczii, showing the pseudo-polythalamous foot. Fig. 5. Ibid, (group of specimens in situ). Fig. 6. Haplophragmwm agglutinans (a, lateral b, longitudinal section). Fig. 7. II. nanurn (a, superior aspect; b, peripheral aspect). Fig. 8. Textularia gramen (a, lateral aspect; b, oral aspect). Fig. 9. T. gramen (peripheral aspect). Fig. 9 a. Pavonina, Jiabell if or mis (a, lateral aspect; b, oral aspect). Fig. 10. Bulndnia spinulosa. Fig. 11. Chilostomella oroidea (a and b, lateral aspects; c, specimen mounted in Canada balsam and seen with transmitted light). PLATE XVII Fig. 12. Lag cm a sulcata. Fig. 13. L. sulcata. Fig. 14. L. sulcata. Fig. 15. L. sulcata (a, lateral aspect; b, oral aspect). Fig. 16. Nodosaria raph an vs. Fig. 17. Cristellaria calcar (a, b, c, lateral aspects). Fig. 18. Itamulina globulifera. Fig. 19. It. globulifera. Fig. 20. Globigerina bnlloides (var. triloba, pelagic specimen). Fig. 21. G. biilloides (a, b. c, adult typical shell). FORAMINIFERA EXPLANATION OF PLATES Fig. 22. Rvtalia Beccarii. Fig. 23. Polgstomella craticulata. Fig. 24. Amphistegina Lesson 'd(«, superior lateral aspect; b, inferior lateral aspect; e, peripheral aspect). Fig. 25. NummnUtes Icevigat.a (b, lateral aspect; e, vertical section). Fig. 26. Portion of Orbitoides nummulitica. PLATES XVIII, XIX, XX ACARINA All the figures, except fig. 4, Plate XX., are copied from plates drawn by Mr. A. D. Michael, F.L.S., &c. by the kind permission of the respective societies that published them. Figs. 1 to 6, Plate XVIII., and 1 to 3, Plate XIX., are from ‘ British Oribatidae,’ published by the Kay Society; fig. 7, Plate XVIII., from the ‘ Journal of the Linnean Society ’; fig. 4, Plate XIX., and fig. 3, Plate XX., from the ‘ Journal of the Royal Microscopical Society ’; fig. 5, Plate XIX., and figs. 1 and 2, Plate XX., from the ‘ Journal of the Quekett Micro- scopical Club.’ Fig. 4, Plate XX., is drawn after Fiirstenberg by the Editor. PLATE XVIII ORIBATIDiE Fig. 1. Anatomy of Nothrus theleproctus (male, dorsal aspect, x about 60). The dorsal portion of the chitinous exo-skeleton, and the fat and muscles which underlie it, have been removed from the abdomen. The internal organs are shown protruding, as they usually do when the creature is opened, as though they were too large to be contained in the ventral exo-skeleton. Part of the oesophagus is seen at the top (the brain having been removed). The preventricular glands (brown) lie on each side of the oesophagus. The ventri- culus is coloured pink ; part of it and the whole of the caeca are covered with botryoidal tissue (yellow). The testes (white shaded with blue) show at the sides protruding from beneath the alimentary canal. Fig. 2. Hoplophora magna (female, lateral aspect, x about 50). Thechitin at the side and the fatty tissue and muscles have been removed. Alimentary canal pink ; caeca of the ventriculus spotted; preventricular glands brown ; supercoxal gland white; its vesicles yellow; expulsory vesicle, between supercoxal and ovaries, grey ; ovary and oviducts white shaded with blue and yellow. The genital and anal plates are open, and the genital suckers pro- truding. One maxilla, white, is seen between the legs. Fig. 3. Tegeocranus lotus (female, dorsal aspect, x about 55). Dorsal exo-skeleton, fatty tissue, and muscles removed. Same colours as before. Brain (between preventricular glands) blue grey. Mandibles seen from above and behind, their retractor muscles cut short. The tracheae, which are present in this species, are seen proceeding to their stigmata in the acetabula of the iegs. Fig. 4. Female genital organs of Cepheus tegeocranus ( x about 25), Vigt. Central ovary, oviducts with eggs, vagina and ovipositor. Fig. 5. The same of Danuzus geniculatus ( x about 20). The genital plates and the muscles and tendons which move them, and the genital suckers, are shown. These two figures are reduced from the originals. Fig. 6. Nymph (active pupal stage) of Tegeocranus herichis ( x about 100) carrying its cast dorsal skins). Fig. 7. Hypopial (travelling) nymph of Rliizoglyphus Robini (ventral aspect, x 100). TYROGrLYPHID^® xviii EXPLANATION OF PLATES PLATE XIX ORIBATID.E Fig. 1. Leiosoma palmicinctum ( x about 40). Fig. 2. Nymph of same species, fully grown ( x about 55). The central ellipse with the innermost set of scales attached is the cast larval dorsol abdominal skin. The other rows of scales belong to the successive nympha- skins. v Fig. 3. One of the scales more highly magnified. CHEYLETID^E Fig. 4. Rostrum and great raptorial palpi, with their appendages of Cliey- letus venustissimus ( x about 150). Fig. 5. Myobia chiropteralis (female, x about 125). MYOBIID^E PLATE XX Claw of first leg of same species, being an organ for holding the hair of the bat. GAMAS1D.33 Fig. 2. Gamasus terribilis (male, x 30). A species found in moles’ nests. Fig. 3. Freyana heteropus (male, x about 95, a parasiteof the cormorant). Fig. 4. Sarcoptes scabiei (the itch mite, x about 150, adult female). ANALGINiE Errata Page 375, line 25, for oscillaricce read Oscillariacece. „ 677, „ 7, for Coccodia read Coccidia. „ 702, „ 3 from bottom, for Ophridiutn read Ophrydium. „ 706, „ 3, for Ophryda read OphryrHa. „ 770, „ 14, for single read simple. PLATE 1. 1 2 X1750 X1750 3 4 X110 5 X2000 6 x&ooO X27 O THE MICROSCOPE CHAPTER I ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS To be the owner of a well-chosen and admirably equipped micro- scope, and even to have learnt the general purpose and relations of its parts and appliances, is by no means to be a master of the instrument, nor to be able to employ it to the full point of its efficiency even with moderate magnifying powers. It is an instru- ment of precision, and both on its mechanical and optical sides requires an intelligent understanding of principles before the best optical results can be invariably obtained. We may be in a position, with equal facility, to buy a high-class microscope and a high-class harp ; but the mere possession makes us no more a master of the instrument in the one case than the other. An intelligent understanding and experimental training are needful to enable the owner to use either instrument. In the case of the microscope, for the great majority of purposes to which it is applied in science, the amount of study and experimental training needed is by comparison incomparably less than in the case of the musical instrument. But the amount required is absolutely essential, the neglect of it being the constant cause of loss of early enthusiasm and not infrequent total failure. In the following pages we propose to treat the elementary principles of the optics of the microscope in a practical manner, not merely laying down dogmatic statements, but endeavouring to show the student how to demonstrate and comprehend the applica- tion of each general principle. But in doing this we are bound to remember a large section of the readers who will employ this treatise, and to so treat the subject that all the examples given, or that may be subsequently required by the ordinary microscopist, may be worked out with no heavier demand upon mathematics than the employment of vulgar fractions and decimals. In like manner, although we shall again and again employ the trigonometrical expression ‘ sine,’ its use will not involve a mathe- matical knowledge of its meaning. The sines of angles may be found 2 ELEMENTARY PRINCIPLES OE MICROSCOPICAL OPTICS by published tables. A table to quarter degrees is given in Appendix A of this book, which will, in the majority of cases, suffice ; it is not difficult to find such tables as may be required.1 Of course it is more than desirable that the microscopist should have good mathematical knowledge ; but there are many men who desire to obtain a useful knowledge of the principles of elementary optics who are without time or inclination, or both, to obtain the large mathematical knowledge required. Now just as a man who is without any accurate knowledge of astronomy or mathematics may find time from a sun-dial by applying the equation of time taken from a table in an almanac, so by the use of a table of sines the microscopist may reach useful and reliable results, although he may have no clear knowledge of trigonometry, physical optics, nor the mathematical proof of formulae. All microscopes, whether simple or compound, in ordinary use depend for their magnifying power upon the ability possessed by lenses to refract or bend the light which passes through them. Re- fraction acts in accordance with the two following laws, viz. :— 1. A ray which in passing from a rare medium into a denser medium makes a certain angle with the normal, i.e. the perpendicu- lar to the surface or plane at which the two media join, will, on entering the denser medium, make a smaller angle with the normal. Conversely, a ray passing out from a dense medium into a rarer one, making a certain angle with the normal will, on emergence from the dense medium, make a greater angle with the normal. The ray in one meditim is called the incident ray, and in the other medium the refracted ray. The incident and refracted rays are always in the same plane. 2. The sine of the angle of incidence divided by the sine of the angle of refraction is a constant quantity for any two particular media. When one of the media is air (accurately a vacuum) the ratio of these sines is called the absolute refractive index of the medium. As every known medium is denser than a vacuum it follows that the angle of the refracted ray in that medium will be less than the angle of the incident ray in a vacuum ; consequently, the absolute refractive index of any medium is greater than unity. Further, the absolute refractive index for any particular substance will differ according to the colour of the ray of light employed. The refraction is least for the red, and greatest for the violet. The difference between these refractive values determines what is called the dispersive power of the substance. This will be understood by fig. 1. Let I C, a ray of light travel- ling in air, meet the surface A B of water at the point C. Through C draw N IS-' at right angles to the surface of the water A B. The line 1ST 1ST' is called the normal to the surface A B. The ray I C will not continue its path through the water in a straight line to Q ; but, because water is denser than air, it will be bent to R, that is towards N;. The whole course of the ray will be ICR, of which the part I C is called the incident my, and C R the refracted ray. 1 Vide Chambers’s Mathematical Tables. THE LAW OF SIXES 3 The angle I C makes with the normal N N', viz. I C 1ST, is called the angle of incidence ; and the angle R C makes with the normal N' N, viz. R C N', is called the angle of refraction. Conversely, if a ray R C, travelling in water, meet the surface of air A B in the point C, it will not continue in a straight line, but will be bent to the point I farther away from N. Thus, when a ray passes from a rarer to a denser medium it is bent or refracted towards the normal, and when it passes out of a dense medium into a rarer one it is bent or refracted away from the normal. Further, if the shaded portion of the figure were glass instead of water, the refracted ray R C would be bent still nearer 1ST', and, conversely, if the ray passed out of glass into air, it would be Fig. 1.—The refraction of light. The law of sines. more bent away from the normal than if it had passed out of water into air. The angle of incidence I C N is connected with the angle of re- fraction RCF (as stated above) by what is known as Snell’s Law of Sines. The constant relation between the two sines for two specific media is called the refractive index of the medium, and is usually indicated in problems by the symbol /x. This law, stated with reference to the figure, would be : — a — the refractive index of water. sine RCN In I C take any point, P, and from P draw P T perpendicular to N W. Similarly in It C take any point, F, and draw F H per- pendicular to H V. 4 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS P X F H Now, as sine I C N = , and sine R C N' = =—•, then, by I C -T PT Snell’s law, = a. EH FC As any points may be taken in I C and R C if the points had been more judiciously selected, we might have greatly simplified the above expression. Thus, if we take two other points, K and E, such that K C = E C, and draw the perpendiculars as before, we shall have KS sine I C N = and sine R C 1ST' = , and therefore = u. KC EC ED EC But as KC = EC by construction, we can write K C for E G K S T7- /S _ g thus : m _ a. K C is cancelled, which leaves — = ix. ED ED 1 As fx can be experimentally determined for any two particular media, it follows that if one of the other terms is known, then the remaining term can be found. Thus, if fx and the angle of incidence are known, the angle of refraction can be found ; and if /x and the angle of refraction are known, the angle of incidence can be found. The unknown quantity can be found either geometrically or by cal- culation when the other two terms are given. It will, of course, be understood that, for the same medium in every case, a red ray would be bent or refracted less than a violet ray. The value therefore of /x for a red ray will be less than that of fx' for a violet ray. As a practical illustration : The refractive in- dex for a red ray in crown glass is T5124 = v, and for a violet ray is 1*5288 = ju', the difference being fx'—/x =*0164. The refractive index for a red ray in dense flint glass is T7030 = //, and for a violet ray is 1*7501 = //, the difference being fx' — /x = *0471. Consequently there will be a greater difference between the bend- ing of the refi’acted red and violet rays in the case of dense flint than in the case of crown glass, the angle of the incident ray with the normal being the same in either case. Where air (more correctly a vacuum) is not one of the media, then the refractive index is called the relative refractive index. The normal to a plane surface is always the perpendicular to it; the normal to a spherical surface is the radius of curvature, The angle of the incident ray and the angle of the refracted ray are always measured with the normal, and not with the surface. Fig. 2, a, b, shows the normals A, B to both a plane and a spherical surface, C D. In the case of the spherical surface, B is the centre of curvature,E F PROBLEMS ON REFRACTIVE INDEX 5 is the incident ray in air, F G the refracted ray in crown glass. The angle A F E is the angle of incidence, B F G the angle of refraction. Sine A F E divided by sine B F G is equal to the refractive index of air into crown glass, or, in other words, the absolute refractive index of crown glass, /x ; thus in this particular case : (Problem) I. : sin A F E _ sin 45° _ -707 _ 3 _ sin BTG “ sin 28° “ G72 ~~ 2 ~ This problem, however, is not actually needed by the reader of this book, for a table of absolute refractive indices is given in Appendix B. It will be clear from the above that when the refractive index, absolute or relative, of a ray from any first medium is given, the refractive index from the second to the first may be found. Thus, the absolute re- fractive index p from air into glass being given as ' , find \x\ the refractive A index from glass into air. (Problem) II.: , 1 _ 1 2 r 2 When the absolute refractive indices of any two media are given, the relative refractive indices between the media can be found. Thus, the absolute re- fractive index p of crown glass is 1*5, and the ab- solute refractive index p' of flint glass is TO ; find the relative refractive index p" from crown to flint. (Problem) III. : „ _ / P6 = 1>066 p 1 -5 Fig. 2.—The normals to a plane and a curved surface. The relative refractive index fx'" from flint to crown is determined by (problem) ii. : „"'=JL — _JL = -938 h a" 1-066 6 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS Let us now suppose that in fig. 2 the ray is travelling in the op- posite direction. GF in the denser medium will now lie the incident ray, and FE in the rarer medium will be the refracted ray. Now, if the angle B F G be increased, the angle A F E will also be in- Fig. 8.—The phenomenon of total reflexion. (From the ‘ Forces of Nature,’ published by Macmillan.) creased in a greater proportion, and the ray F E will approach the surface F D. When F E coincides with F D, G F is said to be incident at the critical angle of the medium. When this critical angle is reached, none of the incident light will pass out of the denser medium, but it PKOBLEMS ON REFRACTIVE INDEX 7 will be totally reflected from the surface C D back into the denser medium. A simple illustration of this is shown in fig. 3. It represents a glass of water so held that the surface of the water is above the •eye. If we look obliquely from below at this surface, it appears brighter than polished silver, and an object placed in the water has the upper portion of it brightly reflected. The action on all light incident on C D in the denser medium (fig. 2) at an angle greater than the critical angle is precisely the same in fact as if C D were a silvered mirror. A critical angle can only exist in a denser medium, for obviously there can be no critical angle in the rarer medium, since a ray of any angle of incidence can enter. When the relative or absolute refractive index of the denser medium is given, the critical angle for that medium can be found, thus: The absolute refractive index of water is 1-33 =/.t ; find its critical angle 0. (Problem) IV. : gin 1 = I = ,J6 fx 1'33 e = 48|° (found by table). So the sine of the critical angle is the reciprocal of the refractive index. The connection between the path of an incident ray in a first medium and its refracted ray in a second medium is established by the formula /x sin (f) = ft! sin (f)', where /x is the absolute refractive index of the first medium, the ;angle of the incident ray in it, // the absolute refractive index of the second medium, and <\>' the angle of the refracted ray in it. The angle = 45° of the incident ray in the first medium A F E 3 .(fig. 2) and /x = 1 fi' = the absolute refractive indices of both the media, air and glass respectively,' being given, find ' == 28° (fig. 2, BFG) is given ; find , the refractive indices remaining the same as before. (Problem) V. 2 : Sin

= 45°, find the angle of the refracted ray f or Bra. . 8 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS Problem Y. 3 : c- ,/ _n sin (j> _ 1'5 x sin 45 _ P5 x ‘707 _ P0605 _ ~ 1*6 1-6 1-6 “ * ' = 41-J,° (found by table). As a final instance. Suppose the ray to be travelling in the opposite direction, so that (4 P is the incident ray and B F G, or t]/ = be given, the media being the same as in the last caser /i' = 1-6 and /j. = 1*5, find (f>, or the angle of the refracted ray. (Problem) Y. 4 : Q. t/sind/ 1-6 sin P6 x‘663 P0608 ,_n7 Sui*=e~7 -=—is—=—rs rs—''707 ’ = 45° (found by table). The importance of the prism in practical optics is well known. Its geometrical form in per- spective and in section is shown in fig. 4. By means of the above pro- blems and their solutions we are now able to trace the diver- gence of a ray through a 'prism. In fig. 5 let A B C repre- sent a prism of very dense flint glass whose absolute refractive indices f for red light is 1 *7, and p" for blue light is l-75. Let the refracting angle B A C of the prism = 50°, and let the angle of incidence of a ray of white light D E = 45° = in air, g = 1. The dotted lines show the normals. Then by (problem) v. 1 for red light we have for the angle of refraction (f>r. Fig. 4.—The geometrical form of the prism, (From the ‘ Forces of Nature.’) . ,, ix sin

" of 264°. It should be remembered, however, that if the refracting angle of the prism is known, there is no necessity for this measurement, because it is always the difference between this and the angle of refraction before determined, thus 50° — 244° = 254°. Fig. 5.—Diagram of deviation of luminous ray by a prism. This ray E F now becomes the incident ray on the surface AC; and as the angle it makes with the normal at F is known, and as the refractive indices remain the same, we can, by (problem) v. 2, find the angles of refraction for each colour. If we take red light : a- . u! sin (pf 1*7 sin 251° 1*7 x *43 Sm 0 = H= a = = */ 32 ; /x L 1 0, the angle of refraction = 47° (found by table). If we take blue light : q. a" sin ip" 1*75 sin 264° 1*75 x *442 [XL 1 0, the angle of refraction = 50f° (found by table). This dispersion can now be represented in the diagram, seeing that it amounts to 3f°. In optics it is convenient to use an expression to measure the dispersive power of diaphanous substances, which does not depend on the refracting angle of the prism employed. Further, in order that various substances may be compared, their dispersive powers- are all measured with reference to a certain selected ray. (For this purpose the bisection of the D or sodium lines is the point in the spectrum often chosen.) In the crown and flint glasses mentioned on page 4 the dispersion between the lines C and F, in the spectrum, referred to the bisection of the sodium lines D, is as follows. Crown glass :—refractive index bisection of lines D, 1*5179 = /*; line F, 1*52395 =/*'; line C, 1*51535 = fx". Then the dispersive power w _ - IX." _ 1*52395 - 1*51535 = *0086 = .01661 l*.-1 1*5179-1 -5179 ELEMENTARY PRINCIPLES OE MICROSCOPICAL OPTICS The values of the same lines for the flint glass are as follows : D, 1-7174 = /x; F, 1-73489 = yP; C, 1*71055 = /P'. _/P - /P' _ 1*73489 - 1-71055 _ -02434 _ =1-5 x -573=-86. n sin m=1‘5 x -573=-86=:X.A. of objective. oil u =■ f-5 (/lass /i =/-5 oil n = /-5 Angular aperture of objective = 35° + 35° = 70° in glass,, which is equiva- lent to the angular aperture of the condenser = 60° + 60° = 120° in air. glass n =/-5 air n*= /-0 n*sin u* ju sin ' air /u'= 10 N.A. of condenser=m* sin a*=1-0 x -86=-86. fj.’ sin '—!'0 x ,86=-86=N.A. of condenser. Fig. Al.—Identity of n sin u (German math, form) with M sin (English). Also N A and angular aperture. Abbe’s theories and demonstrations presented in the following pages the Editor has scarcely felt justified in altering this, especially as the German form of symbol ob- EELATIYE APEETUEES 49 from the radiant, and u*, U* the angles of the same rays on their emergence ; then we shall have always sin U* : sin u* :: sin U : sin u ; or, sin IT* sin u* ———— =—; = const. = c ; sin U sin u that is, the sines of the angles of the conjugate rays on both sides of an a/planatic system always yield one and the same quotient c, whatever rays may be considered, so long as the same system and the same foci are in question. This proposition holds good for every arrangement of media, and refracting surfaces that may go to the composition of the system, and for every position of object and image. It is the law upon which de- pends the delineation of an image by means of wide-angled pencils. When, then, the values in any given cases of the expression n sin u (which is known as the 4 numerical aperture ’ and expressed by N.A.) has been ascertained, the objectives are instantly compared as regards their aperture, and, moreover, as 180° in air is equal to TO (since n = l'O and the sine of half 180° or 90° = TO), we see with equal readiness whether the aperture of the objective is smaller or larger than that corresponding to 180° in air. Thus, suppose we desire to compare the relative aperture of three tains in our Universities, and is thoroughly understood amongst University men. But to those unaccustomed to mathematical formulae confusion might easily arise from the juxtaposition of different symbols meaning precisely the same thing. To meet the possible necessity of these this footnote is inserted with an accompanying diagram to illustrate the identity of ‘ n sin u ’ with 1 y sin cp.’ The student who has mastered Snell’s Law of Sines, given and illustrated on p. 3 (fig. 1), will by a glance at the figure A1 on p. 48 understand the meaning and import- ance of the expression ‘ N.A.’ (numerical aperture) and at the same time will grasp wherein it differs from ‘ angular aperture ’ iq.v.). He will also perceive how it comes to pass that an angular aperture of 70° in glass is equivalent to an angular aperture of 120° in air. In the figure the upper hemispherical lens represents the front of a homogeneous immersion objective. It is supposed to be focussed on an object in contact with the lower side of a cover-glass. Between the plane front of the lens and the upper surface of the cover-glass is a drop of oil of cedar-wood, whose refractive index is 1'5, being thus identical with the cover-glass and the front lens. It is understood that no slip is used, and that there is nothing between the object and the front lens of the condenser. In this case the axis A B is the normal (p. 5, fig. 2); on the left-hand side there is a ray which makes an angle of 35° with the normal in glass issuing into air on the right-hand side of the normal. By Snell’s formula (p. 3)— y sin cp = y' sin ) in fig. A 1 (p. 48, note). Now because the areas of circles are to one another in the- RADIATION IN AIR ANI) BALSAM 55 A dry objective was therefore supposed to be placed at a disad- vantage when used upon balsam-mounted objects, its aperture being supposed to be ‘ cut down ’ by the balsam, and the advantage of the immersion objective was considered to rest on the fact that it restored, in the case of the balsam-mounted object, the same condi- tions as subsisted in the case of the dry-mounted object, allowing as. large (but no larger) an aperture to be obtained with the former object as is obtained by the dry objective with the latter. The error here lies in the assumption of the identity of radiation in air and balsam. If there were in fact any such identity, the conclusion above referred to would, of course, be correct, for if in tig. 37 the air pencil of 170° was identical with the balsam pencil of 170° (shown by the dotted lines in tig. 38), there would necessarily be a relative loss of light in the latter case in consequence of SO' much of the pencil being reflected back at the cover-glass. When, however, the increase of radiation with the increase in the refractive index of the medium is recognised, the mistake of the preceding view is appreciated. The 170° in air of tig. 37 is not equal to, but much less than, the 170° in balsam of fig. 38, and not- withstanding that a great part of the latter does not reach the Fig. 38. proportion of the squares of their radii, it follows that if we designate the radius by n sin u (or fx sin ), the area of the circle A will be to the area of the circle B as the square of the radius of A is to the square of the radius of B, or as (n sin u)2 is to (n' sin it')2. ' Area, proportional to (ih'sivL tA)~ \ nsi7b to USlfl (p pc'sin, n we require depth dimension, low and moderate powers must be used ; ‘ and no greater aperture should therefore be used than is required for the effective- ness of these powers—an excess in such a case is a real damage.’5 Moreover, in biological work—constant application of the instru- ment to varied objects—lenses of moderate aperture and suitable power facilitate certainty of action and conserve labour. Increase of aperture involves a diminished working distance in the objective, and it is inseparable from a rapid increase of sensibility of the objectives for slight deviations from the conditions of perfect cor- rection. If it be not necessary to encounter the possible difficulties these things involve, to do so is to lose valuable moments. These difficulties, of course, are diminished by the use of homogeneous, and especially apocliromatic objectives, but even with these they are not, in practice, done away where the best results are sought. Employ the full aperture suitable to the power used. This is the practical maxim taught in effect by the Abbe theory of microscopic vision. It has been suggested that all objectives be made of relatively wide apertures, and that they be ‘ stopped down ’ by diaphragms when the work of ‘ lower apertures ’ has to be done/ But this is i A micron is p = mm. Vide Journ. B.M.S. 1888, pp. 502 and 526; and Nature, vol. xxxviii. pp. 221, 244. 2 See p. 83. 3 Abbe’s explanation of the reason of the non-stereoscopic perception of these is given (see pp. 93 et seq.). 4 ‘ The Relation of Aperture to Power,’ Journ. B.M.S. series ii. vol. ii. p. 304. 5 Ibid. ’ ' * PENETRATING- POWER IN OBJECTIVES 83 not a suggestion that commends itself to the working biologist. If there were no other defects in such a method, the fact that the working distance remains unaltered would be fatal ; and we may safely adopt the statement of Abbe,1 that ‘ scientific work with the microscope will always require, not only high power objectives of the widest attainable apertures, but also carefully finished lower powers of small and very moderate apertures.’ We complete this section with a table of numerical apertures, which will be found on the following page. As already stated, the resolving powers are exactly proportional to the numerical apertures, and the expressions for this latter will allow the resolving power of different objectives to be compared, not only if the medium be the same in each, but also if it be different. The first column gives the numerical apertures from •40 to 1‘52. The second, third, and fourth, the air-, water-, and oil- (or balsam-) angles of aperture, corresponding to every ‘02 of N.A. from 47° air- angle to 180° balsam-angle. The theoretical resolving power in lines to the inch is shown in the sixth column ; the line E of the spectrum about the middle of the green = 0'5269/z), being taken. The column giving ‘ illuminating power,’ we have already seen, is of less importance ; while it must be borne in mind in using the column of ‘penetrating power ’ that several data besides — go to make up the total depth of vision with the microscope. Penetrating Power in Objectives.—Intelligibility and sequence, more than custom, suggest the consideration of this subject at this point. The true meaning and real value of £ depth of focus,’ or what is known as ‘penetrating power,’ follows logically upon the above considerations. That quality in an objective which was supposed to endow it with a capacity of visual range in a vertical direction, that is, in the direction of the axis of vision, has been called ‘ penetration,’ it being supposed that by this ‘ property ’ parts of the object not in the focal plane could be specially presented, so as to enable their perspective and other relations with what lies precisely in the focal plane to be clearly traced out. Concerning the manner in which this quality of the objective operated, there have been most diverse opinionsindeed, the whole matter was involved in obscurity. The remarkable insight and learning of Professor Abbe have, however, found for this important subject a sound scientific basis. The delineation of solid objects by a system of lenses is, by virtue of the most general laws of optical delineation, subject to a pecular disproportion in amplification. The linear amplification of the cfeptfA-dimension is, when both the object and the image are in the same medium (air), found to be always equal to the square of the linear amplification of the dimensions at right angles to the optical axis ; but if the object be in a more highly refracting medium than air, it is equal to this square divided by the refractive index of the medium. In proportion to the lateral amplification there is 1 ‘ The Relation of Aperture to Power,’ Journ. R.M.S. series ii. vol. ii. p. 309. 84 VISION WITH THE COMPOUND MICROSCOPE 1 Corresponding Angle (2 u) for Limit of Resolving Power, in Lines to an Inch Illu- minating Power (N.A.)2 Pene- “ Numerical Aperture (N.A.= ?! sin ii) Air 0=i-oo) Water 0=1-33) Homo- geneous Immersion 0=l-52) White Light (A=0-52GV, Line E.) Mono- chromatic (Blue) Light |(A=0-4861M, Line P) Photo- graphy. (A=0-4000n, near Line h) trati n g Power (x.A.) Maximum aperture of homogeneous immersion with crown glass covers . . . . 1’52 180° 0' 146.543 158,845 193,037 2-310 •658 Powell and Lealand’s lenses constructed with 1*51 — — 166° 51' 145,579 157,800 191,767 2-280 •662 the Abbe-Schott optical glassi-inch, i-incb, anc1 JL-inch objectives . . . _> 1-50 _ 161° 23' 144,615 156,755 190,497 2-250 •667 Powell and Lea-land s A, inch homogeneous 1-49 — — 157° 12' 143,651 155.710 189,227 2-220 •671 objective. . . . . . , —» T48 — — 153° 39' 142,687 154,665 187,957 2-190 •676 1-47 — 150° 32' 141,723 153,620 186,687 2-161 •680 1-46 — — 147° 42' 140,759 152,575 185,417 2-132 •685 1’45 — — 145° 0' 139,795 151,530 184,147 2-103 •690 1’44 — 142° 39' 138,830 150,485 182,877 2-074 •694 Zeiss’ homogeneous and apochromatic objec- 1’43 — 140° 22' 137,866 149.440 181,607 2-045 •699 1’42 — — 138° 12' 136,902 148,395 180,337 2-016 •704 fives ; Powell and Lealand’s apochromatic 1-41 — — 136° 8' 135,938 147,350 179,067 1-988 •709 i inch objective 1-40 — — 134° 10' 134,974 146,305 177,797 1-960 •714 Powell and Lealand’s i inch and A- inch homo- 1*89 — — 132° 16' 134,010 145,260 176,527 1-932 •719 geneous objectives 1-38 — ‘— 130° 26' 133,046 114,215 175,257 1-904 •725 1-37 — — 128° 40' 132,082 143,170 173,987 1-877 •729 1-36 — — 126° 58' 131,H8 142,125 172,717 1-850 •735 1-35 — — 125° 18' 130,154 141,080 171,447 1-823 •741 Maximum aperture for water immersion . -» 1-34 — — 123° 40' 129,189 140,035 170,177 1-796 •746 1*88 — 180° O' 122° 6' 128,225 138,989 168,907 1-769 •752 1-32 — 1(55° 56' 120° 33' 127,261 137,944 167,637 1-742 •758 1-31 — 160° 6' 119° 3' 126,297 136,899 166,367 1-716 •763 1’30 — 155° 38' 117° 35' 125,333 135,854 165,097 1-690 ■769 1-29 — 151° 50'116° 8' 124,369 134,809 163,827 1-664 •775 1-28 — 118° 42' 114° 44' 123,405 133,764 162,557 1-638 •781 1-27 — 145° 27' 113° 21' 122,441 132,719 161,287 1-613 •787 1’26 —< 1 12° 39' 111° 59' 121,477 131,674 160,017 1-588 •794 1-25 — 140° 3' 110° 39' 120,513 130,629 158,747 1-563 •800 1 1-24 i it7° :>.<•/ -’<><> - 20' 1 10,5 IH 5 20, r, hi '157,477 1-538 •806 I.—NUMERICAL APERTURE TABLE, 85 N.A. TABLE Etc. Limit of tlie resolution of Nobert’s 19tli band 1-23 — 435° 17''108° 2' 118,584 128,539 1-56.207 1-513 •813 1-22 — 133° 4' 106° 45' 117,620 127,494 154,937 1-488 •820 = 113,000 lines to the inch with vertical 1-21 — 130° 57' 105° 30' 116,656 126,449 153,668 1-464 ■826 illumination. It has been resolved to the 1-20 — 128° 55' 104° 15' 115,692 125,404 152,397 1-440 •833 19th band by objectives of N.A. 1-4 achro- 119 — 126° 58' 103° 2' 114,728 124,359 151,128 1-416 •840 matic . . . . . • • -» 1-18 — 125° 3' 101° 50' 113,764 123,314 149,857 1-392 •847 1-17 — 123° 13' 100° 38' 112,799 122,269 148,588 1-369 •855 1-16 — 121° 26' 99° 29' 111,835 121,224 147,317 1-346 •862 1-15 — 115)° 41' 98° 20' 110,872 120,179 146,048 1-323 •870 1-14 — 118° 0' 97° 11' 109,907 119,134 144,777 1-300 •877 1*13 — 116° 20' 96° 2' 108,943 118,089 143,508 1-277 •885 1-12 — 114° 44' 94° 55' 107,979 117,044 142,237 1-254 •893 111 — 113° 9' 93° 47' 107,015 115,999 140,968 1-232 •901 1-10 —- 1110 36' 92° 43' 106,051 114,954 139,698 1-210 •909 ro9 — 110° 5' 91° 38' 105.087 113,909 138,428 1-188 ■917 ro8 — 108° 36' 90° 34' 104,123 112,864 137,158 1-166 •926 ro7 — 107° 8' 89° 30' 103,159 111,819 135,888 1-145 •935 roe — 105° 42' 88° 27' 102.195 110,774 134,618 1-124 •943 1-05 — 104° 16' 87° 24' 101.231 109,729 133,348 1-103 •952 1-04 — 102° 53' 86° 21' 100,266 108.684 132,078 1-082 •962 103 — 101° 30' 85° 19' 99,302 107,639 130,808 1-061 •971 102 — 100° 10' 84° 18' 98,338 106,593 129,538 1-040 •980 l’Ol — 98° 50' 83° 17' 97,374 105,548 128,268 1-020 •990 Maximum aperture of * Dry ’ or air objectives -» TOO 180° 0' 97° 31' 82° 17' 96,410 104,503 126,998 1-000 1 -ooo 0-99 163° 48' 96° 12' 81° 17' 95,446 103,458 125,728 •980 1-010 0'98 157° 2' 94° 56' 80° 17' 94,482 102,413 124,458 •960 1-020 Limit of resolution of A mp/t Ipleura pellucida, 097 151° 52' 93° 40' 79° 18' 93,518 101,368 123,188 •941 1-031 5)2,000 to 95,000 lines to the inch . . _> 0-96 147° 29' 92° 24' 78° 20' 92,554 100,323 121,918 •922 1-042 In actual practice this diatom in a 0*95 143° 36' 91° 10' 77° 22' 91,590 99,278 120,648 •903 1-053 medium of 2-4 refractive index has been re- 0-94 140° 6' 89° 56' 76° 24' 90,625 98,233 119,378 •884 1-064 solved to 93,000 striae per inch. The trans- 093 136° 52' 88° 44' 75° 27' 89,661 97,188 118,108 ■865 1-075 • verse striae may be easily resolved, and are 092 133° 51' 87° 32' 74° 30' 88,697 96,143 116,838 ■846 1-087 just discoverable with a new apochromatic 0-91 131° 0' 86° 20' 73° 33' 87,733 95,098 115,568 ■828 1-099 dry N.A. 1*0 by Powell and Lealand. 0-90 128° 19' 85° 10' 72° 36' 86,769 94,053 114,298 •810 1-111 Limit of resolution of the form of Navicula 0-89 125° 45' 84° 0' 71° 40' 85,805 93,008 113,028 •792 1-124 rhomboides known as Frustulia saxonioa 0-88 123° 17' 82° 51' 70° 44' 84,841 91,963 111,758 •774 1-136 and IV. crassinervis, 78,000 to 87,000 lines 0-87 120° 55' 81° 42' 69° 49' 83,877 90,918 110,488 •757 1-149 to the inch. In N. crassinervis transverse 0‘86 118° 38' 80° 34' 68° 54' 82,913 89,873 109,218 •740 1-163 striae 80,500 to the inch have been seen 6-85 116° 25' 79° 37' 68° 0' 81,949 88,828 107,948 •723 1-176 with Powell and Lealand’s dry i N.A, l-0-> 084 114° 17' 78° 20' 67° 6' 80,984 87,783 106,678 •706 1190 86 VISION WITH THE COMPOUND MICROSCOPE Corresponding Angle (2 u) for Limit of Resolving Power in Lines to an Inch Pene- Resolution, of Colletonema vulgarc in balsam, obj. apochr. \ -95 N.A. and Surirella gemma Numerical Aperture (N.A.= 11 sin u) Air 0=1-00) | Water 0=1-33) Ilomo- \ geneous [Immersion 0=1-52) White Light. (A=0-5269/a, Line E.) Mono- chromatic (Blue) Light. (A=0-4861 /a, Line E.) Photo- graphy. ( A=0-4000 g., near Line h.) minating Power (N.A.)" trating Power (&) 083 112° 12' 77° 14' 66° 12' 80,020 86,738 105,408 •689 1-205 (dry) into checks -> 0-82 110° 10' 76° 8' 65° 18' 79,056 85,693 104,138 •672 1-220 0-81 108° 10' 75° 3' 64° 24' 78,092 84,648 102,868 •656 1-235 0-80 106° 16' 73° 58' 63° 31' 77,128 83,603 101,598 •640 1-250 0-79 104° 22' 72° 53' 62° 38' 76,164 82,558 100,328 •624 1-266 0-78 102° 31' 71° 49' 61° 45' 75,200 81,513 99,058 ■608 1-282 077 100° 42' 70° 45' 60° 52' 74,236 80.468 97,788 •593 1-299 0-76 98° 56' 69° 42' 60° 0' 73,272 79,423 96,518 •578 1-316 0-75 97° 11' 68° 40' 59° 8' 72,308 78,378 95,248 •563 1-333 0-74 95° 28' 67° 37' 58° 16' 71,343 77,333 93,979 •548 1-351 073 93° 46' 66° 34' 57° 24' 70,379 76,288 92,709 •533 1-370 Resolution of Surirella gemma, fid,000 to 69,000 0-72 92° 6' 65° 32' 56° 32' 69,415 75,242 91,439 •518 1-389 0-71 90° 28' 64° 32' 55° 41' 68,451 74,197 90,169 ■504 1-408 lines per inch 0-70 88° 51' 63° 31' 54° 50' 67,487 73,152 88,899 •490 1-429 0-69 87° 16' 62° 30' 53° 59' 66,523 72,107 87,629 •476 1-449 068 85° 41' 61° 30' 53° 9' 65,559 71,062 86,359 •462 1-471 0-67 84° 8' 60° 30' 52° 18' 64,595 70,017 85,089 •449 1-493 066 82° 36' 59° 30' 51° 28' 63,631 68,972 83,819 •436 1-515 0'65 81° 6' 58° 30' 50° 38' 62,667 67,927 82,549 ■423 1-538 Resolution of N. rhomboides from Cherry field 064 79° 36' 57° 31' 49° 48' 61,702 66,882 81,279 •410 1-562 in balsam by apochr. \ in N.A. -65 . _> 0 63 78° 6' 56° 32' 48° 58' 60,738 65,837 80,009 •397 1-587 0-62 76° 38' 55° 34' 48° 9' 59,774 64,792 78,739 •384 1-613 Resolution of Pleurosigma fasciola, 55,000 to 061 75° 10' 54° 36' 47° 19' 58,810 63,747 77,469 •372 1-639 58,000 lines per inch . . . . 0-60 73° 44' 53° 38' 46° 30' 57,846 62,702 76,199 •360 1-667 059 72° 18' 52° 40' 45° 40' 56,881 61,657 74,929 •348 1-695 0-58 70° 54' 51° 42' 44° 51' 55,918 60,612 73,659 •336 1-724 0-57 69° 30' 50° 45' 44° 2' 54.954 59,567 72,389 •325 1-754 0-56 68° 6' 49° 48' 43° 14' 53,990 58,522 71,119 •314 1-786 0-55 66° 44' 49° 51' 42° 25' 53,026 57,477 69,849 •303 1-818 4 . • ...../ _ J 0 54 65° 22' 47° 54' 41° 37' 52,061 56,432 68,579 •292 1-852 Numerical Aperture Table—continued, N.A. TABLE Etc. 87 Resolution of Pleurosigma angulation, 44,000 to j 0'53 64° 0' 46° 58' 40° 48' 51,097 55,387 67,309 •281 1-887 49,000 lines per inch when mounted dry. 0'52 62° 40' 46° 2' 40° 0' 50,133 54,342 66,039 •270 1-923 < Dots ’ shown with axial illumination and 0‘51 (54° 20' 45° 6' 39° 12' 49,169 53,297 64,769 •260 1-961 wide cone. Obj. apochr. £ N.A. -65 . -> 0'50 60° 0' 44° 10' 38° 24' 48,205 52,252 63,499 •250 2-000 0-48 57° 22' 42° 18' 36° 49' 46,277 50,162 60,959 •230 2-083 0-46 54° 47' 40° 28' 35° 15' 44,349 48,072 58,419 •212 2-174 045 53° 30' 39° 33' 34° 27' 43,385 47,026 57,149 •203 2-222 0-44 52° 13' 38° 38 33° 40' 42,420 45,981 55,879 T94 2-273 042 49° 40' 36° 49' 32° 5' 40,492 43,891 53,339 •176 2-381 1 0-40 47° 9' 35° 0' 30° 31' 38,564 41,801 50,799 T60 2-500 0-38 44° 40' 33° 12' 28° 57' 36,636 39,711 48,259 •144 2-632 036 42° 12' 31° 24' 27° 24' 34,708 37,621 45,719 •130 2-778 0-35 40° 58' 30° 30' 26° 38' 33,744 36,576 44,449 T23 2-857 0-34 39° 44' 29° 37' 25° 51' 32,779 35,531 43,179 T16 2-941 0-32 37° 20' 27° 51' 24° 18' 30,851 33,441 40,639 T02 3-125 Resolution of Nitzscliia scalaris in balsam with 0‘30 34° 58' 26° 4' 22° 46' 28,923 31,351 38,099 ■090 3-333 apochr. obj. 1 in N.A.-30 . . . 0‘28 32° 32' 24° 18' 21° 14' 26,995 29,261 35,559 •078 3-571 0-26 30° 9' 22° 33' 19° 42' 25,067 27,171 33,019 •068 3-846 0*25 28° 58' 21° 40' 18° 56' 24,103 26,126 31,749 •063 4-000 0-24 27° 46' 20° 48' 18° 10' 23,138 25,081 30,479 •058 4-167 0-22 25° 26' 19° 2' 16° 38' 21,210 22,991 27,940 •048 4-545 020 23° 4' 17° 18' 15° 7' 19,282 20,901 25,400 •040 5-000 048 20° 44' 15° 34' 13° 36' 17,354 18,811 22,860 •032 5-555 0-16 18° 24' 13° 50' 12° 5' 15,426 16,721 20,320 •026 6-250 0-15 17° 14' 12° 58' 11° 19' 14,462 15,676 19,050 •023 6-667 044 16° 5' 12° 6' 10° 34' 13,498 14,630 17,780 •020 7143 012 13° 47' 10° 22' 9° 4' 11,570 12,540 15,240 ■014 8-333 010 11° 29' 8° 38' 7° 34' 9,641 10,450 12,700 •010 10-000 0-08 9° 11' 6° 54' 6° 3' 7,713 8,360 10,160 •006 12-500 0-06 6° 53' 5° 10' 4° 32' 5,785 6,270 7,620 •004 16-667 005 5° 44' 4° 18' 3° 46' 4,821 5,225 6,350 •003. 20-000 Example illustrating the accompanying Table. The apertures of four objectives, two of which are dry, one water-immersion, and one oil-immersion, would be compared on the angular aperture view as follows : Air Air Water Oil 106° ir>7° 142 ° 130° Their angular apertures are, however, as -80 •98 1-26 1-38 or as their numerical apertures. 88 VISION WITH THE COMPOUND MICROSCOPE a progressive, and with high powers a rapidly increasing, over- amplification of the depth of the three-dimensional image. If a transverse section of an object is magnified 100 times in breadth the distance between the planes of parts lying one behind the other is magnified 10,000 times at the corresponding parts on the axis when the object is in air, 7500 times when it is in water, and 6600 times when it is in Canada balsam. This excessive distortion in the case of high amplifications is not, however, of itself so complete a hindrance to correct appreciation of solid forms in the microscopical image as at first appears. The appreciation of solid form is not a matter of sensation only ; it is a mental act—a conception—and, therefore, the peculiarity of the optical, image, however great the amplification, would not prevent the conception of the solidity of the object so long as salient points for the construction of a three-dimensional image were found. But for this the solid object, as such, must be simultaneously visible ; a single layer of inappreciable depth can convey no conception of the three space dimensions possessed by the object. Owing to the disproportional amplification of the depth-dimension normal to the action of optical instruments, the visual space of the microscope loses more and more in depth as the amplification increases, and thus constantly approximates to a bare horizontal section of the object. The visual space, which at one adjustment of the focus is plainly visible, is made up of two parts, the limits of which as regards the depth are determined in a very different manner. First, the accommodation of the eye embraces a certain depth different planes being successively depicted with perfect sharpness of image on the retina, whilst the eye, adjusting itself by conscious nr unconscious accommodation, obtains virtual images of greater or less distance of vision. This depth of accommodation, which plays the same part in microscopical as in ordinary vision, is wholly determined by the extent of power in this direction possessed by the particular eye, the limits being the greatest and the least distance nf distinct vision. Its exact numerical measure is the difference between the reciprocal values of these twro extreme distances. The depth of distinct vision is directly proportional to this numerical equivalent of the accommodation of the eye, directly proportional to the refractive medium of the object, and inversely proportional to the square of the amplification when referred always to the same image-distance. For example, a moderately short-sighted eye sees distinctly at 150 mm. as its shortest distance, and at 300 mm. as its longest distance ; then the numerical equivalent of the extent of accommodation would be equal to ’y7T mm. ; the calculation for an object in air would give a depth of vision by accommodation amounting to 2-08 mm. with 10 times amplification 0-23 „ 30 0-02 „ 100 0-0023 „ 300 0-00021 „ 1000 0-00002 3000 PRINCIPLES OF STEREOSCOPIC VISION 89 These figures are modified by the medium in which the object is placed and by the greater or less shortness and length of vision. Secondly, the perception of depth is assisted by the insensi- bility of the eye to small defects in the union of the rays in the optic image, and therefore to small circles of confusion in the visual image. Transverse sections of the object which are a little above and below the exact focal adjustment are seen without prejudicial effects. The total effect so obtained is the so-called penetration or depth of focus of an objective. This may be determined numerically by defining the allowable magnitude of the circles of confusion in the micro- scopical image by the visual angle under which they appear to the eye. It is found that one minute of arc denotes the limit of sharply defined vision, two to three minutes for fairly distinct vision, and five to six minutes the limits of vision oidy just tolerable. This being determined, the focal depth depends only on the refractive index of the medium in which the object is placed, the amplification, and the angle of aperture, and it is directly proportional to the refractive index of the object medium, and inversely proportional to the ‘ numerical aperture ’ of the objective, as also to the first power of the amplification. These assume the visible angle of allowable indistinctness to be fixed at 5', the aperture angle of the image- forming pencils to be 60° in air ; the depth of focus of an object in air will then be— 0‘073 mm. for 10 times amplification 0-024 „ 30 0-0073 „ 100 0-0024 „ 300 0-00073 „ 1000 000024 „ 3000 By limiting or enlarging the allowable magnitude of indistinctness in the image we correspondingly modify these figures, as we should do with media of different refractive indices and increased aperture- angle. It is plain, then, that the actual depth of vision must always be the exact sum of the accommodation depth and focal depth. The former expresses the object space through which the eye by the play of accommodation can penetrate and secure a sharp image : the latter gives the amount by which this object-space is extended in its limits—reckoning both from above and below—because without perfect sharpness of image there is still a sufficient distinctness of vision. As the amplification increases the over-amplification of the depth-dimension presents increasingly unfavourable relation between the depth and width of the object-space accessible to accommodation. When low powers are employed we have relatively great depth of vision, because we have large accommodation-depth ; but we pass to medium powers, the accommodation-depth diminishes in rapid ratio, becoming equal to only a small depth of focus ; while when the magnifying power is great!}7 increased the accommodation depth is a factor of no moment, and we have vision largely, indeed almost wholly, dependent on depth of focus. VISION WITH THE COMPOUND MICROSCOPE 90 The following table shows the total depth of vision from ten to 3,000 times :— Amplification Diameter of Field Accommoda- tion Depth Focal Depth Depth of Vision, Accommodation Depth, and Focal Depth liatio of Depth of Vision to Diameter of Field 10 mm. 25*0 mm. 2-08 mm. 0 073 mm. 2*153 1 IT-7; 30 8-3 023 0021 0-254 l W-7 100 2-5 0-02 0-0073 0-0273 1 917; 300 0-83 0-0023 00024 0-0047 i 176-6 1000 0-25 0-00021 0-00073 0-00094 1 266 3000 0-083 0-00002 0-00024 0-00023 1 Till* It has been pointed out by Abbe that this over-amplification of depth-dimension, though it limits the direct appreciation of solid forms, yet is of great value in extending the indirect recognition of space relations. When with increase of magnifying power the depth of the image becomes more and more flattened, the images of different planes stand out from each other more perfectly in the same ratio, and in the same degree are clearer and more distinct. With an increase of amplification the microscope acquires increasingly the property of an optical microtome, which presents to the observer’s eye sections of a fineness and sharpness which would be impossible to a mechanical section. It enables the observer by a series of adjustments for consecutive planes, to construe the solid forms of the smallest natural objects with the same certainty as he is accustomed to see with the naked eye the objects with which it is concerned. This is a large advantage in the general scientific use of the instrument ; a greater gain, in fact, than could be expected from the application of stereoscopic observation. Stereoscopic Binocular Vision.—This subject has been elaborately considered and partially expounded and demonstrated by Professor Abbe ; his exposition differs in some important particulars from that of the original author of this book, but in its present incomplete form it appears to the editor to be the wiser way to allow Dr. Car- penter’s treatment of the subject to stand, and to place below it as complete a digest of Professor Abbe’s theory and explanation of the same subject as the data before us will admit. The admirable invention of the stereoscope by Professor Wheat- stone has led to a general appreciation of the value of the conjoint use of both eyes in conveying to the mind a notion of the solid forms of objects, such as the use of either eye singly does not generate with the like certainty or effectiveness ; and after several attempts, which were attended with various degrees of success, the principle of STEREOSCOPIC BINOCULAR VISION 91 the stereoscope has now been applied to the microscope, with an advantage which those only can truly estimate who (like the Author} have been for some time accustomed to work with the stereoscopic- binocular1 upon objects that are peculiarly adapted to its powers. As the result of this application cannot be rightly understood with- out some knowledge of one of the fundamental principles of binocular- vision, a brief account of this will be here introduced. All vision depends in the first instance on the formation of a picture of the object upon the retina of the eye, just as the camera obscura forms, a picture upon the ground glass placed in the focus of its lens. But the two images that are formed by the two eyes respectively of any solid object that is placed at no great distance in front of them are far from being identical, the perspective projection of the object varying with the point of view from which it is seen. Of this the reader may easily convince himself by holding up a thin book in such a position that its back shall be at a moderate distance in front of the nose, and by looking at the book, first with one eye and then with the other ; for he will find that the two views he thus obtains- are essentially different, so that if he were to represent the book as he actually sees it with each eye, the two pictures would by no- means correspond. Yet on looking at the object with the two eyes conjointly, there is no confusion between the images, nor does the mind dwell on either of them singly ; but from the blending of the two a conception is gained of a solid projecting body, such as could only be otherwise acquired by the sense of touch. Now if, instead of looking at the solid object itself, we look with the right and left eyes respectively at pictures of the object, corresponding to those which would be formed by it on the retime of the two eyes if it were placed at a moderate distance in front of them, and these visual pictures are brought into coincidence, the same conception of a solid projecting form is generated in the mind, as if the object itself were- there. The stereoscope—whether in the forms originally devised by Professor Wheatstone or in the popular modification long subse- quently introduced by Sir D. Bi’ewster—simply serves to bring to- the two eyes, either by reflexion from mirrors or by refraction through prisms or lenses, the two dissimilar pictures which would accurately represent the solid object as seen by the two eyes respec- tively, these being thrown on the two retinae in the precise positions, they would have occupied if formed there direct from the solid object, of which the mental image (if the pictures have been correctly taken) is the precise counterpart. Thus in fig. 69 the upper pair of pictures (A, B) when combined in the stereoscope suggest the idea of a projecting truncated pyramid, with the small square in the centre- and the four sides sloping equally away from it ; whilst the combi- nation of the lower pair, C, D (which are identical with the upper, but are transferred to opposite sides), no less vividly brings to the mind the visual conception of a receding pyramid, still with the- 1 It has become necessary to distinguish the binocular microscope which gives, true stereoscopic effects by the combination of two dissimilar pictures from a binocular which simply enables us to look with both eyes at images which are essentially identical Ip. 106). 92 VISION WITH THE COMPOUND MICROSCOPE small square in the centre, but the four sides sloping equally to- wards it. Tlius we see that by simply crossing the pictures in the stereo- scope, so as to bring before each eye the picture taken for the other, a ‘ conversion of relief ’ is produced in the resulting solid image, the projecting parts being made to recede and the receding parts brought into relief. In like manner, when several objects are com- bined in the same crossed pictures, their apparent relative distances are reversed, the remoter being brought nearer and the nearer carried backwards ; so that (for example) a stereoscopic photograph Fig. 69. representing a man standing in front of a mass of ice shall, by the crossing of the pictures, make the figure appear as if imbedded in the ice. A like conversion of relief may also be made in the case of actual solid objects by the use of the pseudoscope, an instrument devised by Professor Wheatstone, which has the effect of reversing the perspective projections of objects seen through it by the two eyes respectively ; so that the interior of a basin or jelly-mould is made to appear as a projecting solid, whilst the exterior is made to appear hollow. Hence it is now customary to speak of stereoscopic vision as that in which the conception of the true natural relief of an object is called up in the mind by the normal combination of the two perspective projections formed of it by the right and left eyes respectively ; whilst by pseudoscopic vision we mean that ‘ conver- sion of relief ’ which is produced by the combination of two reversed perspective projections, whether these be obtained directly from the object (as by the pseudoscope) or from ‘ crossed ’ pictures (as in the stereoscope). It is by no means every solid object, however, or every pair of stereoscopic pictures which can become the subject of this conversion. The degree of facility with which the ‘ converted 5 form can be apprehended by the mind appears to have great influence on the readiness with which the change is produced. And while there are some objects—the interior of a plaster mask of a face, for ex- ample—which can always be ‘ converted ’ (or turned inside out) at CARPENTER'S V. ABBE’S VIEW OF STEREOSCOPIC VISION 93 once, there are others which resist such conversion with more or less of persistence.1 Now it is easily shown theoretically that the picture of any projecting object seen through the microscope with only the right hand half of an objective having an even moderate angle of aperture,, must differ sensibly from the picture of the same object received through the left hand of the same objective ; and, further, that the difference between such pictures must increase with the angular- aperture of the objective. This difference may be practically made apparent by adapting a 1 stop ’ to the objective in such a manner as. to cover either the right or the left half of its aperture, and then by carefully tracing the outline of the object as seen through each half. But it is more satisfactorily brought into view by taking two photo- graphic pictures of the object, one through each lateral half of the objective ; for these pictures when properly paired in the stereo- scope give a magnified image in relief, bringing out on a large scale the solid form of the object from which they were taken. What is needed, therefore, to give the true stereoscopic power to the micro- scope is a means of so bisecting the cone of rays transmitted by the objective that of its two lateral halves one shall be transmitted to the right and the other to the left eye. If, however, the image thus formed by the right half of the objective of a compound microscope were seen by the right eve, and that formed by the left half were seen by the left eye, the resultant conception would be not stereo- scopic but pseudoscopic, the projecting parts being made to appear receding, and vice versa. The reason of this is, that as the microscope itself reverses the picture, the rays proceeding through the right and the left hand halves of the objective must be made to cross to the left and the right eyes respectively, in order to correspond with the direct view of the object from the two sides ; for if this second reversal does not take place, the effect of the first reversal of the images produced by the microscope exactly corresponds with that produced by the ‘ crossing ’ of the pictures in the stereoscope, or by that reversal of the two perspective projections formed direct from the object, which is effected by the pseudoscope. It was from a want of due appreciation of this principle (the truth of which can now be practically demonstrated) that the earlier attempts at pro- ducing a stereoscopic binocular microscope tended rather to produce a £ pseudoscopic conversionJ of the objects viewed by it than to represent them in this true relief. In contradistinction to this explanation of binocular vision Dr. Abbe, as we have seen, has demonstrated that oblique vision in the microscope is wholly unlike ordinary vision ; there is, in fact, no perspective. The perspective shortening of lines and surfaces by oblique projection is entirely lost in the microscope, and, as a con- sequence, it is contended that the special dissimilarity which is the raison d’etre of ordinary stereoscopic effects does not exist, but that an essentially different mode of dissimilarity is found between the two pictures. The outline or contour of a microscopic object is 1 For a fuller discussion of this subject see the Author’s Mental Physiology §§ 168-170. VISION WITH THE COMPOUND MICROSCOPE 94 unaltered, whether viewed by an axial or an oblique pencil; there is no foreshortening, there is simply lateral displacement of the images of consecutive layers. But Abbe contends that, whilst the manner in which dissimilar pictures are formed in the binocular microscope is different from that by which they are brought about in ordinary .stereoscopic vision, yet the activities of the brain and mind by which they are so blended as to give rise to sensations of solidity, depth, and perspective are practically identical. The fact that lateral displacements of the image are seen in the microscope depends on a peculiar property of microscopic amplifica- tion, which is in strong contrast to the method of ordinary vision. It depends entirely on the fact, enunciated above, that the amplifi- cation of the depth is largely exaggerated. Hence solid vision in the binocular microscope is confined to large and coarse objects, the dimensions of which are large multiples of the wave-length. It therefore follows that when moderate or large apertures have to be 'employed—that is to say, whenever delineation requires the employ- ment of oblique rays the elements of the object are no longer depicted as solid objects seen by the naked eye or through the telescope would be depicted ; nevertheless the brain arranges them so that the characteristics of solid vision are still presented. Professor Abbe demonstrates 1 that in an aplanatic system pencils of different obliquities yield identical images of every plane object, or of a single layer of a solid object. This is true however large the aperture may be. This carries with it, as we have said, a total absence of perspec- tive and an essential geometrical difference between vision with the binocular microscope and vision with the unaided eye. An object, not quite flat, as a curved diatom, when observed with an objective of wide aperture will present points of great indistinct- ness. This has been by some supposed to arise from the assumption that there was a dissimilarity between the images formed by the axial and oblique pencils ; but this is not so. It is wholly explic- able by the fact that the depth of the object is too great for the small depth of vision attendant upon a large aperture. It will be seen, then, that so long as the depth of the object is within the limits of the depth of vision, corresponding to the aperture and amplification in use, we obtain a distinct parallel projection of all the successive layers in one common plane perpendicular to the axis of the microscope—a ground plan, as it were, of the object. Manifestly, then, since depth of vision decreases with increasing aperture, good delineation with these must be confined to thinner ■objects than can be successfully employed with objectives of narrow apertures. Stereoscopic vision with the microscope, therefore, is due solely to difference of projection exhibited by the different parallactic dis- placements of the images of successive layers on the common ground plane and to the perception of depth, not to the delineation of the plane layers themselves. For, if there were dissimilar images per- 1 Journ. B.M.S. series ii. vol. iv. pp. 21-24. ABBE ON STEREOSCOPIC VISION 95 ceptible at different planes, the out-of-focus layers must appear con- fused and no vision of depth would be possible. Now stereoscope vision requires, as shown by Dr. Carpenter, that the delineating pencils shall be so divided that one portion of the admitted cone of light is conducted to one eye and another portion to the other eye. If this division of the image is effected in a symmetrical way, the cross section of, e.g. a circle must be reduced to two semicircles representing one of these two arrangements seen in O and P, fig. 70. Fig. 70. Dr. Abbe’s theory is that the only condition necessary for ortho- scopic effect in any binocular system is that these semicircles or their equivalents should be depicted according to diagram O, fig. 70, and for pseudoscopic effect according to diagram P in the same figure ; and he demonstrates that all other circumstances, such, e.g. as the crossing of the images, are wholly immaterial. Orthoscopic vision is always obtained when the right half of the right pupil and the left half of the left pupil only are employed ; pseudoscopic vision in the opposite conditions. ‘ It is quite indif- ferent whether the effect is obtained with crossing or non-crossing rays, whether the image be erect, or inverted, or semi-inverted, and whatever may be components of the optical arrangement.’ The observant reader will perceive that it is at this point that there is a radical divergence from the interpretation given by Dr. Carpenter, who, as we have seen above, insisted that orthoscopic vision is not to be obtained in a binocular with non-erecting eye-pieces unless the axes of the two halves of the admitted cone cross each other. Of course we must keep clearly before us the fact that in micro- scopic vision it is not the object but its virtual image only that we see. This apparently solid image is placed in the binocular micro- scope under circumstances similar to those of common objects in ordinary vision. Clearly, then, it is the perspective projections of this image which require to be compared to the projections of solid objects in ordinary vision, in respect to which the criteria of ortho- scopic and pseudoscopic vision have been defined. But it can be geometrically demonstrated that right-eye perspective of the ap- parently solid image is always obtained from the right-hand portion of the emergent pencils, left-eye perspective from the left-hand portion ; and it is quite immaterial, as regards this result, which portion of the emergent rays is admitted by the right or the left part of the objective. The manner in which the delineating pencils are transmitted through the system may be such as to require crossing over of the rays from the right-hand half of the objective to the left eye-piece, and vice versa. But it is not essential to binocular effect. In the Wenham and Nachet binocular (pp. 98, 99) crossing over is required because the inversion of the pencils is not changed by two reflexions. If the delineating pencils have been reflected an even number of times 96 VISION WITH THE COMPOUND MICROSCOPE Fig. 72. Fig. 71 BINOCULAR MICROSCOPES in the same plane, it will be necessary for the rays to cross ; but if they have been reflected an odd number of times, it is not only un- necessary, but is destructive of orthoscopic effect, provided ordinary ■■eye-pieces (non- erecting) are employed. Hence in the Stephenson bin- ocular it is not only not required, but would give pseudoscopic effect. 97 Principal Forms ot Binocular Microscopes.—The first binocular of a practical character was the arrangement of Professor J. L. Riddell, of New Orleans. It was devised in 1851 and constructed in 1852, and a description of its nature and its genesis was given by him in the second volume of the first series of the c Quarterly Journal of Microscopical Science ’ in the year 1851.1 A representation of his original instrument is presented in fig. 71, ■and the arrangement of the prisms by which, the binocular effect was obtained is shown in fig. 72. It will be seen that the pencil of rays emerging from the back lens of the combination l is divided into two, each half passing re- spectively into the right and left prisms ; the path of the rays is indicated at a, b, c, d, the object being at o. To secure coincidence of the images in the field of view for varying widths between the eyes Professor Riddell devised (1) a means of regulating the inclination of the prisms by mounting them in hinged frames, so that, while their lower edges, near a, fig. 72, remain always in parallel contact, the inclination of the internal refiecting surfaces can be varied by the action of the milled head in front of the prism box; (2) tin ■lower ends of the binoculai tubes are connected by travel- ling sockets, moving on one and the same axis, on which are cut corresponding right - and left-handed screws, so that the width of the tubes may ■correspond with that of the prisms ; and (3) the uppei -ends of the tubes are con- nected by racks, one acting ■above and the other below the same pinion, so that right- and left-handed movements are communicated by turning the pinion. This instrument could only be used in a vertical position, as shown in the figure (71) ; to obviate this considerable -drawback Riddell mounted two right-angled prisms in brass caps, which could be slipped over the eye-pieces. This arrangement in- verts the image in both planes, and it is seen through the instru- ment as in nature. This system of binocular excited much interest in England im- Fig. 78.—Arrangement of prisms in Nachet’s stereoscopic binocular microscope. i P. 13. 98 VISION WITH THE COMPOUND MICROSCOPE mediately after its publication, and Mr. Wenliam in London and MM. Nachet, of Paris, soon suggested and devised a variety of binocular systems. Nachet’s Binocular.—One of these (not now, we have reason to believe, advocated or employed by its inventor) was that devised by MM. Nachet, constructed on the method shown in tig. 73. The cone of rays issuing from the back lens of the objective meets the Hat surface of a prism (p) placed above it, whose section is an equi- lateral triangle, and is divided by reflexion within this prism into two lateral halves, which cross each other in its interior. The rays a b that form the right half of the cone, impinging very obliquely on the internal face of the prism, suffer total reflexion, emerging through its left side perpendicularly to its surface, and therefore undergoing no refraction ; whilst the rays a' b', forming the left half of the cone, are reflected in like manner towards the right. Each of these pencils is received by a lateral prism, which again changes its direc- tion, so as to render it parallel to its original course, and thus the two halves a b and a' b' of the original pencil are completely separated from each other, the former being received into the left-hand body of the microscope (fig. 73), and the latter into its right-hand body. These two bodies are parallel ; and, by means of an adjusting screw at their base, which alters the distance between the central and the lateral prisms, they can be separated from or approximated towards each other, so that the difference between their axes can be brought into exact coincidence with .the distance between the axes of the eyes of the individual observer. This instrument gives true ‘ stereo- scopic ’ projection to the conjoint image formed by the mental fusion of the two distinct pictures, and with low powers of moderate angular aperture its performance is highly satisfactory. There are, however, certain drawbacks to its general utility. First, every ray of each pencil suffers two reflexions, and has to pass through four surfaces: this necessarily involves a considerable loss of light, with a further liability to the impairment of the image by the smallest want of exactness in the form of either of the prisms. Secondly, the mechanical arrangements requisite for varying the distance of the bodies involve an additional liability to derangement in the adjust- ment of the prisms. Thirdly, the instrument can only be used for its own special purpose; so that the observer must also be provided with an ordinary single-bodied microscope for the examination of objects unsuited to the powers of his binocular. Fourthly, the paral- lelism of the bodies involves parallelism of the axes of the observer’s eyes, the maintenance of which for any length of time is fatiguing. Wenham’s Stereoscopic Binocular.—All these objections are overcome in the admirable arrangement devised by the ingenuity of Mr. Wenliam, in whose binocular the cone of rays proceeding up- wards from the objective is divided by the interposition of a prism of the peculiar form shown in fig. 74, so placed in the tube which carries the objective (figs. 75, 76, a), as only to interrupt one half, o c, of the cone, the other half, a b, going on continuously to the eye- piece of tlie principal or right-hand body, R, in the axis of which the objective is placed. The interrupted half of the cone (figs. 74, 75, a). WENHAM’S BINOCULAR PRISM on its entrance into the prism, is scarcely subjected to any refraction, since its axial ray is perpendicular to the surface it meets; but within the prism it is subjected to two reflexions at b and c, which send it forth again obliquely in the line d towards the eye-piece of the secondary or left-hand body (tig. 75, L); and since at its emergence its axial ray is again perpendicular to the surface of the glass, it suffers no more refraction on passing out of the prism than on entering it. By this arrangement the image received by the right eye is formed by the rays which have passed through the left half of the objective, and have come on without any interruption whatever; whilst the image received by the left eye is formed by the rays which have passed through the right half of the objective, and have been sub- jected to two reflexions within the prism, passing through only two surfaces of glass. The adjustment for the variation of distance between the axes of the eyes in different in- dividuals is made by drawing out or pushing in the eye-pieces, which 99 Fig. 74.—Wenliam’s prism. Wenliam’s stereoscopic binocular microscope. Fig. 75. Fig. 76. are moved consentaneously by means of a milled-head, as shown in tig. 76. Now, although it may be objected to Mr. Wen ham’s method (1) that, as the rays which pass through the prism and fire obliquely reflected into the secondary body traverse a longer distance than VISION WITH THE COMPOUND MICROSCOPE those which pass on uninterruptedly into the principal body, the picture formed by them will be somewhat larger than that which is formed by the other set; and (2) that the picture formed by the rays which have been subjected to the action of the prism must be inferior in distinctness to that formed by the uninterrupted half of the cone of rays ; these objections are found to have no practical weight. For it is well known to those who have experimented upon the phenomena of stereoscopic vision (1) that a slight differ- ence in the size of the two pictures is no bar to their perfect com- bination ; and (2) that if one of the pictures be good, the full effect of relief is given to the image, even though the other picture be faint and imperfect, provided that the outlines of the latter are sufficiently distinct to represent its perspective projection. Hence if, instead of the two equally half-good pictures which are obtainable by MM. Nachet’s original construction, we had in Mr. Wenham’s one good and one indifferent picture, the latter would be decidedly preferable. But, in point of fact, the deterioration of the second picture in Mr. Wenham’s arrangement is less considerable than that of both pictures in the original arrangement of MM. Nachet; so that the optical performance of the Wen- ham binocular is in every way superior. It has, in addition, these further advantages over the preceding : First, the greater com- fort in using it (especially for some length of time together), which results from the convergence of the axes of the eyes at their usual angle for moderately near objects; secondly, that this binocular arrangement does not necessitate a special instrument, but may be applied to any microscope which is capable of carrying the weight of the secondary body, the prism being so fixed in a movable frame that it may in a moment be taken out of the tube or replaced there- in, so that when it has been removed the principal body acts in every respect as an ordinary microscope, the entire cone of rays passing uninterruptedly into it; and thirdly, that the simplicity of its construction renders its derangement almost impossible.1 Pig. 77.—Riddell’s binocular prisms, as applied by Mr. Stephenson. Stephenson’s Binocular.—A new form of stereoscopic binocular lias been introduced by Mr. Stephenson,2 which lias certain dis- tinctive features, and at the time Mr. Stephenson devised it he was entirely unaware that any part of the method lie employed had been used by another. He had, however, independently conceived Rid- dell’s device for dividing the beam as a part of his very ingenious instrument. This he discovered and acknowledged about three 1 The Author cannot allow this opportunity to pass without expressing his sense of the liberality with which Mr. Wenham freely presented to the public this im- portant invention, by which, there can be no doubt, he might have largely pro- fited if he had chosen to retain the exclusive right to it. 8 Monthly Microscopical Journal, vol. iv. (1870), p. 61, and vol. vii. (1872), p. 167, STEPHENSON’S BINOCULAR years after the full description and completion of his binocular.1 The cone of rays passing upwards from the object-glass meets a pair of prisms (A A, fig. 77) fixed in the tube of the microscope imme- diately above the posterior combination of the objective, so as to catch the light-rays on their emergence from it; these it divides into two halves and behaves as described in the Riddell prisms, which, in fact, they are. As the cone of rays is equally divided by the two prisms, and its two halves are similarly acted on, the two pictures are equally illuminated, and of the same size ; while the close approximation of the prisms to the back lens of the objective enables even high powers to be used with very little loss of light or of definition, provided that the angles and surfaces of the prisms are worked with exactness ; and as the two bodies can be made to converge at a smaller angle than in the Wenham arrangement, the observer looks through them with more comfort. But Mr. Ste- phenson’s ingenious arrangement is liable to the great drawback of not being convertible (like Mi-. Wenham’s) into an ordinary monocular by the withdrawal of a prism, so that the use of this form of it will be probably restricted to those who desire to work with a binocular when employing high powers. In order to avoid slight errors arising from the impinging of the central ray of the cone, at its emergence from the objective, against the double edge of the prism-combination, Mr. Stephenson has devised a special form of sub-stage condenser (also made by Mr. Browning), which causes the illuminating rays to issue from the object in two separate pencils, which will strike the surfaces of the two prisms. This consists of two deep cylindrical lenses A and B, fig. 78, whose focal lengths are as 2’3 to 1, having their curved faces opposed to each other, as shown in section at C ; the larger and less convex being placed with its plane side downwards, so as to receive light from the mirror, or (which is preferable) direct from a lamp. Under this combination slides a movable stop, with two circular openings, as shown in fig. 79. The lamp being placed in front of the instrument, the two apertures admit similar pencils of light from it, so that each eye receives a completely equal illumination, and no confusion can occur from the impinging of the rays on the lower edges of the prisms. With this arrangement the Podura markings are shown as figured by the late Richard Beck, while the curvatures of the scale come out with the distinctness peculiar to binocular vision. Fig. 78. Fig. 79. 1 Monthly Microscopical Journal, vol. x. p. 41. 102 VISION WITH THE COMPOUND MICROSCOPE But one of the greatest advantages attendant on Mr. Stephen- son’s construction is its capability of being combined with an erecting arrangement, which renders it applicable to purposes for which' the Wenham binocular cannot be conveniently used. By the interposition of a plane silvered mirror, or (still better) of a reflecting prism (fig. HO), above the tube containing the binocular prisms, each half of the cone of rays is so deflected that its image is reversed vertically, the rays entering the prism through the surface C B, being reflected by the surface A B, so as to pass out again by the surface A C in the direc- tion of the dotted lines. Tlius the right and the left half-cones are directed respec- tively into the right and the left bodies, which are inclined at a convenient angle, as shown in fig. 81 ; so that— the stage being horizontal—the instru- ment becomes a most useful compound dissecting microscope, and as thus ar- ranged by Swift, with well adjusted rests for the hands, has but few equals for the purposes of minute dissectio'ns and delicate mounting operations ; indeed, the value of the erecting binocular consists in its applic- ability to the picking out of very minute objects, such as Diatoms, Polycystina, or Foraminifera, and to the prosecution of minute dissections, especially when these have to be carried on in fluid. No one who has only thus worked monocularly can appreciate the guidance derivable from binocular vision when once the habit of working with it has been formed. Tolies’ Binocular Eye-piece. An ingenious eye-piece has been constructed by Mr. Tolies (Boston, U.S.A.), which, fitted into the body of a monocular microscope, converts it into an erecting stereo- scopic binoculnr. This conversion is effected by the interposition of a system of prisms similar to that originally devised by MM. Nachet, but made on a larger scale, between an ‘erector’ (re- sembling that used in the eye-piece of a day-telescope) and a pair of ordinary Huyghenian eye pieces, the centraI or dividing prism being placed at or near the plane of the secondary image formed by the erector, while the two eye-pieces are placed immediately above the two lateral prisms, and the combination thus making that division in the pencils forming the secondary image which in the Fig. HO.—Stephenson’s erecting prism. Fig. 81.—Stephenson’s erecting binocular. AUBE’S BINOCULAR EYE-PIECE 103 (Nachet binocular it makes in the pencils emerging from the objective. As all the image-forming rays have to pass through the two surfaces of four lenses and two prisms, besides sustaining two internal re- flexions in the latter, it is surprising that Professor H. L. Smith, while admitting a loss of light, should feel able to speak of the definition of this instrument as not inferior to that of either the Wenham or the Nachet binocular. It is obviously a great advantage that this eye-piece can be used with any microscope and with objectives of high power ; but as its effectiveness must depend upon extraordinary .accuracy of workmanship its cost must necessarily be great.1 Fig. 82.—Abbe’s binocular eye-piece. Abbe's Stereoscopic Eye-piece.—Fig. 82 represents this in- strument in section. The body A A' contains three prisms of crown glass, «, b, and b'. The two eye-pieces B, B' are let into the top plate, the former being fixed, whilst the latter has a lateral sliding movement; the bottom plate carries the tube C for inserting the eye-piece into the microscope tube like an ordinary eye-piece. The two prisms a and b are united so as to form a thick plate with parallel sides, their continuity, however, being broken by an 1 See American Journal of Science, vol. xxxviii. (1864), p. Ill, and vol. xxxix. (1865), p. 212; and Monthly Microsc. Journal, vol. vi. (1871), p. 45. 104 VISION WITH THE COMPOUND MICROSCOPE exceedingly thin stratum of air less than O'Ol mm.—inclined to*, the axis at an angle of 38‘5°. The cone of rays from the objective is divided into two parts, one being transmitted and the other reflected. The transmitted rays pass through a, b without deviation, and form an image of the object in the axial eye-piece /3. The rays- reflected at the angle shown in the figure pass through the second, surface of the prism b (upon which they are incident at right angles), and emerging at an inclination of 13° with the horizontal are- totally reflected into the eye-piece [3' at an angle of 90° by the hypothenuse surface of the right-angled equilateral prism b', the- axis of which also makes an angle of 13° with the axis of the microscope. Adjustment for different distances between the eyes is effected by the screw D, which moves the eye-piece f3', together with the- prism b\ in a parallel direction. The tubes of the eye-pieces can also be drawn out if greater separation is required. The eye-pieces have the usual two lenses, but are of special con- struction in order to equalise the length of the direct axis and the doubly reflected axis, and in spite of this inequality obtain sharply defined images of equal amplification with the same focus. Stereoscopic vision is obtained by halving the cones of rays- above the eye-pieces. This is effected by stopping off half of the real image of the objective opening formed above the eye-pieces at the so-called ‘eye-point’ (3 or /?', which represents the common cross-section of all the pencils emerging from the eye-piece. A cap with a semicircular diaphragm is fitted to the eye-piece (shown in the figure over /?'), the straight edge of which is exactly in the- optic axis of the eye-piece, and can be raised or lowered by screwing so as to obtain a uniform bisection of the cones of rays from every point of the field. The height of the diaphragm is regulated once for all for the- same length of the microscope-tube by finding the position for which the aperture-image (which on withdrawing the eye from the eye- piece is visible as a bright circle above it) shows no parallax against the straight edge of the diaphragm, i.e. so that on moving the eye- laterally the image always appears to adhere to the edge. In addition to the above caps with diaphragms the instrument is supplied with ordinary caps with circular apertures, as in /?.. They taper slightly and simply slide into the eye-piece, so that they can be readily changed. The special feature of the instrument is 1st, that it is capable of being used with the highest powers; and ■Jndly, that it is not necessary to cover up half of each of the eye-piece tubes, thus losing half the total amount of light. It is sufficient if one only (the lateral one) is half obscured, leaving the other free. As the normal division of light between the two tubes is two-thirds (in the axial) and one-third (in the lateral), the total loss of light is reduced to one-sixth. The field of view in the axial eye-piece in this arrangement in any case necessarily appears brighter than that of the lateral one seen with the same eye, and in regard to this Dr. Abbe re- marks that the difference between the brightness of the two fields in binocular observation £ is not only no defect, but, on the contraryr USE OF THE BINOCULAR 105 a decided advantage.’ For experience has long proved that, to- obtain a good stereoscopic effect, it is only necessary that one image should be as perfect and clear as possible, whilst the other may,, without appreciable disadvantage, be of sensibly less perfection. It might therefore be anticipated that this would apply (as in fact it does) in the same way to difference of luminosity. Moreover, an additional fact must be taken into account—that the two eyes, especially of microscopists, always show unequal sensibility to light as the result of constant unequal use. The less used eye, whose acuteness of vision is always less than that of the one more fre- quently exercised, shows a greater sensibility to light, and the difference is so considerable that the less luminous image of the lateral eye-piece, when viewed with the less exercised (generally the left) eye, seems even brighter than the other when viewed with the- exercised eye. The unequal division of the light is therefore a welcome element, as it serves to equalise this physiological difference. The observer has only to take care that the less used eye is applied to the lateral eye-piece ; and 3rdly, the ingenious arrangement whereby, by simply turning the caps with the diaphragms, orthoscopic or pseudoscopic effect can be produced instantaneously at pleasure. It is more particularly available for tubes of short length, for which the Wenham prism is inapplicable. The stereoscopic binocular is put to its most advantageous use- when applied either to opaque objects of whose solid forms we are desirous of gaining an exact appreciation or to transparent objects which have such a thickness as to make the accurate distinction between their nearer and their more remote planes a matter of im- portance. All stereoscopic vision with the microscope, so far as it is anything more than mere seeing with two eyes, depends, as already seen, exclusively upon the unequal inclination of the pencils which form the two images to the plane of the preparation, or the axis of the microscope. By uniform halving of the pencils—whether by prisms above the objective or by diaphragms over the eye-pieces— the difference in the directions of the illumination in regard to the preparation reaches approximately the half of the angle of aperture of the objective, provided that its whole aperture is filled with rays. By the one-sided halving we have been considering, the direct image is produced by a pencil the axis of which is perpendicular to the plane of the preparation, and the deflected image by one whose axis is inclined about a fourth of the angle of aperture. With low powers, which allow of a relatively considerable depth-perspective, the slight difference of inclination, which remains- in the latter case, is quite sufficient to produce a very marked dif- ference in the perspective of the successive layers in the images. But with high powers the difference in the two images does not keep pace—even when both eye-pieces are half-covered with the increase of the angle of aperture, so long as ordinary central illumi- nation is used. For in this case the incident pencil does not fill the whole of the opening of the objective, but only a relatively small central part, which, as a rule, does not embrace more than 40 of angle, and in most cases cannot embrace more without the clearness of the microscopic image being affected and the focal 106 VISION WITH THE COMPOUND MICROSCOPE depth also being unnecessarily decreased. But as those parts of the preparation which especially allow of solid conception are always formed by direct transmitted rays in observation with transmitted light, it follows that under these circumstances the difference of the two images is founded, not on the whole aperture-angle of the ob- jective, but on the much smaller angle of the incident and directly transmitted pencils, which only allow of relatively small differences of inclination of the image-forming rays to the preparation. It is evident, however, that when objectives of short focus and correspondingly large angle are used, a considerably greater differentiation of the two images with re- spect to parallax can be produced if, in place of one axial illuminating pencil, two pencils are used oppositely inclined to the axis in such a way that each of the images is produced by one of the pencils. This kind of double illumination, though it, cannot be obtained by the simple mirror, can be easily produced by using with the condenser a diaphragm with two opening s (fig. 83), placed in the diaphragm stage under the con- denser. We then have it in our power to use, at pleasure, pencils of narrower or wider aperture and of greater or less inclination towards the axis by making the openings of different width and different distance apart. With diaphragms of this form (which can easily be made out of card-board) the larger aperture angles of high-power objectives may be made use of to intensify the stereoscopic effect without employing wide pencils, which are prejudicial both as diminishing the clearness of the image and the focal depth. Of course with this method of illumination both eye-pieces must be half covered in order that one image may receive light only from one of the two illuminating cones, and the other only from the other. The division of light in both the aperture- images will then be as shown in fig. 84; and it is evident that in this case the brightness of the image for both eyes together is exactly the same as would be given by one of the two cones alone without any covering. The method of illumination here referred to—which was origin- ally recommended by Mr. Stephenson for his binocular microscope— has, in fact, proved itself to be by far the best when it is a question of using higher powei’s than about 300 times. It necessai'ily requires very well corrected and properly adjusted objectives if the sharpness of the image is not to suffer; but if these conditions are satisfied it yields most striking stereoscopic effects, even with objectives of 2 mm. and less focal length, provided the preparation under obser- vation pi’esents within a small depth a sufficiently characteristic structure. Fig. 83. Fici. 84. Non-Stereoscopic Binoculars.—The great comfort which is ex perienced by the microscopist from the conjoint use of both eyes has POWELL AND LEALAND’S HIGH-POWER BINOCULAR 107 led to the invention of more than one arrangement by which this comfort can be secured when those high powers are required which cannot be employed with the ordinary stereoscopic binocular. This is accomplished by Messrs. Powell and Lealand by taking advantage of the fact already adverted to, that when a pencil of rays falls obliquely upon the surface of a refracting medium a part of it is reflected without entering that medium at all. In the place usually occupied by the Wenham prism, they interpose an inclined plate of glass with parallel sides, through which one portion of the rays proceeding up- wards from the whole aperture of the objective passes into the principal body with very little change in its course, whilst another portion is reflected from its sur- face into a rectangular prism so placed as to direct it obliquely upwards into the secondary body (fig. 85). Although there is a decided difference in brightness be- tween the two images, that formed by the reflected rays being the fainter, yet there is marvellously little loss of definition in either, even when the 50th of an inch objec- tive is used. The disc and prism are fixed in a short tube, which can be readily substituted in any ordinary binocular microscope for the one containing the Wenham prism. Other arrangements were long since devised by Mr. Wenham,1 and subsequently by Dr. Schroder, for securing binocular vision with the highest powers. We have used the latter of these with perfect satisfaction, but all that is required is at the disposal of the student in the arrangement of Powell and Lealand. To those who have used these forms of binocular habitually it has been a frequent soui’ce of surprise and perplexity that, although theoretically such a form as that of Powell and Lealand’s is non- stereoscopic, yet objects studied with high powers have appeared as if in relief, and the effect upon the mind of stereoscopic vision has been distinctly manifest. The Editor was conscious of this for many years in the use of the Powell and Lealand form, with even the of an inch power -of the achromatic construction; at the time he inter- preted it as a conceptual effect ; but it always arose when the pupils fell upon the outer halves of the Ramsden circles. The explanation, Dr. A. C. Mercer considers,2 is due to Abbe. Since (fig. 86) when the eye-pieces are at such a distance apart that the Ramsden circles correspond exactly with the pupils of the eyes, centre to centre, the object appears fiat. But if the eye-pieces be racked down, so as to be nearer together, the centres of the pupils fall upon the outer halves of the Ramsden circles, and we have the conditions of orthoscopic effect; while if they be racked up so as to be more separated, the centres of the pupils fall on the inner halves, and we have pseudoscopic effect. Fig. 85. Fig. 86. 1 Transactions of the Microsc. Soc. N.S. vol. xiv. (1866), p. 105. 2 Journ. B. M. S. ser. ii. vol. ii. p. 271. io8 VISION WITH THE COMPOUND MICROSCOPE The Optical Investigations of Gauss.—Before leaving this sections of our subject, in which we have endeavoured, with as much clear- ness as we could command, to* enable the general reader to com- prehend with intelligibility the ■ principles of theoretical and ap- ])lied optics as they relate to the microscope, we believe we shall serve the higher interests of' microscopy, and the wants or de- sires of the more advanced micro- scopical experts, if we endeavour to present in a form either devoid of technicality or with inevitable - technicalities explained a general' outline and then an application of tlte famous dioptric investiga- tions of Gauss, an eminent Ger- man mathematician, who amongst many other brilliant labours in applied mathematics expounded the larvs of the refraction of light in the case of a co-axial system of' spherical surfaces, having media, of various ref ractive indices lying between them. Although the assumptions- upon which the fonnula? of Gauss • rest are not coincident with the conditions presented by the lens- combinations which are employed in the construction of modern' objectives of great aperture, the- results, nevertheless, furnish an admirable presentation of the path of the rays and the positions, of cardinal points, eA’en in the- microscope as we know and use it.. We remember that the micro- scope is largely used in England and America by men who can only employ it in their more or- less brief recessions from profes- sional and commercial pursuits, but who often employ it with en- thusiasm and intelligent purpose. Much scientific work may be done- by such men, and it will promote- the accomplishment of this, in our judgment, if the frequently ex- pressed desire be met which will enable such students to understand? Fig. 87.1 1 This figure is greatly exaggerated for the sake of clearness. DIOPTRIC INVESTIGATION I3Y GAUSS in a general but thoroughly intelligent manner the principles in- volved in the employment of systems of lenses. Many such either have scanty knowledge of algebra, or in the -continuous pressure of other claims have lost much that they once possessed. We believe that in these cases the following exposition -of the dioptric system of Gauss, with a following example worked -out in full and with every step made clear, will be of real and practical value. Without some intelligible understanding of the ultimate principles of the microscope no results of the highest order can, at least with moderate and high-power lenses of the best modern construction, be anticipated. On this ground we commend the study to the earnest reader. Let RN, SN' (fig. 87) be the spherical surfaces of a lens of density greater than air, and let PRSj; be the course of a ray of light passing through it; C, C', the centres of the spherical surfaces. Let PR, RS be produced to meet the perpendiculars through C and C' in A and A'. Let C R = r, C' S = r',1 y = index of refraction out of air into the medium. NN' ■= d, the thickness of the lens. N R = b, N' S = b'. These may be considered as straight lines. Let the equation to P R be y — b = to (x — O N) . . (1) „ „ RS „ w — b = to' (x — O N) . . (2) -or, y- bf = to' (x -ON') . . (3) „ „ „ y — b' = to" (x — O N') . . (4) From (2) and (3) b' —b = ml (ON7 — O N) = m! N' N = to'd , . (5) Now sin CRA = jn. sin CRB; -or, ~ sin CAB, =/*.??. sinCBR. C V JLV; Now C A and C B are the values of y in equations (1) and (2) when x = O C j C A = b + m (O C — O N) = b + m r ; -and similarly C B = b + ml r ; (b + m r) sin C A ft = y (b + to' r) sin CBR. Now C A K, C B R do not in general differ much from each •other, so that for a first approximation we may consider them to be -equal. b + mr = fx (b + to' r), i.e. /*. ml = m — —^ Let f1 ~~ = u ; then p, m! = to — b u . . . (6) r Similarly, sin C' S B' = /t. sin C' S A' j C'B' . n,-p/cj C'A! sin O' A'S • -or, — . sm O B S = • sin o a o , OS t o 1 If either of the curvatures be turned in the opposite direction the sign of the -corresponding r must he changed. VISION WITH THE COMPOUND MICROSCOPE and, as before, C' B' = b' + m" r', C' A' = b' + in' r' from equations (4) and (3); as before we may take b' -f m" r' = g (J)' + m' r'), or y. in' = m" — ~ b'. Let —-— = u', then y. in' = m" — b'u'. . . (7) From (5) and (6) b' = b + m~hu- d=b (1 - —+ m d. ft \ ft 1 y. „ this and (7) in'' = ftm' -f b u' (\ — —d n. \ y- 1 ft and from (6) = in — b u + bit' fl — -j- mdu \ ft J ft = m(l + + b(u'~ u-dutA. Assume d 7-» d it , d it j / d it it' 7 — = h, 1 — —. = y, ] 4- - = t, it — u — — = k /x y. ft y. then b' = g b + h m) , , 7 , , /0. m" = * 6 + / >4' where »'1 ~ h k = 1 • ' (8>' Now let X, Y be the coordinates of P, the point from which the ray of light proceeds ; then by (1) b = Y — in (X — 0 N) ; substituting in (8) b' = g Y + in (It — g . X — 0 N); in" = k Y + m (/ _ k X — O N) ; whence m = 71 f ~ ; b' = gY + (m"- k Y) J ~ V (X ~ 0 y v I — k (X — O N) Now substituting in (4) the equation to the refracted ray becomes J V l — & (X — O N)J = in" (x -OF + A - g(X -ON)\ . V l — k (X - O N j J ’ or by (8) y - ——- = m" (x - O Nr + h - u (X - 0 N) I /o W y /-fc(X-ON) V. t/-/'(X-ON)J First : If X be taken such that l — k (X — O N) = 1, i.e.. X = O N - — = O E, suppose; k then when * = O N' - h + g = 0 S' + L - ? = o E', suppose, y = Y, or P and p are equally distant from the axis. DIOPTRIC INVESTIGATION I3Y GAUSS Also, if Y = 0, y = 0 ; or it a ray proceed from E, it will after “ m» y l’efraction pass through E . Also m = - ——— = in", thatis„ I -i(X-ON) the ray will be equally inclined to the axis before and after refrac- tion. E and E' are called the ‘ principal points.’ 111 du' O E = O N - 1 =ON + k u' — u — du n' /* TVT . d U' — 0 + —j—, r ; ; y (u —u ) — d u u d u OE' =ON'+If? :=ON' -4- /* k v! — u — d u n' = 0N'+ - T d -• /x (ii — u) — duu Secondly : If m" = 0, or the ray be parallel to the axis after refraction, we have from (8) b = — \ in, and the equation to the incident ray becomes k y + - m = m (x — O N), or y = m x — O N — ; , . i + when )/ = 0,a;=ON + = ON + k- k u' — u — d u ul = O F, suppose. k- Tf m = 0, or the ray be parallel to the axis before refraction, we* have from (8) b' = gb = 'j m", and the equation to the refracted ray becomes k y - | in" = m" (x - O N'), or y = m" (x -ON' + ; du .*. when y = 0, x = ON'~ = O N' — tL k u' — u — dnu' = O F', suppose. F and F' are called the ‘ focal points.’ OF = ON+ - / + ) /x (u' — u) — a u u . OF' = ON' — , /x ~?U, \ [x (n — u) — du u ) VISION IVITH THE COMPOUND MICROSCOPE The focal distance -/=OF-OE = OE' —OF' P = 1 p (it' — u) — du u k Similarly, it may be shown that if there be two lenses, and sub- script numbers refer to the first and second lens respectively, while E, E', F, F' refer to the entire system, and if 3 = OE, -OE,', VX = - 9 (Ml' — Mj) - dy ux U/, /1 V, = - 'y! = p, ( < - «,) - d, u.2 u.f OE = OE! + ] p-i vx + y\ v2 +cvxV2\ oe' = oe;- ——— r y.2Vx + yx V.2+ <>VX V.J OF = OE, + . . P\0'*+Jv*) ) pdvx + pxv.2 + hvxvp OF' = 0 E./ — fbp'i f V\ + px V.2 + vj We are now prepared to work out an example of the Gauss system Toy tracing a ray through two or more lenses on an axis, showing how any conjugate may be found through two or more lenses on that axis.1 * The Gauss system of tracing a ray through two or more lenses on an axis illustrated by means of a worked-out example. Two lenses, 1 and 2, fig. 88, or an axis x y are given. No. 1 is a double convex of crown \ inch thick, the refractive index y being 1 Remembering our object, and tlie assumed conditions of some for whom we write, we do not hesitate to preface this with the following notes to remind the reader of the sense attached to certain mathematical expressions. go means infinity. A plane surface of a lens is considered a spherical surface of an infinite radius. Any number divided by oo = 0; any number divided by 0 - x ; any number multiplied by 0 — 0. x plus, or minus, or multiplied by any number is -still x. The following are the rules for the treatment of algebraical signs : In the multiplication or division of like signs the result is always plus ; but if the signs are dissimilar it is always minus. In addition, add all the terms together that have a plus sign; then all the terms with a minus sign; subtract the less from the greater and affix the sign of the Example: + 3—4 + 2—5 = -4. In subtraction change the sign of the term to be subtracted and then add in with the previous rule. Example : — 8 + 2 — 5 An example occurs in the annexed equations (x) and (xi), p. 114 of — — — = -t- but then the + is changed into a — by the negative sign in front’ of the fraction’ In (xii), p. 114, however, there being a + in front of the fraction, the result remains positive. EXAMPLE AFTER GAUSS 113 the radius of the surface A is | and that of B 1 inch. No. 2 lens is a plano-concave of flint inch thick, the refractive index p being 8, the radius of the surface C is -|, and the surface D is plane. The -distance between the lenses, that is, from B to C measured on the axis, is } inch. The problem is to find the conjugate focus of any given point Y. In order to accomplish this two points have first to be found with regard to each lens. These points are called principal points (see P P', Q Q' in fig. 88). When the radii of curvature r and r', d, the thickness, and p l p2, the refractive indices of the respective lenses,1 are known, the distance of these points from the vertices, i.e. the points where the axis cuts the surfaces of the lens, can be found. Thus by applying Professor Fuller’s formuhe to lens 1 the distance of P from the vertex A can be determined—see p. 116 (i)—similarly P' from B —p. 116 (ii). In the same way the points Q Q' from C and D in lens 2 can be measured off—(v) (vi), p. 117. It must be particulai’ly noticed that in measuring off any dis- tance if the number is + it must be measured from left to right, ■and if — from right to left. Thus in (i) p. 116 because the sign of •158 is + P lies T58 of an inch to the right of A. And in (ii) because ‘21 is — P' lies -21 of an inch to the left of B. The same rule applies to the radii; thus the radius of A, being measured from the vertex to the centre or from left to right, is + ; but the radius of B, being measured from the vertex to its centre or from right to left, is —. Similarly with the concave surface, C being measured from right to left is —. In both the examples before us the points P P', Q Q' fall inside their respective lenses, but it does not follow that they will do so in •every instance. In some forms of menisci, for example, they will fall ■outside the lens altogether. With regard to the focus of the lens it follows the same rule ; thus, f in lens 1 is measured to the left from P, and f' to the right from P' ; similarly in lens 2, f" is measured to the right from Q, and f" to the left from Q'. Having determined the focal length of each lens, the distance between the right hand principal point of the first lens P' and the left hand principal point of the second lens Q must next be found. It manifestly is the distance of B from P' + the distance B C between the lenses, Q being at the point C. Therefore P' Q = -21 + -25 = -46 = c. When these three data have been obtained—that is, the focal length of each lens, and the distance between them—we are in a position to apply the formuhe (ix) and (x), p. 118, to find the principal points E and E' of the combination. 1 In the worked out example no distinction has been made between the r, r' of one lens and the r, r' of the other lens, as well as of /j. and cl, because when the principal points and focal length are determined for one lens those expressions are not again needed, so the same letters with different values assigned to them may be equally well used for the next lens. Too many different terms are apt to confuse the student, while those who are familiar with mathematical expressions will under- stand the arrangement. 114 VISION WITH THE COMPOUND MICROSCOPE In selecting the value of the focus to be put into the equations, for both lenses, the last must be taken, that is, in lens 1 (iv), or + •947, and in lens 2 (viii), or —1'875. It will be noticed that the value of E being negative, it will be measured -314 inch to the left from P. Similarly, E' is measured •622 inch to the left from Q'. (f) also is 1*28 to the left from E, and (p' P28 to the right from E'. These four points, E, E' and and ‘2, and as x = 1 inch, y = - _ This numerically determines the position of the conjugate plane. If the rays incident on the combination are parallel, then x = x> d>9‘ and y = - - = 0, which means that y is coincident with cuts the axis, to meet E, and from the point where it meets E draw a line parallel to the axis, cutting the other line in W. W will be the- conjugate of Y, which was required. If it is required to find the conjugate of a ray passing through three lenses on an axis, two of the lenses must be combined and their four cardinal points found. The principal points and the focal length of the third lens must then be calculated, and then combined in their turn by formula} (ix), (x), (xi), and (xii), p. 118, with the cardinal points of the double com- bination. 8 is taken as the distance of the first principal point of the combination, nearest the third lens, to the second principal point of the lens, nearest the combination. A fresh set of cardinal points, is determined in this manner for the three lenses. So also with four lenses ; the cardinal points of each pair being- found, they are combined by the same formula}, and new cardinal points for the whole combination of four lenses are obtained. Simi- larly, the cardinal points of five, six, or any number of lenses can. be found and the conjugate of any point localised. Finally, no one need be discouraged by the appearance of the length of the calculation ; the example is given in full, so that any- A PRACTICAL EXAMPLE AFTER GAUSS one acquainted only with vulgar fractions and deci- mals can work it, or any other similar problem, out. In lens No. 1, for in- stance, the numerators of the fractions are all very simple, and the denomina- tors of the four equations are all alike; so, too, in the equations for No. 2 and in those for both lenses. Further, f is the same as f f" as f", and ' as c£. Hence the problem is much shorter that it looks. If the conjugate of a point on the axis is only required, and if the prin- cipal points and foci of each lens have been de- termined, it will not be necessary to enter into the further calculation to find E, E', and cf>, ', the cardi- nal points of the combina- tion. The method of proce- dure is as follows : If x is the given point, its dis- tance from f the focus of lens No. 1, must first be measured. Call this dis- tance x. Then the distance of o its conjugate from the other focus, f, supposing lens No. 2 to be removed, can be found by formula ox— f2, o = -, x P = -897, x = 1-65 ; •897 K.Q :.o — = -o48. 1-65 This is the distance from f to o. As the distance from x to f is positive, the dis- tance between f and o is also positive ; so o is to the right of/7. Fig. 88, VISION WITH THE COMPOUND MICROSCOPE Before proceeding it will be as well to examine other possible cases which might occur. Suppose that x was at the point f then x would equal 0, and o — co ; that is, o would lie at an infinite distance from f. If, on the other hand, the point x was to the right of f x would be nega- tive, and o would be also negative, because /2 is always positive ; o would then be measured oli‘ to the left of and the conjugate would be virtual. This means that there will be no real image, because the rays will be divergent on the f side of the lens, as if they had come from some focus on the f side of the lens. But to return. The point o having been found to be the conjugate of x, due to the sole influence of No. 1 lens, we have next to measure the distance between o and f", and, by applying the same formula, find the distance of its conjugate hom/1'1, owing to the exclusive effect of No. 2 lens now replaced. This distance Of" may be found thus : Po = Pf+/'o = -947 + -543 = P49 ; P'/"=P'B + BC + Q/"= -21 4--25 + 1-875 = 2-335 ; Pf" - P>0=of"= 2-335 - 1-49 = -845. Calling this distance O, then, by formula y O = f" 2, we shall find f"2 3-515 the distance of y from f", which we shall call y. y = * = 4-16, which is positive ; therefore y lies 4-16 inches from f" to the right hand, y is therefore the conjugate of x, due to the influence of both lenses 1 and 2. Similarly, the conjugate of any point on the axis may be found through any number of lenses. 3 Lens Xo. 1 : Data.—Radius A = = r ; radius B = — 1 = r' ; 1 3 foci, ff) thickness = = d ; p = ; P = principal point mea- sured from A ; P' = principal point measured from B 3 - 1 ? _ ! n _ IX - 1 _ 2 2 . , _ /, - 1 _ 2 _ _ 1 T 3 3, r' — 1 2 * ,.(«'-«) = |(-1) = - \; <»«*' = o * 3 x - i =_ y (u' — u) — duu’ — — \ + -J = — = — 1-583 : 4 b 12 1 _ 1 P = A + —= A -f H = A + A p (u — u) — duu' 19 19 “ 12 = A + -L58 . . . , • (i) 1 2 P-B+ , , = B + ll2 = B-i p {id — u) — duu 19 19 ~ 12 = B - -21 . . s . . . (ii) A PRACTICAL EXAMPLE AFTER GAUSS 117 3 / — p _l_ p i _z p _ 18 H (u' — u) — duu' 19 19 “ 12 = P - -947 (iii) 3 /•/ _ p/ 1* p' 2 p/ . 18 /.t(u' — u) — duu 19 19 ” 12 = P' + '947 (iv) 9 Lens No. 2 : Data.—Radius C = — = r; radius D = oo = r'; O foci, f" ; thickness = ~ = d ; ft = „ ; Q = principal point measured from C ; Q' = principal point measured from D. 8-i 8-i „ - 1 _ 5 * _ __8 . u, _ n - 1 _ 5 = 0. r 9 ’15 r' oo “ 8 /•(«'-«)=8(o + ;i)=|i; X 0=0; n (u' — u) — d u u' = — 0 = ~ = *853 ; x ' 75 15 Q = C + - , **"' ) = c + ii = c+0 . . (v) /.t (w — w) — dun' o4 75 1 _ _8_ Q' = D + , , ~dX 7 = D + = 11 - u ;t (a — u) — duu b4 lb 75 = D — -0825 (vi) 8 f"=Q + 7 -7 7 = Q + ~t = Q + = Q + 1-875 (vii) yn [u — u) — du u b4 8 75 8 /" = Q'- -j-r y——t = = H [u — u) — du u b4 o 75 = Q'- 1-875 .... (viii) Both Lenses.—Distance apart = BC = > P Q — '21 + '2-> = -46 = a ; /= focus of No. 1 lens = '947 ; f = focus of No. 2 lens = — 1 -87 5. VISION WITH THE COMPOUND MICROSCOPE E _ p i { J — p i '46 x ‘947 _ p , ‘4^6 / +/' - 8 *947 - 1-875 — -46 - 1 -388 = P - 314 (ix) E' = Q' - (-r = Q' - -4G x “ 1-875 /+/' - i> -947 - 1-875 - -46 = Q'--iS = Q'-'621 • • • M ■t-B-.' //' = E — •M7 x - v /+/' _ a -947 - 1*875 - *46 =e-:!S=e-1-28 • • • <”> ' = E' + - ff = E' + -947-X _ 1-875 r T/■+/'_ 3 -947 - 1-875 - -46 = E' + -ri§= E' + 1’28- • • ,o - 1-6384 . .... xy = 2 y= = (XU1) cc a? 119 CHAPTER III THE HISTORY AND EVOLUTION OF THE MICROSCOPE The historic progression of the modern microscope from its earliest inception to its most perfect form is not only full of interest, but is -also full of the most valuable instruction to the practical micro- scopist. In regard to the details of this, our knowledge has been greatly enriched during recent years. The antiquarian knowledge and zeal in this matter possessed by Mr. John Mayall, jun., and the unique and valuable collection of microscopes made by Prank Crisp, Esq., LLB., ranging as they do through all the history of fihe instrument, from its earliest employment to its latest forms, have furnished us with a knowledge of the details of its history not possessed by our immediate predecessors. We may obtain much insight into the nature of what is indis- pensable and desirable in the microscope, both on its mechanical and optical sides, by a thoughtful perusal of these details. It will do more to enable the student to infer what a good microscope should be than the most exhaustive account of the varieties of instrument -at this time produced by the several makers (always well presented in their respective catalogues) can possibly do. Availing ourselves of the material placed at our disposal by the generosity of these gentlemen, we shall therefore trace the main points in the origin -and progress of the microscope as we now know it. Mr. Mayall1 gives what we must consider imanswerable reasons for looking upon the microscope, ‘as we know and employ it,’ as a strictly modern invention. Its occurrence at the period when the spirit of modern scientific research was asserting itself, and when the necessity for all such aids to physical inquiry and experimental research were of the highest value, is as striking as it is full of interest. It may be held as fairly established that magnifying lenses were not known to the ancients, the simplest optical instruments as we understand them having no place in their civilisation. A large number of passages taken from ancient authors, and having an apparent or supposed reference to the employment of magnifying instruments, have been collected and carefully criticised, with the result that all such passages can be explained without in- volving this assumption. We learn from Pliny the elder and others, that crystal globes filled with water were employed for cauterisation by focussing the 1 Cantor Lectures on the Microscope, 1886, p. 1. THE HISTORY ANI) EVOLUTION OF THE MICROSCOPE 120 sun’s rays as a burning-glass, and that these were used to produce- ignition ; but there is no trace of suggestion that these refracting globes could act as magnifying instruments. Seneca (‘ Quest. Nat.’ i. 6, § 5) states, however, that ‘ letters though small and indistinct are seen enlarged and more distinct through a globe of glass filled with water.’ He also states that ‘ fruit appears larger when seen immersed in a vase of glass.’ But he only concludes from this that all objects seen through water appear larger than they are. In like manner it could be shown that Archimedes, Ptolemy, and others had no knowledge of the principles on which refraction took place at curved surfaces. Nor is there any ancient mention of spectacles or other aids to vision. Optical phenomena wrere treated of ; Aristotle and the Greek physician Alexander dealt with myopy and presbyopy ; Plutarch treated of myopy, and Pliny on the sight. But no allusion is made to even the most simple optical aids ; nor is there any reference to any such instruments by any Greek or Roman physician or author. In the fifth century of the Christian era the Greek physician Actius says that myopy is incurable ; and similarly in the thirteenth century another Greek physician, Actuarius, says that it is an in- firmity of sight for which art can do nothing. But since the end of the thirteenth century, which is after the invention of spectacles, they are frequently referred to in medical treatises and other works. If we turn to the works of ancient artists we find amongst their cut gems some works which reveal extreme minuteness of detail and delicacy of execution, and some have contended that these could only have been executed by means of lenses. But it is the opinion of experts that there is no engraved work in our national collection in the gem department that could not have been engraved by a qualified modern engraver by means of unaided vision ; and in reference to some very minute writing which it was stated by Pliny that Cicero saw, Solinus and Plutarch, as well as Pliny, allude to these marvels of workmanship for the purpose of proving that some men are naturally endowed with powers of vision quite exceptional in their excellence, no attempt being made to explain their minute- details as the result of using magnifying lenses. These and many other instances in which reference to lenses must have been made had they existed or been known are con- clusive ; for it is inconceivable that even simple dioptric lenses, to say nothing of spectacles, microscopes and telescopes, could have been known to the ancients without reference to them having been made by many writers, and especially by such men as Galen and Pliny. The earliest known reference to the invention of spectacles is found in a manuscript dating from Florence in 1299, in which the writer says, ‘I find myself so pressed by age that I can neither read nor write without those glasses they call spectacles, lately in- vented, to the great advantage of poor old men when their sight grows weak.’1 Giordano da Rivalto in 1305 says that the invention 1 Smith’s Optics, Cambridge, 1738, 2 vols. ii. pp. 12, 13. A ‘ LENS ’ FROM SARGON’S PALACE of spectacles dates back ‘twenty years, which would be about 1285. It is now known that they were invented by Salvino d’Armato degli Armati, a Florentine, who died in 1317. He kept the secret for profit, but it was discovered and published before his death. But there is a singular evidence that a lens used for the purpose of magnification was in existence as early as between 1513 and 1520 for at that time Raphael painted a portrait of Pope Leo X. which is in the Palazzo Pitti, Florence. In this picture the Pope is drawn holding a hand magnifier, evidently intended to examine carefully the pages of a book open before him. But no instruments com- parable to the modern telescope and microscope arose earlier than the beginning of the seventeenth century and the closing years of the sixteenth century respectively. It is, of course, known that there is in the British Museum a remarkable piece of rock crystal, which is oval in shape and ground to a plano-convex form, which was found by Mr. Layard during the excavations of Sargon’s Palace at Ximroud, and which Sir David Brewster believed was a lens de- signed for the purpose of magnify- ing. If this could be established it would of course be of great interest, for it has been found possible to fix the date of its pro- duction with great probability as not later than 721-705 B.c. A drawing of this £ lens ’ in two aspects is shown in figs. 89 and 90; but Mr. Mayall gives strong and clear reasons for con- cluding that its lenticular cha- racter as a dioptric instrument has certainly not been made out. There are cloudy stripe in it, which would prove fatal for optical purposes, but would be even sought for if it had been intended as a decorative boss; while the grinding of the ‘ convex ’ surface is not smooth, but produced by a large number of irregular facets, making the curvature quite unfit for optical purposes. In truth, it may be fairly taken as established that there is no evidence of any kind to justify us in believing that lenses for optical purposes were known or used before the invention of spectacles. From the simple spectacle-lens, the transition to lenses of shorter and shorter focus, and ultimately to the combination of lenses into a compound form, would be—in such an age as that in which the invention of spectacles arose—only a matter of time. But rt is almost impossible to fix the exact date of the production of the first microscope, as distinguished from a mere magnifying lens. There is nevertheless a consent on the part of those best able to judge that it must have been between 1590 and 1609 ; while it is probable (but bv no means certain) that Hans and Zacharias Janssen, Fig. 89. Fig. 90.—An Assyrian ‘ lens ’ (?). THE HISTORY AND EVOLUTION OF THE MICROSCOPE -spectacle makers, of Middleburg, Holland, were the inventors. But it would appear that the earliest microscope was constructed for observing objects by reflected light only. At the Loan Collection of Scientific Instruments in London in 1876 an old microscope, which had been found at Middleburg, was shown, which, Professor Harting considered, might possibly have been made by the Janssens. It is drawn in fig. 91, and consists of a combination of a convex object-lens and a convex eye- lens, which form was not published as an actual con- struction until 1646 by Fontana, which, as Mr. Mayall points out, does not harmonise with the assumption that this instrument was constructed by one of the Janssens. It is strictly a compound microscope, and the dis- tance between the lenses can lie regulated by two draw-tubes. There are three diaphragms, and the eye- lens lies in a wood cell, and is held there by a wire ring sprung in. The object-lens, a, is loose in the actual instrument, but was originally fixed in a similar way to b. It cannot be an easy task—if it be even a pos- sible one- to definitely determine upon the actual indi- vidual or individuals by whom the compound micro- scope was first invented. Recently some valuable evidence has been adduced claiming its sole invention for Galileo. In a memoir published in 1888 1 Pro- fessor G. Govi, who has made the question a subject of large and continuous research, certainly adduces evidence of a kind not easily waived. Huyghens and, following him, many others assign the invention of the compound microscope to Cornelius Drebbel, a Dutchman, in the year 1621 ; but it has been suggested that he derived his know- ledge from Zacharias Janssen or his father, Hans Janssen, spectacle makers, in Holland, about the year 1590; while Fontana, a Nea- politan, claimed the discovery for himself in 1618. It is said that the Janssens presented the first microscope to Charles Albert, Arch- duke of Austria ; and Sir D. Brewster states, in his £ Treatise on the Microscope,’ that one of their microscopes which they presented to Prince Maurice was in 1617 in the possession of Cornelius Drebbel, then Mathematician to the Court of James I., where ‘ he made microscopes and passed them off as his own invention.’ Nevertheless we are told by Viviani, an Italian mathematician, in his ‘ Life of Galileo,’that ‘this great man was led to the discovery of the microscope from that of the telescope,’ and that ‘in 1612 he sent one to Sigismund, King of Poland.’ We now receive evidence through the researches of Govi that the invention was solely due to Galileo in the year 1610. Professor Govi understands by ‘ simple microscope ’ an instrument ‘ consisting of a single lens or mirror,’ and by ‘compound microscope’ one ‘con- Fig. 91. ‘ Janssen’s ’ compound microscope. 1 AM B. Acad. Sci. Fis. Na t. No noli. vol. ii. series ii. ‘II microscopio composto inventato da Galileo,’ Journ. B.M.S. Pt. IV. 1881), p. 574. DID GALILEO INVENT TIIE COMPOUND MICROSCOPE ? sisting of several lenses or a suitable combination of lenses and mirrors.’ 123 In a pamphlet published in 1881, treating of the invention of the binocular telescope, Govi pointed out that Ghorez, a spectacle maker, in 1625, used the Dutch telescope as a microscope, and stated that with it 1 a mite appeared as large as a pea; so that one can distinguish its head, its feet, and its hair a thing which seemed in- credible to many until they witnessed it with admiration.’ To this quotation he added :— ‘ This transformation of the telescope into a microscope (or, as ■opticians in our own day would say, into a Briicke lens) was not an invention of the French optician. Galileo had accomplished it in the year 1610, and had announced it to the learned by one of his pupils, John Wodderborn, a Scotchman, in a work which the latter had just published against the mad ‘ Peregrinazione ’ of Horky. Here are the exact words of Wodderborn (p. 7) : ‘ Ego nunc aclmirabilis huius perspicilli perfectiones explanare no conabor: sensus ipse iudex est integerrimus circa obiectum pro- prium. Quid quod eminus mille passus et ultra cum neque videre iudicares obiectum, adhibito perspicillo, statim certo cognoscas, esse hunc Socratem Sophronici filium venientem, sed tempus nos docebit -et quotidian* nouarum rerum cletectiones quam egregie perspicillum suo fungatur munere, nam in hoc tota omnis instrumenti sita est pulchritudo. ‘ Audiueram, paucis ante diebus authorem ipsum Excellentissimo D. Cremonino purpurato philosopho varia narrantem scitu dignissima ■et inter csetera quomodo ille minimorum animantium organa motus, •et sensus ex perspicillo ad vnguem distinguat; in particulari autem de quodem insecto quod utrumque habet oculum membrana crassius- •cula vestitum, quse tamen septe foraminibus ad instar larvae ferreae militis cataphracti terebrata, viam praebet speciebus visibilium. En tibi [so says Wodderborn to Horky] nouum argumentum, quod per- spicillum per concentrationem radiorum multiplicet obiectu; sed audi prius quid tibi dicturus sum: in caeteris animalibus eiusdem magnitudinis, vel minoris, quorum etiam aliqua splendidiores habent oculos, gemini tantum apparent cum suis superciliis aliisque partibus annexis.’ To this Govi adds :—- ‘ I have wished to quote this passage of Wodderborn textually, ;so that the honour of having been the first to obtain from the Dutch telescope a compound microscope should remain with Galileo, which the later called occhiolino, and that the glory of having reduced the Keplerian telescope to a microscope (in 1621) should rest with Drebbel. The apologists of the Tuscan philosopher, by attributing to him the invention of the microscope without specifying with what microscope they were dealing, defrauded Drebbel of a merit which really belongs to him ; but the defenders of Drebbel would act un- justly in depriving Galileo of a discovery which incontestably was his.’ I turn now to Wodderborn’s account, published in 1610 (the ■date of the dedication to Henry Wotton, English Ambassador at Venice, is October 16, 1610), which reads thus :— 124 THE HISTORY AND EVOLUTION OF THE MICROSCOPE ‘ I will not now attempt to explain all the perfections of this, wonderful occhiale ; our sense alone is a safe judge of the things which concern it. But what more can I say of it than that by pointing a glass to an object more than a thousand paces off, which does not even seem alive, you immediately recognise it to be Socrates, son of Sophronicus, who is approaching ? But time and the daily discoveries of new things will teach us how admirably the glass does its work, for in that alone lies all the beauty of that instrument. ‘ I heard a few days back the author himself (Galileo) narrate to the Most Excellent Signor Cremonius various things most desirable to be known, and amongst others in what manner he perfectly dis- tinguishes with his telescope the organs of motion and of the senses in the smaller animals ; and especially in a certain insect which has each eye covered by a rather thick membrane, which, however, per- forated with seven holes, like the visor of a warrior, allows it sight. Here hast thou a new proof that the glass concentrating its rays enlarges the object ; but mind what I am about to tell thee, viz. in the other animals of the same size and even smaller, some of which have nevertheless brighter eyes, these appear only double with their eyebrows and the other adjacent parts.’ After reading this document Govi judges that it is impossible to refuse Galileo the credit of the invention of a compound microscope in 1610, and the application of it to examine some very minute animals ; and if he himself neither then nor for many years after made any mention of it publicly, this cannot take away from him or diminish the merit of the invention. It is not to be believed, however, that Galileo after these first experiments quite forgot the microscope, for in preparing the ‘ Saggiatore ’ between the end of 1619 and the middle of October, 1622, he spoke thus to Lotario Sarsi Segensano (anagram of Oratio Grassi Salonense) :— ‘ I might tell Sarsi something new if anything new could be told him. Let him take any substance whatever, be it stone, or wood, or metal, and holding it in the sun examine it attentively, and he will see all the colours distributed in the most minute particles, and if he will make use of a telescope arranged so that one can see very near objects, he will see far more distinctly what I say.’ It will not therefore be surprising if, in 1624 (according to some letters from Rome, written by Girolamo Aleandro to the famous M. de Peiresc), two microscopes of Kuffler, or rather Drebbel, having been sent to the Cardinal of S. Susanna, who at first did not know how to use them, they were shown to Galileo, who was then in Rome, and he, as soon as he saw them, explained their use, as Aleandro tvrites to Peiresc on May 24, adding, ‘Galileo told me that he had invented an occhiale which magnifies things as much as 50,000 times, so that one sees a fly as large as a hen.’ This assertion of Galileo, that he had invented a telescope which magnified 50,000 times, so that a fly appears as big as a hen must, without doubt, be referred to the year 1610, and from the measure given of the amplification by the solidity or volume the GALILEO’S ‘ OCCHIALE ’ 125 linear amplification (as it is usually expressed now) would have, been equal to something less than the cubic root of 50,000, that is, about 36, and that is pretty fairly the relative size of a fly and a hen. Aleandro’s letter of May 24 (1624) does not state at what time Galileo saw the telescope and explained the use of it, but another letter of Faber’s to Cesi, amongst the autograph letters in the possession of D. B. Boncompagni, says (May 11) : ‘I was yesterday evening at the house of our Signor Galileo, who lives near the Madalena ; he gave the Cardinal di Zoller a magnificent eye-glass for the Duke of Bavaria. I saw a fly which Signor Galileo him- self showed me. I was astounded, and told Signor Galileo that he was another creator, in that he shows things that until now we did not know had been created.’ So that even on May 10, 1624, Galileo had not only seen the telescope of Drebbel, and explained the use of it, but had made one himself and sent it to the Duke of Bavaria. We lack documents to show how this microscope of Galileo was made, that is, whether it had two convergent lenses like those of Di’ebbel. A letter of Peiresc of March 3, 1624, says that ‘the effect of the glass is to show the object upside down . . . and so that the real natural motion of the animalcule, which, for example, goes from east to west, seems to go contrariwise, that is, from west to east,’ or whether it was not rather composed of a convex and a concave lens, like that made earlier by him, and used in 1610, and then almost forgotten for fourteen years. It is, however, very probable that this last was the one in question, for Peiresc, answering Aleandro on July 1, 1624, wrote :— ‘ But the occhiale mentioned by Signor Galileo, which makes Hies like hens, is of his own invention, of which he made also a copy for Archduke Albert of pious memory, which used to be placed on the ground, where a fly would be seen the size of a hen, and the instrument was of no greater height than an ordinary dining-room table.’ Which description answers far better to a Dutch tele- scope used as a microscope, in the same way exactly as Galileo had used it, rather than to a microscope with two convex lenses. One cannot find any further particulars concerning Galileo’s occhialini (so he had christened them in the year 1624), either in Bartholomew Imperiali’s letter of September 5, 1624, in which he thanks Galileo for having given him one in every way perfect, or in that of Galileo to Cesi of September 23, 1624, accompanying the gift of an occkialino, or in Federico Cesi’s answer of October 26, or in a letter of Bartholomeo Balbi to Galileo of October 25, 1624, which speaks of the longing with which Balbi is awaiting ‘ the little occhiale of the new invention,’ or in that of Galileo to Cesar Marsili of December 17 in the same year, in which Galileo says to the learned Bolognese ‘that he would have sent him an occhialino to see close the smallest things, but the instrument maker, who is making the tube, has not yet finished it.’ This, however, is how Galileo speaks of it in his letter to Federico Cesi, written from 126 THE HISTORY AND EVOLUTION OF THE MICROSCOPE Florence on September 23, 1624, more than three months after his departure from Home : ‘ I Excellency an occhialino, by which to see close the smallest things, which I hope may give you no small pleasure and entertainment, as it does me. I have been long in sending it, because I could not perfect it before, having experienced some difficulty in finding the way of cutting the glasses perfectly. The object must be placed on the movable circle which is at the base, and moved to see it all, for that which one sees at one look is but a small part. And because the distance between the lens and the object must be most exact, in looking at objects which have relief one must be able to move the glass nearer or further, according as one is looking at this or that part; therefore the little tube is made movable on its stand or guide, as we may wish to call it. It must also be used in very bright, clear weather, or even in the sun itself, remembering that the object must be sufficiently illuminated. I have contemplated very many animals with infinite admiration, amongst which the flea is most horrible, the gnat and the moth the most beautiful ; and it was with great satisfaction that I have seen how flies and other little animals manage to walk sticking to the glass and even feet upwards. But your Excellency will have the opportunity of observing thousands and thousands of other details of the most curious kind, of which I beg you to give me account. In fact, one may contemplate endlessly the greatness of Nature, and how subtilely she works, and with what unspeakable diligence.—P.S. The little tube is in two pieces, and you may lengthen it or shorten it at pleasure.’ It would be very strange, knowing Galileo’s character, that in 1624, and after the attacks made on him for having perhaps a little too much allowed the Dutch telescope to be considered his invention, he should have been induced to imitate Drebbel’s glass with the two convex lenses, and have wished to make them pass as his own invention, whilst he had always used, and continued to use to the end of his days, telescopes with a convex and a concave lens without showing that he had read or in the least appreciated the proposal made by Kepler, ever since 1611, to use two convex glasses in order to have telescopes with a large field and more powerful and convenient. In any case it is impossible to form a decided opinion on such a matter, the data failing; but the very fact that from 1624 onwards Galileo thought no more of the occhialino (probably because he found it less powerful and less useful than the occhiale of Drebbel), as he had not occupied himself with it or had scarcely remembered it from the year 1610 to 1624, seems sufficient to show that the occhialino, like the microscope of 1610, was a small Dutch telescope with two lenses, one convex and one concave, and not a reduced Keplerian telescope like that invented by Drebbel in 1621. The name of microscope, like that of telescope, originated with the Academy of the Lincei, and it was Giovanni Faber who invented it, as shown by a letter of his to Cesi, written April 13, 1625, aud which is amongst the Lincei letters in the possession of D. B. Bon- compagni. Here is the passage in Faber’s letter :— ‘ I only wish to say this more to your Excellency, that is, that GALILEO THE INVENTOR OF THE MICROSCOPE IN 1610 I2p you will glance only at what I have written concerning the new in- ventions of Signor Galileo ; if I have not put in everything, or if anything ought to be left unsaid, do as best you think. As I also- mention his new occhiale to look at small things and call it micro- scope, let your Excellency see if you would like to add that, as the- Lyceum gave to the first the name of telescope, so they have wished to give a convenient name to this also, and rightly so, because they are the first in Rome who had one. As soon as Signor Rikio’s. epigram is finished, it may be printed the next day ; in the mean- while I will get on with the rest. I humbly reverence your Excel- lency.—From Rome, April 13, 1625. Your Excellency’s most humble servant, Giovanni Faber (Lyneeo).’ The Abbe Rezzi, in a work of his on the invention of the micro- scope, thought that he might conclude from the passage of Wodderborn, reproduced above, that Galileo did not invent the com- pound microscope, but gave a convenient form to the simple micro- scope, and in this way as good as invented it, for the Latin word used by Wodderborn,perspicillum, 1 signified at that time, it is clear,’ Rezzi says, ‘ no other optical instrument than spectacles or the telescope, never the microscope, of which there is no mention whatever in any book published at that time, nor in any manuscript known till then.” But Bezzi was not mindful that on October 16, 1610, the date of Wodderborn’s essay, the name of microscope had not yet been invented, nor that of telescope, which, according to Faber, was the idea of Cesi, according to others of Giovanni Demisiano, of Cephalonia, at the end, perhaps, of 1610, but more probably at the time of Galileo’s journey to Home from March 29 to June 4, 1611. If, therefore, the word microscope had not yet been invented, and if the telescope, or the occliiale, as it was then called, was by all named perspicillum, one cannot see why Wodderborn’s per.spirillum cannot have been a cannoccliiale (telescope) smaller than the usual ones, so that it could easily be used to look at near objects, but yet a cannoccliiale with two lenses, one convex and one concave, like the others, and, therefore, a real compound microscope, although not mentioned by that name either by Wodderborn or others. And, besides that, how could it be that Wodderborn beginning to treat ‘ admirabilis huius perspicilli,’ that is, of the telescope in the first line, should then have called perspicillum a single lens in the eleventh line of the same page ? Rezzi’s mistake is easily explained, remem- bering that he had not under his eyes Wodderborn’s essay, but only knew a brief extract reported by Yenturi. It thus appears as in the highest degree probable that Galileo, in 1610, was the inventor of the compound microscope; it was subsequently invented, or introduced, and zealously adopted in Holland ; and when Dutch invention penetrated into Italy in 1624 Galileo attempted a reclamation of his invention (which was undoubt- edly distinct from that of Drebbel); but as these were not warmly seconded and responded to abroad he allowed the whole thing to pass. Nevertheless the facts Govi gives are as interesting as they are important. In regard to the discovery of the simple lens Govi points out 128 THE HISTORY AND EVOLUTION OF THE MICROSCOPE that after the year 1000, minds having reopened to hope and in- tellects to study, there began to dawn some light of science, so that in 1276 a Franciscan monk, Roger Bacon, of Ilchester, in his ‘ Opus Majus,’ dedicated and presented by him to Clement IV., could show many marvellous things, and amongst these the efficacy of crystal lenses, in order to show things larger, and in this wise he says make of them ‘ an instrument useful to old men and those whose sight is weakened, who in such a way will be able to see the letters suf- ficiently enlarged, however small they are.’ As long as no documents anterior to him are discovered, Roger Bacon may be considered the first inventor of convergent lenses, and therefore of the simple micro- scope, however small the enlargement by his lenses may have been. As, however, that man of rare genius, the initiator of experi- mental physics, had brought on himself the hatred of his contempo- raries, they kept him for many years in prison, then shut him up in a convent of his order to the end of his long life of nearly eighty years. His writings had to be hidden, at least those treating on natural science, to save them from destruction, and so the invention of lenses, or the knowledge of their use to enlarge images and to alleviate the infirmities of sight, remained unknown or forgotten in the pages of the famous ‘Opus Majus,’ which only came to light in 1733 by the care of Samuel Jebb, a learned English doctor. A Florentine, by name Salvino degli Armati, at the end of the thirteenth century (1 1280) (in Bacon’s lifetime), had therefore the glory of inventing spectacles, and it was a monk of Pisa, Alexander Spina, who suddenly charitably divulged the secret of their construction and use. Perhaps Salvino degli Armati and Spina reaily discovered more than Roger Bacon had discovered ; that is, they found out the use of converging lenses for long-sighted people, and of diverging lenses for short sight, whilst the English monk had only spoken of the lenses for long sight, and perhaps they added to this first invention the capability of varying the focal lengths of the lenses according to need, and the other of fixing them on to the visor of a cap to keep them firm in front of the eyes, or to fasten them into two circles made of metal, or of bone joined by a small elastic bridge over the nose. However it may be, the discovery of spectacles, or, as it may be called, of the simple microscope, may be equally divided between Roger Bacon and Salvino degli Armati, leaving especially to the latter the invention of spectacles. The earliest known illustration of a simple microscope is given by Descartes in his ‘Dioptrique’ in 1637 : fig. 92 reproduces it. It is practically identical with one devised by Lieberkiihn a century after and shown Fig. 92.—Descartes’ simple microscope with reflector. on p. 138. A lens is mounted in a central aperture in a polished concave metal reflector. Descartes apparently devised another and much more pretentious instrument, but it appears impracticable and could never have existed save as a suggestion. But he appears to have been the first to publish figures and descrip- tions for grinding and polishing lenses. In the Museo di Fisica there * GALILEO’S ’ AND CAMPANI’S MICROSCOPES 129 Fig. 94.—Campani’s microscope (? 1686). are two small microscopes whicl it is affirmed have been liandec down from generation to gene ration since the dissolution oi the Accademia del Cimento ir 1667, with the tradition ol having been constructed by Galileo. They are shown in fig. 93, but from the superiority of construction of these instru- ments it is very improbable that they belong to the days of Galileo, who died in 1642 ; and there is a specially interesting compound Fig. 93.— Galileo’s microscopes. THE HISTORY AND EVOLUTION OF THE MICROSCOPE microscope, by Giuseppe Campani, which was published first in 1G86, which is presented in fig. 94 ; its close similarity to ‘ Galileo micro- scopes ’ is plainly apparent, making it still more improbable that these could be given a date prior to 1642. In a journal of the travels of M. de Monconys, published in 1665, there is a description of his microscope which is of much interest. He states that the distance from the object to the first lens is one inch and a half ; the focus of the first lens is one inch the distance from the first lens to the second is fifteen inches ; the focus of the second lens, one inch and a half ; distance from the second to the third, one inch and eight lines ; the focus of the third lens, one inch and eight lines ; and the distance from the eye to the third lens, eight lines. This would form the data of a practical com- pound microscope with a field lens ; and as Mon- conys had this instrument made in 1660 by the ‘son-in-law of Viselius,’ it becomes probable in a very high degree that to him must be attributed the earliest device of a microscope with a field- lens. In 1665 Hooke published his ‘ Micro- graphia,’ giving an account and a figure of his compound microscope. He adopted the field-lens employed by Monconys and gives details as to the mode and object Fig 95.—Hooke’s compound microscope (1605), DIVINI’S COMPOUND MICROSCOPE 131 oi its employment, winch are at once interesting and instruc- tive ; for they show quite clearly that it was not employed by him to correct the spherical aberration of the eye-lens, but merely to increase the size of the held of view. He tells us that he used it ‘only when he had occasion to see much of an object at once. . . . But whenever I had occasion to examine the small parts of a body more accurately I took out the middle glass (field-lens) and only made use of one eye-glass with the object-glass.’ Fig. 95 is a reproduction of the ori- ginal drawing, and the general design ap- pears to be claimed by Hooke. There is a ball-and-socket movement to the body, ■of which he writes : ‘ On the end of this arm (D, which slides on the pillar C C) was a small ball fitted into a kind of socket F, made in the side of the brass ring G, through which the small end of the tube was screwed, by means of which contri- vance I could place and fix the tube in whatsoever posture I desired (which for many observations was exceedingly neces - sary), and adjusted it most exactly to any object.’ It need hardly be remarked that, useful as the ball-and-socket joint is for many purposes in microscopy, it is not advan- tageously employed in this instrument. Hooke devised the powerful illuminat- ing arrangement seen in the figure, and employed a stage for objects based on a practical knowledge of what was required. He described a useful method of estimat- ing magnifying power, and was an in- dustrious, wide, and thoroughly practical observer. But he worked without a mirror, and the screw-focussing arrange- ment seen in the drawing must have been as troublesome as it was faulty. But as a microscopist, Hooke gained a European fame, and gave a powerful stimulus to microscopy in England. In 1668 a description was published in the ‘Giornale dei Letterati’ of a com- pound microscope by Eustachio Divini, which Fabri had previously commended. It was stated to be about 161 inches high, and adjustable to four different lengths bv draw-tubes, giving a range of Fig. 96.—Divini’s compound microscope. 132 THE HISTORY AND EVOLUTION OF THE MICROSCOPE magnification from 41 to 143 diameters. Instead of the usual bi- convex eye-lens, two plano-convex lenses were applied with their convex surfaces in contact, by which he claimed to obtain a much flatter field. Mr. Mayall found in the Museo Copernicano at Rome a microscope answering so closely to this description that he does not hesitate to refer its origin to Divini. He made the sketch of it given in fig. 96. But the optical con- struction had been tampered with and could not be esti- mated. Ch6rubin d’Orleans published, in 1671, a treatise containing a design for a micro- scope, of which fig. 97 is an illustration. The scrolls were of ebony, firmly at- tached to the base and to the collar encircling the fixed central portion of the body-tube. An ex- terior sliding tube carried the eye-piece above on the fixed tube, and a similar sliding tube carried the object-lens below, these sliding tubes serving to focus the image and regulate (within certain limits) the magnification. He also suggested a screw arrangement to be applied beneath the stage for focus- sing. He devised, or recommended, seve- ral combinations of lenses for the optical . part of the micro- scope, and refers to combinations of three or four separate lenses, by which objects could be seen erect, which he considered ‘ much to be preferred.’ He also invented a binocular form of microscope and published it in his work, ‘La Yision Parfaite,’ in 1677. It consisted of two compound microscopes joined together in one setting, so as to be Fig. 97.—Clierubin d’Orleans’ compound microscope. EARLY BINOCULAR MICROSCOPE 133 applicable to both eyes at once; a segment of each object-lens (supposed to be of one-inch focus) was ground away to allow the convergent axes starting from the two eyes to meet at about 16 inches distance at the common focus. Mechanism was provided for regulating the width of the axes to correspond with the observer’s eyes. Fig. 99. —Cherubin d’Orleans’ binocular microscope (1685). Fig. 98. IV. 98, showing the optical construction, is copied from the original diagram (‘ La Vision Parfaite,’ tab. i. hg. 2, p. SO). _ ‘ A drawing of this binocular, as known to Zahn, was given m the first edition of his £ Oculus Artificialis ’ m lGSo (Fundamen III. p. 233), and is reproduced in fig. 99. . a . In 1672 Sir Isaac Newton communicated to the Royal Society 134 TIIE HISTORY AND EVOLUTION OF THE MICROSCOPE a note and diagram tor a reflecting microscope: we have, however,, no evidence that it was ever constructed. But in 1673 Leeuwenhoek began to send to the Royal Society his microscopical discoveries. Nothing was known of the construction of his instruments, except that they were simple microscopes, even down to so late a period as 1709. We know, however, that his microscopes were mechanically rough, and that optically they consisted of simple bi-convex lenses, with worked surfaces mounted between two plates of thin metal with minute apertures through which the objects were directly seen. At his death Leeuwenhoek bequeathed a cabinet of twenty-six of his microscopes to the Royal Society; unhappily, they have mysteriously disappeared. But Mr. Mayall was enabled to figure one lodged in the museum of the Utrecht University, which is given in figs. 100 and 101 in full size. It is seen on both sides. The lens is seen in the upper third of the plate. It has a focus. The object is held in front of the lens, on the point of a short rod, with screw arrangements for ad- justing the object under the lens. Many modifications of this and the preceding in- struments are found with some early English forms, but no important construc- tive or optical modification immediately presents itself. But some ingenious arrange- ments are found in the simple microscopes devised by Musschenbroek in the early years of the eighteenth century. Grindl figured a microscope in his ‘ Micrographia Nova ’ in 1687, in which optical modifications arise. Divini had, as was stated, combined two plano-convex lenses, with their convex surfaces facing, to form an eye-piece: this idea was carried further in 1668 by "a London optician, who used two pairs of these lenses; Grindl did this also, but in addition Ire used two similar (but smaller) lenses in the same mannei as an objective. The form of the microscope itself was copied from that of Cherubin d’Orleans (fig. 97), but was modified by the application of an external screw. In 1691 Bonanni modified preceding arrangements by devising a means of clipping the object between two plates pressed away from the object-lens by a spiral spring, the focussing being then effected by a ‘screw barrel.’ Fig. 100. Leeuwenhoek's microscope. Fig. 101. HARTSOEKER’S MICROSCOPE 135 This system of focussing was employed in a more practical form by Hartsoeker in 1694 and was adopted by Wilson in 1702. It became a very popular form for the microscope in the eighteenth century. We are indebted to Bonanni also for originating a horizontal form of microscope, which is interesting and which, in a drawing of the instrument, is shown to possess a sub-stage compound condenser fitted with focussing arrangements for illuminating transparent objects. In Hartsoeker’s microscope ‘ the lens-carrier A B, fig. 102 (on which the cell P, containing the lens, is screwed), screws into the body O C, Q D at O Q ; the thin brass plates E and F fit within the body, the portions cut out allowing them to slide on the short pillars O C and Q D, and the spiral spring pressing them towards C D ; the object-slides, or an animalcule cage G H (hinged at a & to allow the Fig. 102.—Hartsoeker’s simple microscope (1694). lid G to fit into H, enclosing the objects between strips of talc), slide between the plates E and F when in position, and the “screw barrel ” I K fits into the screw-socket C D and regulates the focus- sing ; a condensing lens, N, fits on a second “ screw-barrel,'’ L M, which is applied in the screw socket of I K. This arrangement of the condenser is better than the plan adopted by Wilson, as it allows the illumination to be focussed on the object independently of the focal adjustment of the object to the magnifying lens ; whereas in Wilson’s microscope, the condenser being mounted in IK, without facility of adjustment, remained at a fixed distance from the object, and hence the control of the illumination was very limited.’ In Harris’s ‘Lexicon Technicum ’ (1704, 2 vols. fol.), under the word microscope, Marshall’s compound microscope (fig. 103) is de- scribed and figured. Several important innovations in microscopical construction were here embodied. (1) A fine-adjustment screw F Iohn Marshall’s New Invented DoubleMicroscope, For Viewing the Cl RC UL ATI ON oF the B L O O D Made bv him at die Archimedes Golden Speeiacles in Slreef. fLu. tJnj under y H microscopes were taken. But Martin originated a large number of improvements both in the optical arrangements and the mechanism of the microscope, and was an excellent maker. He applied rack-and- pinion focussing adjust- ments, to the compound microscope^!le added in- clining movements to the pillar carrying the stage and mii’ror, and he furnished the stage with rectangular move- ments. It wms to this maker that the late Professor Quekett was indebted for an early microscope, of which he evidently to the last thought highly, and which he subse- quently gave to the Royal Microscopical So- ciety. A drawing of this instrument is given in fig. 106, and should be described in Quekett's own words. He says : ‘ It stands about two feet in height, and is supported on a tripod base, A; the central part or stem, B, is of tri- angular figure, having a rack at the back, upon which the stage, O, and frame, D, supporting the mirror, E, are capable of being moved up or down. The compound body, F, is three inches in diameter ; it is composed of two tubes, the inner of which contains the eye-piece, and can be raised or de- pressed by rack and pinion, so as to increase or diminish the mag- nifying power. At the base of the triangular bar is a cradle joint* G, by which the instrument can be inclined by turning the screw- head, H [connected with an endless screw acting upon a worm- 139 Fig. 106.—Martin’s large universal microscope as used by Queliett. 140 THE HISTORY AND EVOLUTION OF THE MICROSCOPE wheel]. The arm, I, supporting the compound body, is supplied with a rack and pinion, K, by which it can be moved backwards and for- wards, and a joint is placed below it, upon which the body can be turned into a horizontal position ; another bar carrying a stage and mirror can be attached by the screw, L N, so as to convert it into a horizontal microscope. The stage, O, is provided with all the usual apparatus for clamping objects, and a condenser can be applied to its under surface ; the stage itself may be removed, the arm, P, sup- porting it, turned round on the pivot C, and another stage of exquisite workmanship placed in its stead, the under surface of which is shown at Q. ‘ This stage is strictly a micrometer one, having rectangular movements and a tine adjustment, the movements being accom- plished by fine-threaded screws, the milled heads of which are graduated. The mirror, E, is a double one, and can be raised or depressed by rack and pinion ; it is also capable of removal, and an apparatus for holding large opaque objects, such as minerals, can be substituted for it. The accessory instruments are very numerous, and amongst the more remarkable may be mentioned a tube, M, con- taining a speculum, which can take the place of the tube, R, and so form a reflecting microscope. The apparatus for holding animalcules or other live objects, which is represented at S, as well as a jflate of glass six inches in diameter, with four concave wells ground in it, can be applied to the stage, so that each well may be brought in succession under the magnifying power. The lenses belonging to this microscope are twenty-four in number ; they vary in focal length from four inches to one-tenth of an inch ; ten of them are supplied with Lieberkuhns. A small arm, capable of carrying single lenses, can be applied at T, and when turned over the stage the in- strument becomes a single microscope ; there are four lenses suitable for this purpose, their focal length varying from to of an inch. The performance of all the lenses is excellent, and no pains appear to have been spared in their construction. There are numerous other pieces of accessory apparatus, all remarkable for the beauty of their workmanship.’ 1 Cuff designed and made a microscope, which Baker figured and described in his ‘Employment for the Microscope ’ in 1753, which possessed several conveniences and improvements. Not the least of these is that which gives greater delicacy to the fine adjust- ment than is found in any preceding model. It was subsequently further improved by the addition of a cradle joint at the bottom of the pillar by Adams. Cuff also designed a simple form of micrometer. There were three designs of microscopes by George Adams, of London, in 1746 and 1771, which have many points of interest, but scarcely contribute enough of distinctive improvement to the modern forms of the microscope to detain us long. That designed in 1771 is figured in the Adams ‘ Micrographia Illustrata,’ and is reproduced in fig. 107. In this instrument Adams claims to have embodied a number of 1 A Practical Treatise on the Use of the Microscope, 3rd ed London 1855 8vo pp. 25, 26. the Variable Microscope J3y George StneetyLONDGN'- Fig. 107 THE HISTORY AND EVOLUTION OF THE MICROSCOPE 142 improvements on all previous constructions. He applied ‘ two eye- glasses at A, a third near B, and a fourth in the conical part between B and C,’ by which he increased ‘ the field of view and of light ’; draw-tubes were at A and B, by which these lenses could be separated more or less. He also arranged the object-lenses, or ‘buttons,’ a and b, to be combined ; seven ‘ buttons ’ were provided, ‘ also six silver specula [‘ Lieberkiihns ’] highly polished, each having a magni- fier adapted to the focus of its concavity, one of which is represented •at inches, Fig. 150.—Beck’s economic microscope. and is supplied with a beautifully made sliding ledge, which will move easily and firmly with pressure from one side only. INEXPENSIVE MICROSCOPES 197 The stage is fastened to the upper side of two brackets which are oast in one piece with the limb ; on the under side of these brackets there is another plate which holds the sub-stage tube. Tins instrument is supplied with large plane and concave mirrors ; and considering that it constitutes a sixth class of microscope has very much in its favour as a secondary instrument for the work-table. Like all these makers’ instruments the feet are plugged with cork Fig. 151.—Swift's fourth-class microscope. and we know of some of these microscopes that have been in use for forty years, and are still the trusted ‘journeymen instruments of mounters and other workers of various orders in many depart- ments of microscopy. ..... The Messrs. Beck also make a microscope of kind called a ‘histological dissecting microscope. It is illustrated in tig. 152 ; the body is removable, and ‘ loups ’ or simple lenses can be used instead. 198 THE HISTORY AND EVOLUTION OF THE MICROSCOPE ihe two modes or use are seen m the figure. In this case, however the body does not incline • but in simplicity and price for an in strument of this class it deserves commendation. Portable Microscopes.—Microscopes that may be readily taken from place to place, and which are yet provided with the arrange- ments required for using the principal apparatus, are of importance in some investigations, and are desirable by the majority of those who have a living interest in microscopic work. The earliest, and still the best form of this kind of microscope was made by Powell and Lealand. As opened for use it is illustrated in fig. 153 ; but the tripod foot folds into what becomes practically a single bar, and is bent by means of a joint to occupy the least space. The body unscrews, and the whole lies in a very small space, giving at the same time fittings in the cabinet for lenses, condensers, and all needful apparatus. The coarse and fine adjustments to the body are as in the No. 1 stand, so are the stage movements; and the sub-stage Fig. 152.—Beck’s histological and dissecting microscope. has rack-and-pinion movements and rectangular sector centring, while all the apparatus provided with the largest instrument can be employed with it. We have used this instrument for delicate and critical work for fifteen years and there is no falling off in its quality ; and when packed with the additional apparatus required the case is 12x7 x 3 inches. Swift and Son subsequently made an instrument on similar lines. The tripod and stage are packed practically as was Powell and Lea- land s, but the stage in this case is plain. It carries a very con- venient achromatic condenser, to which we call attention in its proper PORTABLE MICROSCOPES •place ; but its tine adjustment is so unsteady that it cannot be used with high powers or for critical work. This instrument ‘set up’ is seen in tig. 154, and in its packed Pig. 158.—Powell and Lealand’s portable microscope. condition it is illustrated in tig. 155. The case in which this instru- ment is packed is 10§ x 6 x inches. The Messrs. Beck were the next in order of time to manufacture an instrument of this kind. This is illustrated in fig. 156. It differs 200 THE HISTORY AND EVOLUTION OF THE MICROSCOPE from the two preceding forms in being a Jackson model. The bin- ocular body and the coarse adjustment have to be lifted and lowrered by the fine adjustment whenever it is used. The stage is plain, but it rotates, and the suh-stage has no centring gear. The instrument packs into a box 1(H x X inches. There is a condenser specially made for this instrument. Mr. Rousselet has designed an admirable little instrument of port- able form but of the sixth class. It is binocular. The tripod folds the stage is plain, with a sliding ledge. The condenser focusses by Fig. 154.—Swift’s portable microscope. means of a spiral tube, within which an inner tube slides, carrying stops, diaphragms, etc. The mirror is jointed so as to be used above the stage ; and as its focus is only IP inch can be used as a side reflector. It is also arranged so that eye-pieces with large field glasses may be employed. It packs in a box 1(H X x 3P and weighs six pounds complete. Microscopes employed for the purpose of minute dissection are of considerable importance in certain kinds of work. Many instru- ments specially adapted are made, although the majority are arranged for simple lenses. But an instrument of great value arranged for DISSECTING MICROSCOPES 201 use with compound lenses has been devised by employing the bin- ocular of Mr. Stephenson. This instrument is illustrated in fig. 157. It is made by Swift and Son. The stage is a large, flat table, with special rests for the arms. The objective and binocular part of the body remain vertical and focus vertically by a rack-and-pinion coarse adjustment, there being no fine adjustment. The bodies above the binocular prisms are suitably inclined, mirrors being placed inside them to reflect the image. This reflexion also causes the erection of the image, which is valuable to the majority engaged in insect dissection or the dissection of very delicate and minute organ- isms or organs. We have now to consider the most primitive stands adopted for simple microscopes. That in the form of a bull’s eye stand is the Fig. 155. least complex form possible. This instrument holds an intermediate- place between the hand-magnifier and the complete microscope,, being, in fact, nothing more than a lens supported in such a manner as to be capable of being readily fixed in a variety of positions suitable for dissecting and for other manipulations. It consists of a circular brass foot, wherein is screwed a short tubular pillar (fig. 158), which is ‘ sprung ’ at its upper end, so as to grasp a second tube,, also £ sprung,’ by the drawing out of which the pillar may be elon- gated by about three inches. This carries at its upper end a jointed socket, through which a square bar about 31 inches long slides rather stiffly : and one end of this bar carries another joint, to which is. attached a ring for holding the lenses. By lengthening or shortening the pillar, by varying the angle which the square bar makes with its summit, and by sliding that bar through the socket, almost any posi- THE HISTORY AND EVOLUTION OF THE MICROSCOPE tion and elevation may be given to the lens that can be required for the purposes to which it may be most usefully applied, care being Fig. 156.—Beck’s portable microscope. taken in all instances that the ring which carries the lens should (by means of its joint) be placed horizontally. At a is seen the position SIMPLE LENS Oil LOUP-HOLDERS 203 which adapts it best for picking out minute shells or for other similar manipulations, the sand or dredgings to be examined being spread upon a piece of black paper and raised upon a book, a box, or some other support to such a height that when the lens is adjusted thereto the eye may be applied to it continuously without unnecessary fatigue. It will be found advantageous that the foot of the micro- scope should not stand upon the paper over which the objects are spread, as it is desirable to shake this from time to time in order to bring a fresh portion of the matters to be examined into view ; and, generally speaking, it will be found convenient to place it on the opposite side of the object, rather than on the same side with the observer, At b is shown the position in which it may be most conveniently set for the dissection of objects contained in a plate or trough, the sides of which, being higher than the lens, would prevent the use of any magnifier mounted on a horizontal arm. The powers usually supplied with this instrument are one of an inch focus, and a second of either a half or a quarter of an inch. By unscrewing Fig. 157.—Swift’s Stephenson’s binocular, arranged for dissecting purposes. the pillar, the whole is made to pack into a small, Hat case, the extreme portability of which is a great recommendation. There is, however, a form of mounting for this instrument, which was devised by Quekett, which has superior advantages. In the form illustrated we are obliged to mechanically arrange the horizontality of the lens, which, of course, is important. In Quekett’s form the loup or lens is so hung in a ring that it has a pendulous motion, and with every change in the position or angle of the bar, the lens, by the action of gravity, becomes perfectly horizontal. This is by far the best form of mounting. Although the uses of this little instru- ment are greatly limited by its want of stage, mir- ror, Arc. yet, for the class- of purposes to which it is suited, it has advantages over perhaps every other form that has been de- vised. Where, on the other hand, portability may be altogether sacri- ficed, and the instrument is to be adapted to the making of large dissec- tions under a low magni- fying power, some such form as is represented in tig. 159—constructed by Messrs. Baker on the basis of thatdevised byProfessor Huxley for the use of his Practical Class at South Kensington — will be found decidedly prefer- able. The framework of the instrument is solidly constructed in mahogany, all its surfaces being blackened, and is so ar- ranged as to give two up- rights for the support of the stage and two oblique rests for the hands. Close to the summit of each of these uprights is a groove into which the stage-plate slides 5 and this may be either a square of moderately thick glass or a plate of ebonite having a central perforation into which a disc of the same material may be fitted, so as to lie flush with its surface, one of those being readily 204 THE HISTORY AND EVOLUTION OF THE MICROSCOPE Fig. 158. DISSECTING INSTRUMENTS 205 substituted for the other, as may best suit the use to be made of it. The lens is carried on an arm working on a racked stem, which is raised or lowered by a milled-head pinion attached to a pillar at the further right-hand corner of the stage. The length of the rack is sufficient to allow the arm to be adjusted to any focal distance between two inches and a quarter of an inch. But as the height of the pillar is not sufficient to allow the use of a lens of three inches focus (which is very useful for large dissections), the arm carrying the lenses is made with a double bend, which, when its position is reversed, as in the dotted outline (which is readily done by unscrewing the milled head that attaches it to the top of the racked stem), gives the additional inch required. As in the Quekett micro- scope, a compound body may be easily fitted, if desired, to a separate arm capable of being pivoted on the same stem. The mirror frame is fixed to the wooden basis of the instrument, and places for the Fig. 159.—Laboratory dissecting microscope. lenses are made in grooves beneath the hand-supports. The ad- vantages of this general design have now been satisfactorily demon- strated by the large use that has been made of it; but the details of its construction (such as the height and slope to be given to the hand-rests) may be easily adapted to individual requirements. A very portable simple microscope stand has been designed by Mr. Ward to take a Steinheil x 10 aplanatic loup. It consists of a circular table on three legs ; it has a mirror, one side of which is arranged for white cloud illumination ; it also has a bull’s-eye for opaque. The focussing is performed by a sliding arm, and the whole packs into the remarkably small area of a case whose outside dimen- sions are 3| X 2| x 1 inches. But the very best form of dissecting microscope for loups or simple lenses which we believe to be at present constructed is made by Zeiss, We illustrate this form, fig. 160. It has a large firm stage 4 inches square and 41 inches from the table, to which wooden arm-rests can Fig. 160.—Zeiss dissecting microscope. DISSECTING MICROSCOPES be attached, or rot, as may be desired. The stage has a large opening, 3 x 3§ inches, into which can be placed either a Hat brass plate, or a glass substitute, or a metal plate with a half-inch hole in it. Underneath the stage are black and white screens which can readily be turned aside. The arm which is focussed by an excel- lent spiral rack-work adjustment carries either a Zeiss dissecting microscope, which with and without its concave eye-lens yields six different powers, varying from 15 to 100 diameters, or the arm will receive the very fine Zeiss-Steinheil loups. The instrument is provided with a large plane and concave mirror on a jointed arm. The utility of this simple microscope is. very great, and we do not hesitate to pronounce it the best thing of its class we have ever seen. A substantial and elaborate form of dissecting microscope, devised Fig. 161.—Beck’s binocular dissecting microscope. by the late Mr. JR. Beck, is represented in tig. 161. From the angles of a square mahogany base there rise four strong brass pillars, which support at a height of four inches a brass plate 61T inches square, having a central aperture of one inch across ; upon this rests a circular brass plate, of which the diameter is equal to the side of the preceding, and which is attached to it by a revolving fitting that surrounds the central aperture, and can be tightened by a large milled head beneath ; whilst above this is a third plate, which slides easily over the second, being held down upon it by springs which allow a move- ment of H inch in any direction. The top plate has an aperture of 1-JT inch for the reception of various glasses and troughs suitable for containing objects for dissection ; and into it can also be fitted & spring-holder, suitable to receive and secure a glass slide of the ordinary size. By turning the large circular plate the object under observation may be easily made to rotate, without disturbing its 208 THE HISTORY AND EVOLUTION OF THE MICROSCOPE relation to the optical portions of the instrument; whilst a traATers- ing movement may be given to it in any direction by acting upon the smaller plate. The left-hand back pillar contains a triangular bar Avith rack-and-pinion movement for focal adjustment, which carries the horizontal arm for the support of the lenses ; this arm can be turned aAvay towards the left side, but it is provided Avitli a stop which checks it in the opposite direction, when the lens is exactly over the centre of the stage-aperture. Beneath this aperture is a concave mirror, which, Avhen not in use, lies in a recess in the mahogany base, so as to the space beneath the stage entirely free to receive a box containing apparatus ; Avhilst from the right- hand back corner there can be raised a stem carrying a side condens- ing lens, Avith a ball-and-socket movement. In addition to the single lenses and special combinations ordinarily used for the purposes of dissection, a binocular arrangement Avas devised by Mr. R. Beck, on the principle applied by MM. Nachet, about,the same date, in their stereo-pseudoscopic microscope. Adopting Mr. "VVenham’s method of alloAving half the cone of rays to proceed to one eye without inter- ruption, he caused the other half to be intercepted by a pair of prisms, and to be by them transmitted to the other eye. But Ave find its utility to be practically limited by the narroAvness of its field of ATieAV, by its deficiency of light and of magnifying poAver, and by the inconvenience of the manner in Avhich the eyes ha\Te to be applied to it. The Continental Model.—Our one purpose in this treatise is to endeavour to promote what we believe to be the highest interests of the microscope as a mechanical and optical instrument, as well as to further its application to the ever-widening area of physical investigation to which, in research, it may he directed. To this end throughout the volume, and especially on the subject of the value and efficiency of apparatus and instruments, we have not hesitated to state definitely our judgment, and, where needed, the basis on which it rests. Incidentally we have expressed perhaps more than ■once our disapproval, and with ourselves that of many of the leading English and American microscopists, of the form of microscope known ■as the Continental model ; and we cherish strong hopes in the in- terests of the science of microscopy that so enterprising and eminent a firm as that of Zeiss, of Jena, will bring out a model that will comport more completely with the needs of modern microscopical research than even the best of the models that they now produce. It is to this house, under the cultivated guidance of Dr. Abbe, that Ave are indebted for the splendid perfection to which the optical side of the microscope has been recently brought; and when Ave know that the ‘ Continental model’ has in the hands of the firm of Zeiss passed from an instrument Avithout inclination of the body into an instrument that does so incline, and from an instrument without sub-stage or condenser into one provided Avith the latter of these abso- lutely indispensable appendages, and finally from an instrument Avitli a perfectly plain stage Avith ‘ clips ’ into Avhat is iioav a stage Avith me- chanical movements—Ave can but hope that these concessions to what has belonged to the best English models for over forty years may lead A CRITICISM OF THE ‘CONTINENTAL’ STAND 209 to an entire reconstruction of tlie stand—a wholly new model—in- tended to meet all the requirements of modern high-class work in all departments, and with a line adjustment of the most relined class. We cannot doubt, if this were so, that the same genius which has so nobly elevated the optical requirements of the instrument would act with equal success on its construction and mechanism. We have written throughout this book too frankly of the eminent services of Messrs. Zeiss to the furtherance of the interests and pro- gression of the microscope as a scientific instrument to be misunder- stood in making a plain estimate of the quality of the model on which their elaborate and in some senses beautiful stands are built. It will be seen that we everywhere justify our judgments by plain and easily comprehended reasons, and the very eminence of the makers renders it incumbent that practical microscopists should, without a shade of bias, assess the value of a stand which is certainly not built on lines that contribute to a higher and still more efficient microscopy. At the same time we do not blind ourselves to the fact that an English market for the ‘ Hartnack ’ model has had very much to do with the perpetuation of the errors which that form contains. The reason of this it is not difficult to trace. The inductive method advanced but slowly, in practice, upon the professional activities and even the professional training of medical men. The country which was the home of Bacon and Newton, and Harvey and Hunter theoretically accepted, but was not quick to apply, the methods of induction to the work of its medical schools. Theory and empiricism held a powerful place in both the teaching and practice of medicine in England until the earlier years of the present century. Medicine was absolutely unaffected by Bacon until the latter half of the seventeenth century. It was not until the early years of this cen- tury that the modern school of medicine began its beneficent career. But at that time the microscope—one of the most powerful instru- ments which can be thought of in the application of experimental and deductive methods to the science of medicine—was looked upon and treated by the faculty as a philosophical toy, a mere plaything for the rich dilettanti. But in spite of this the microscope was brought gradually to a high state of perfection, and by the end of the first third of the century was remarkably advanced as a practical instrument, all its essentials being more or less completely developed. Meanwhile, on the Continent, the microscope was regarded by the Faculty as a scientific instrument of great and increasing value, being used to good purpose in making important discoveries in anatomy, histology, and biology generally. This was gradually realised in this country, and there arose slowly a desire to employ the same instrument in England. But, although English instruments of the most practical and relatively perfect kind, representing the large experience of many careful amateurs, were easily accessible to our medical men in their own country—because it was on the Continent that the investigations referred to had been made—it was nothing less than the Continental microscope that was sought after and obtained. Because early 210 observations of a histological character (and therefoi*e of a nature to lie beyond the sphere of the lay amateur) had been successfully made with a certain form of microscope on the Continent, it was practi- cally argued that this must be the most suitable instrument for such a purpose ; but this was an inference made without know- ledge of or reference to the well- known English models. Let us carefully examine this instrument. The typical form was that made by Hartnack. Seen in its best primitive state, we have it in one of Zeiss’ in- struments represented in tig. 162. It is a non-inclining instrument with a short tube on a narrow horse-shoe foot, in which steadi- ness is obtained by sheer weight. It has a sliding tube as a coarse adjustment, and a direct-acting screw for the tine adjustment. The stage is small, and the aper- ture in it is relatively still smaller, of no service in reaching the focus of an object by touch with a high power. It is provided with spring clips, and a diaphragm im- mediately below the stage, and a concave mirror. A sub-stage condenser was rarely used, because up to a com- paratively late date (1874) it was regarded by many on the Con- tinent as a mere elegant play- thing ; its true value was not perceived. On this model all the micro- scopes of the firm of Zeiss, of Jena, are constructed, as they are used almost exclusively on the Continent, and are regarded in many of the universities and medical schools, both here and in America, as possessing all tjm qualities required for the best biological research. If we examine the finest of these instruments made up to 1885, we are impressed, as we THE HISTORY AND EVOLUTION OF THE MICROSCOPE Fig. 1(2. CRITICISM OF THE ‘ CONTINENTAL ’ STAND always are, with the beauty and care of the workmanship and finish of this firm ; but there is the same heavy horse-shoe foot, steady enough while the instrument is non-inclining, only needlessly heavy, requiring common ingenuity alone to get equal steadiness with one-fourth the weight. But since this instrument has been adapted to the English form by being made to incline to any angle up to the horizontal, the foot but insecurely balances the instrument; it is not difficult as it is not uncommon to topple it over. Indeed, in their photo-micrographic outfit the Messrs. Zeiss practically see this, for they supply another foot to which the microscope is clamped. It must not be forgotten that this want of balance is with the short, not the long body. The diameter of the tube is small, being slightly over seven-eighths of an inch. That this is seen to be a disadvantage would appear certain, because the photographic microscope model of Zeiss has a larger body-tube ; and in their recent ‘ Appendix ’ to their latest catalogue they admit that for certain purposes other stands made by them, ‘ owing to the limited diameter of their tubes, cut off the field ; ’ a significant fact for those who would narrow theEnglishbody. At this present date out of eighteen models ten are made with inclining bodies, and three have sliding coarse adjustment. But in the twelve models for 1889 ten incline, while only two are rigid, and eight have rack-work against four having sliding tubes for coarse adjustment. This is a manifest conformity of the primitive model to the English type. The direct-acting screw, only slightly modified, obtains universally in these models. We have already plainly said that this is not suf- ficiently delicate in its action for critical work with an apochromatic objective of 1*4 or 1-5 numerical aperture, especially as a micrometer screw with a necessarily delicate thread is bound to carry the com- bined weight of the body, limb, coarse adjustment, and the opposing spring ; that it will wear loose under the stress of constant work is inevitable, and thus its utility must be wholly gone. The 1889 model has a new form of fine adjustment, the alteration being that the micrometer screw acts on a hardened steel point; this may cause it to work smoother, but as no weight is taken off, there is difficulty in discovering any reason for its admitting of more pro- longed use without injurious wear. The stage of this instrument, in common with all built on the same model, has three fundamental errors of design :—- i. The stage is so narrow that the edges of the 3x1 slips are, in some Continental stands, allowed to project over the edges. Messrs. Zeiss have profitably departed from this fault, by giving to their larger stands a stage in size more like the English type. ii. The stages have an aperture so small as to limit their useful- ness in focussing with high powers. iii. Instead of a sliding ledge they provide, what still more efficiently militates against easy and rapid focussing, viz. spring- clips. It is unfortunate that no stage on this model admits of the use of the finger to aid in reaching the focus. This gentle tilting up of the object, as we approach the focal point, would save hundreds THE HISTORY AND EVOLUTION OF THE MICROSCOPE 212 of cover-glasses and objective fronts ; and we have reason to know that not a few are broken with this form of stage ; but we have never seen put forward, and do not know, a single reason in justifi- cation of a small aperture in the stage. Another important point is the absence of rotation in the ordinary Continental stand. True rotation is a strictly English feature, but its value is great, and it is an indispensable adjunct to practical work. Messrs. Zeiss have introduced a substitute for a rotary stage; they have done this by making the body and limb solid with the stage, so that the whole rotates together. Practically there is only one point in favour of such a movement, and that is, that the object remains exactly in the same position in regard to the field. But against this arrangement there is— 1. The liability of throwing the optic axis above the stage out of centre with that below the stage, and this though the workmanship be, as it is, of the highest order. 2. The rotation of a microscope object for ordinary examination is really unimportant, as there can be no top or bottom to it. Even for oblique illumination it is not required, as it is always easier to rotate the illuminating pencil. The only instances in which rotation of the object is important are : (a) When the object is polarised, and then it is a distinct disadvantage not to be able to rotate the object independently of the body which carries the analyser. In short, the stage rotating independently of the body would be preferable because, if it is required to rotate the object on a dark polarised field, the polarising and analysing prisms can be set at the proper angles, and then the object rotated without disturbing the relative positions of the prisms. But this cannot be done with the arrangement of the Zeiss model, which rotates body and stage. (/3) For photo-micrographic purposes.—In this case in the Zeiss stand the head of the fine-adjustment screw is geared to the focussing rod; so, manifestly, rotation of the body becomes impossible. Thus, by adopting rotation in the form chosen, the highest ends for which the microscope stage should revolve cannot be accomplished. The sub-stage is often quite wanting in the common Continental forms. This was true of the Hartnack stands, with rare exceptions ; the Nachet instruments were provided with an elementary form. As we have seen, until quite recent times, the condenser was regarded on the Continent as a superfluous, if not a foolish appliance, but that prejudice has been killed by the light thrown on the whole question by (1) the chromatic (1873), and now (2) the achromatic condenser of Abbe. But even a compound condenser was in use in England in the year 1691, and the best work in England since the invention of achromatism has never been done without one. In the mounting of the Abbe condenser every possible ingenuity has been displayed to make it do its work without a sub-stage ; but a permanent centring and focussing sub-stage into which this optical arrangement could, amongst others, fit, might be made for half the labour, ingenuity, and cost. But rather than this,we have the con- HOW TO PUECHASE A MICEOSCOPE 213 denser made to slide on the tail-piece, and to be jammed with a screw. It has therefore neither centring nor focussing gear, but, striking as it may appear, a diaphragm, which cannot be used with, and is no part of, the condenser, is supplied with mechanical centring and rack-work focussing moArements ! That is to say, the delicate centre of an optical combination may take cure of itself; but a diaphragm aperture must be centred by mechanism and focussed by rack ! We know that the idea involved in a rack-work diaphragm is the graduation in the angle of the cone of illumination from the plane mirror by racking a certain sized diaphragm up or down. But this can be better done by an iris diaphragm or, more perfectly still, by a wheel of diaphragms. Now, in reality, nothing is so important as the centring and focussing of the condenser, after we are once provided with perfect objectives ; so that whilst the iris diaphragm or a rotating wheel of diaphragms would meet all the case of the racking and centring of these, there is nothing in the best stands of what is doubtless the largest and most enlightened house for the manufacture of micro- scopes in the world, to supply the indispensable needs of the modern condenser. We observe with pleasure advances in every direction to which we call attention to defects. The more recent instruments are marvels of ingenuity ; we present, in fig. 163, the latest and finest form of Zeiss's best microscope. There is no fault in the workmanship, it is the best possible. The design only is faulty ; there is nothing to command commenda- tion in any part of the model ; and, seeing that the Messrs. Zeiss have now progressed so far as to furnish their first-class stand with the English mechanical movement, and even stage rotation, we can but believe that the advantages of these improvements will make plain the greater advantage that "would accrue from an entirely new model. The Purchase of a Microscope.—A desire to possess a good but not costly microscope is extremely common, but as a rule the intend- ing purchaser has little knowledge of the instrument and does not profess to know what are the indispensable parts of such an apparatus, nor what parts may, in the interests of economy and his special ob- ject, be dispensed with, leaving him still possessed of a sound and well-made instrument. We may briefly consider this matter. The first question to be asked when a microscope is to be pur- chased is, ‘ What is the order of importance of the various parts of a microscope ? ’ In answering this query it will be to some extent true that subjectivity of judgment will appear. But we believe that the following table of the relative order of importance of the parts of a microscope will commend itself to all workers of large and broad experience :— 1. A coarse adjustment by rack and pinion. 2. A sub-stage. 3. A fine adjustment. 4. Mechanical movements to sub-stage, i.e. focussing and centring. 5. Mechanical stage. Fig. 103.—Zeiss’s latest stand. WHAT IS DESIRABLE IN A MICROSCOPE <6. Rack-work to draw-tube. 7. Finder to stage. 8. Plain rotary stage. 9. Graduation and rack-work to rotary stage. 10. Fine adjustment to sub-stage. 11. Rotary sub-stage. 12. Centring to rotary stage. 215 This table gives in order the relative values of the several parts : ’thus, a microscope with a rack-and-pinion coarse adjustment and a sub-stage is to be prefered before a microscope with a rack-and- pinion coarse adjustment, a fine adjustment, but no sub-stage. Or, a microscope with a coarse adjustment by rack and pinion, a sub- stage, and a fine adjustment is to be preferred before one with the same coarse adjustment and a mechanical stage movement, but no sub-stage or fine adjustment ; and so on. The last item is of least importance, and the importance of all the others is in the order of their numeration. Another matter of some significance to the tiro is the relative value from the point of view of time consumed and, therefore, of prime cost, in producing the several kinds of microscopes. The No. 1 stands of half-a-dozen makers may be near the same cost, but may nevertheless have involved the consumption of very different quantities of the highest class of skilled labour in their production. Manifestly the first thing to be looked at in a microscope making any pretensions to quality is the character of the workmanship; and this should carry with it the question, how much machine, and how much hand work and fitting, there is in it. Arcs graduated on silver, for example, are very attractive, and with many are most impressive; but they are simply machine work and quite inexpensive. In the two great types of models, the bar movement and the Jackson limb, the bar movement involves more than double the actual hand fitting; while a fine adjustment with a movable nose- piece takes twice the fitting of one in which the whole body is moved by the fine-adjustment screw. In the same way a mechanical stage which is made of machine-planed plates, sliding in a machine-ploughed groove, is much less costly in time and quality of labour than a hand- made sprung stage. So a sub-stage having a movable ring pressed by two screws against a spring has very far less work, and work of a lower class, than one with a true rectangular centring movement. It will follow, then, that a Jackson-limbed microscope with no movable nose-piece, with a machine-made mechanical stage, and a movable ring for sub-stage, will not have involved more, perhaps, than a third of the skilled work which must be expended on a well- made instrument of the same size with a bar movement. But, if we compare the range of prices as presented by English and American makers, we rarely find an equivalent difference in cost. Then the tiro will be warned by this not to purchase a pretentious instrument with a bar movement and mechanical stage for, say, 51, But if a low-priced instrument is to be purchased, if, as is almost certain, it be a Jackson model, see that it has a rack-work coarse adjustment, eschew the short-lever nose-piece, and have a differential 216 THE HISTORY AND EVOLUTION OF THE MICROSCOPE screw fine adjustment, a large plain stage, and an elementary centring sub stage. Such an instrument should be obtained for 51. 10s. Although not frequently used, it would be doing our work im- perfectly not to refer to a special form of microscope devised for chemical purposes. This is an inverted microscope oi’iginally con- structed by MM. Cachet on the plan devised by Dr. J. Lawrence Smith, of Louisiana, U.S.A., for the purpose of viewing objects from their tender side when heat or reagents are being applied to them,1 has lately been improved by its constructor with a special view to Fig. 164.—Nachet’s chemical microscope. meeting the requirements of observers engaged in the ‘ cultivation * of the minute organisms which act as ferments. The general arrangement of this instrument is shown in fig. 164. On the table which forms its base there rests a box containing a glass mirror, silvered on its upper surface, which is placed at such an angle as to reflect the light-rays received through the inverted objective mounted on the top of the box into the body fixed into its oblique face. Over the objective is placed the stage, above which again is the mirror for reflecting light downwards through the object placed upon it. The focal adjustment is made in the first place by means of a sliding tube which carries the objective, and then by the micrometer-screw, 1 This idea was suggested at nearly the same time by Dr. Leeson, and was; carried out in an instrument constructed for him by Messrs. Smith and Beck. ADAPTABLE CHEMICAL AND ORDINARY STANDS Y, which raises or lowers the stage. The platform on which the optical apparatus rests can be moved in rectangular directions by the two milled heads, O, T, and is furnished with two graduated scales by means of which it may be brought with exactness into any posi- tion previously recorded, so that any point of the object may be immediately re-found—an arrangement of special value in cultivation experiments. On the stage is a circular glass cell, C, for holding the fluid to be examined; in the bottom of this is an aperture which is closed by a piece of thin cover-glass well cemented round its edges, thus allowing the use of high magnifying powers having a very short focus; while its top is ground- flat, so that a cover of thin plate glass may be closely fitted to it by the intervention of a little Fig. 165.—Bausch and Lomb’s laboratory microscope, used for chemical work. grease or glycerine, the whole being secured in its place by three small uprights. The cell is furnished also with two small glass taps, R, R, with which indiarubber tubes are connected. By this cell—which may be made to serve as a moist, a warm, and a gas- chamber—experiments on the rarefaction and compression of air, and on the absorption of gases, can be made with great facility. For ‘ cultivation ’ experiments smaller cells are provided, which are attached to brass plates so arranged as to have a fixed position on tllG stcL^G The Bausch and Lomb Optical Company have now combined the above with the ordinary vertical form of microscope, the principle in- volved being (they believe) entirely new. This form of instrument is particularly adapted for chemical investigations, for the reason 218 THE HISTORY AND EVOLUTION OF THE MICROSCOPE that crystals may lie studied as they lie in their natural position in ■any depth of fluid, and the head is sufficiently distant from the stage not to inhale any fumes. Further than this, it is valuable in the examination of diatomacese and other objects in water which are heavier than it, and therefore .sink to the bottom; also in the moist histological preparations, as Fig. 1G6.—The same instrument changed into an ordinary form. they adhere to the surface of the slide, and are therefore in one plane. It is also an excellent dissecting microscope, as it is partially ■erecting, offers no hindrance to manipulation with any power, and makes it convenient to observe the object directly. There are two forms, the ‘Laboratory’ and the ‘University.’ The Laboratory microscope, when used as an inverted instrument, is shown in fig. 165. CHEMICAL MICEOSCOPES 219 The mirror-bar swings on an axis in the plane of the stage to any point above or below it. The mirror and sub-stage are adjustable on the mirror-bar. The sub-stage carries a revolving diaphragm, and is fixed on a pivot, so that it will swing in and out of the optic axis, allowing the polariser to be attached and ready for instant use. On the slide is the arm, to the lower side of which is fastened the prism box. On the upper horizontal surface of this is the nose-piece, with an •extra adapter for high powers, and in the oblique surface is a screw- socket for the body-tube. To transform the instrument into an ordinary microscope (fig. 166), the tube is unscrewed, the milled head at the front of the arm loosened, which releases the prism box, and the arm is swung on its Fig. 167.—The ‘ University ’ microscope as a chemical instrument. •■axis from between the pillars into an upright position. The tube is now attached to the opposite side of the nose-piece, and after the stage-clips are reversed it is ready for work. The ‘ University ; microscope (figs. 167 and 168) is in its general construction similar to the preceding, except that the (single) pillar • and the arm are not japanned, but are of brass, and that the instrument swings on an axis which is the same as that of the mirror-bar. The stage consists of a glass plate mounted in a brass ring. The prism used for inversion is that suggested by Law- rence Smith in 1851, having four faces, with angles of 57°, 150°, 48°, and 105°, the rays being twice totally reflected. Tank microscopes (also called aquarium microscopes) have, for 220 THE HISTORY AND EVOLUTION OF THE MICROSCOPE certain kinds of work, a value of their own. They may be used with low powers outside the glass or above the water; or the object-glass may be protected by a water-tight tube outside it, and with a disc of glass fixed (also water-tight) into that end of the tube which stands be- low the front lens of the objective, at a proper distance for the focus, may then be plunged into the aquarium. Indeed, the tube of the instrument may be so protected as to work for some depth, and have some range in the water of a good-sized tank. A beautiful instru- ment of this class has been devised by Mr. J. W. Stephenson for the examination of living objects in an aquarium. A brass bar is laid across the aquarium, as shown in the woodcut (fig. 169). To adjust it to aquaria of different widths the support on the left is made to slide along the bar, and it can be clamped at any given point by the upper milled head. The milled head at the side, by pressing on a loose plate, fastens the bar securely to the aquarium. Between the ends of the bar slides an arm carrying a sprung socket, and the arm can be clamped at any given point of the bar. Through the socket is passed a glass cylinder, cemented to a brass collar at the upper end, and closed at the lower by a piece of cover- glass. Into this cylinder is screwed the body-tube of the microscope with eye-piece and objective, which are thus protected from the water of the aquarium. The microscope is focussed by rack and Fig. 1G8. The ‘ University ’ microscope fixed for upright use. AQUARIUM MICROSCOPES pinion (milled head just below the eye-piece), and in addition the objective is screwed to a draw-tube, so that its position in the cylinder may be approximately regulated. The arm of the socket is hinged to allow of the microscope being inclined in a plane parallel to the sides of the aquarium. The lower milled head clamps the hinge at any desired inclination. The socket also rotates on the arm, so that the microscope can be inclined in a plane parallel to the front of the aquarium. Thus any point of the aquarium can be reached. A very convenient form for some classes of botanical work with very low powers, and also for aquarium work of a general kind, was 221 Fig. 169. devised by Ross and Co. It may be either monocular or binocular, and has a rack movement on a horizontal bar, giving it considerable range; and a rack focal movement on the upright bar and on the body,' with an additional screw movement in a direction to and from the observer, so that all the principal movements are arranged in its construction. Its general character will be understood by the illustration given in fig. 170. It is extremely useful in the general study of small tanks. Mr. C. Collins’s aquarium microscope (fig. 171) differs from all other forms in that it is applied to the side of the aquarium itself. This is accomplished by making use of a simple pneumatic apparatus. The head of the ‘ sucker ’ is shown on the left of the drawing, with THE HISTORY AND EVOLUTION OF THE MICROSCOPE 222 an indiarubber ring surrounding a central piston. The ring is applied to the glass surface of the aquarium, and the air is exhausted by screwing round the head of the piston seen on the right. Two turns are sufficient to fasten the sucker securely. The rod to which the support of the body-tube is attached passes through the sucker-arm, and can be clamped at any height desired. Professor E. Schultze has designed and Messrs. Klonne and Muller have made the microscope, fig. 172, for the observation of small aqua- tic organisms in an aquarium specially constructed for the purpose. There are three parts : (1) the stand, the greater part of which is nickel-plated ; (2) the aquarium ; (3) the illuminating mirror. Fig. 170. The stand consists essentially of a microscope-tube, which is supported in a horizontal position upon a tripod in such a way that it can be moved in three different directions by rack and pinion. The column of the tripod carries a rack and pinion, by which the tube is moved vertically. On the tube which carries the rack is a slid- ing piece with a second rack for the horizontal movement from right to left ; upon this slide the microscope is fixed in a horizontal posi- tion, and can be moved backwards and forwards in a tube provided with rack and pinion. There are therefore three movements—verti- cal, horizontal-lateral, and horizontal-sagittal—so that the organism AQUARIUM MICROSCOPES 223 observed can be followed by the tube as it moves upon the glass, wall of the aquarium. The aquarium consists of a stand with a frame which carries the- aquarium proper, 10 cm. in breadth and height and 10 mm. in thickness ; this may be replaced by others. The frame is made of brass lacquered black. The aquarium itself consists of a horseshoe- shaped piece of glass, both sides of which are closed by plates of cover-glass, leaving the upper end open. It is thus possible to observe an organism upon either of the two thin sides with an objective giving a linear amplification of 200 to 300. To screen off the superfluous light and the numerous reflexions in the aquarium, the frame carries a diaphragm arrangement which can be applied on either side at pleasure. This consists of a sliding plate which moves the two- horizontal guides ; it is divided into three parts, and has an oblong. Fig. 171.—Collins’s aquarium microscope, applied to the aquarium by a ‘sucker.’ opening in one of the divisions. In this opening a thin plate slides, and can he clamped at any point. In this plate again is a circular- aperture, which can be closed to a greater or less extent by various, diaphragms kept in position by a small spring. If an animal or other small organism is on the upper left-hand corner of the side turned towards the microscope, the sliding plate is first moved so that the vertical longitudinal opening lies in the left-hand third, the small plate is then set so that its opening lies in the upper third. If, on the other hand, the animal is on the right- hand side, the larger sliding plate is moved so that the longitudinal opening lies on the right, and if the animal is towards the bottom, the small slide with its opening is moved downwards. The two sliding plates are now so directed that light may be thrown by the mirror through the aquarium and upon the animal on the front side. The aperture can be further reduced by diaphragms. 224 THE HISTORY AND EVOLUTION OF THE MICROSCOPE The mirror is concave, 10 cm. in diameter, and fixed upon its stand with a ball-and-socket joint so that it can be adjusted in any position. As an adjunct, and admirable aid to the student of the tank and pond, as well as a simple and easy means by which specific forms of microscopic life maybe be found and readily taken, we call attention to the tank microscope of Mr. C. Rousselet. It is illustrated in fig. 173 and scarcely needs further description. One of Zeiss’s Steinheil ‘ loups ’ or aplanatic lenses, to which we have referred, is carried on a jointed arm, which is clamped to the tank,1 the tank being nowhere deeper than the range of focus of the lens employed. The arm moves on a plane parallel to the side Fig. 172.—Professor E. Scliultze’s aquarium microscope. of the tank, and the lens is focussed by means of a rack and pinion, arranged upon the body of the clamp, as seen upon the left-hand corner of the figure. The following points will recom- mend themselves to those who are in the habit of looking at their captures with the pocket lens in the ordinary way When an object of interest is found, it can be followed with the greatest ease and taken up with a pipette, both hands being free for this operation. 1 We prefer to have a stand or 1 rest’ for the tank, and on one side of this a firm pillar to which (and not to the side of the aquarium) the jointed arm is clamped. This enables shallower and deeper tanks to be employed without shifting the rack carrying the ‘ loup.’ It so frequently happens that a minute object is lost simply by removing the pocket lens for an instant to take up the pipette ; in the above apparatus the lens remains in the position in which it has been placed. Microscopes have been arranged in many ways to facilitate class demon- stration in microscopic work, but we have seen none that is more simple, efficient, and inexpensive than that suggested by Dr. Beale. The instru- ment is made by attaching its outer tube on a wooden support to a horizontal board, which also carries a small lamp attached tc it in the required position (tig. 174). The object having been tixed in its place, and the coarse adjustment made by sliding the body in the outer tube, these parts may then be immov- ably secured, nothing being left movable except the eye-tube, by sliding which in or out the tine adjustment may be effected. Thus- AQUARIUM MICROSCOPES 225 Fig. 178.—Rous3elet’s aquarium microscope. Fig. 174. the whole apparatus may be passed from hand to hand with the greatest facility, and without any probability of disarrangement, and every observer may readily ‘ focus ’ for himself, without any risk of injuring the object. 226 CHAPTER IY ACCESSORY APR ABATES This chapter on apparatus accessory to the microscope might be easily made to occupy the whole of the space we propose to devote to the entire remainder of the book ; the ingenuity of successive micro- scopists and the variety of conditions presented by successive improve- ments in the microscope itself have given origin to such a variety of appliances and accessory apparatus that it would be futile in a practical handbook to attempt to figure and describe. We propose, therefore, only to describe, and to explain the mode of successfully employing, the essential and the best accessories now in use, neglect- ing, or only incidentally referring to, those which are either sup- planted, or which present modifications either not important in them- selves, or accounted for by the fact of their production by different ortticians. I. Micrometers and Methods of measuring minute Objects.—It is of the utmost importance to be able with accuracy, and as much simplicity as possible, to measure the objects or parts of objects that are visible to us through the microscope. The simplest mode of doing this is to project the magnified image of the object by any of the methods described under £ Camera Lucida and Drawing’ (p. 233). If we carefully trace an outline of the image, and then, without disturbing any of the arrangements, remove the object from the stage, and replace it with a ‘ stage mi- crometer,’ which is simply a slip of thin glass ruled to any desired scale, such as tenths, hundredths, thousandths of an inch and up- wards. Trace now the projected image of this upon the same paper, and the means are at once before us for making a comparison between the object and a known scale, both being magnified to the same ex- tent. The amount of magnification in no way affects the problem. Thus, if the drawn picture of a certain object exactly fills the in- terval between the drawing representing the -01 inch, the object measures the '01 inch, and whether we are employing a magnifying power of a hundred or a thousand diameters is not a factor that enters into our determination of the size of the object. In fact, all drawings of microscopic objects are rendered much more practically valuable by having the magnified scale placed beneath them, so that measurements may at any time be made. In favour of the above method of micro-measurement, it will be noted (1) that no extra apparatus is required, (2) that it is extremely simple, and (3) that it is accurate. The most efficient piece of apparatus for micro measurement is MICROMETER EYE-PIECE 227 "without doubt the screw-micrometer eye-piece ; it was invented >by Ramsden for telescopes, and if well constructed is a most valu- -able adjunct to the microscope. It is made by stretching across the field of an eye-piece two extremely fine parallel wires, one or both •of which can be separated by the action of a micrometer screw, the circumference of the brass head of which is divided into a convenient number of parts, which successively pass by an index as the milled head is turned : it is seen in fig. 175, B. A portion of the field of view on one side is cut off' at right angles to the filaments by a scale formed of a thin plate of brass having notches at its edge, whose ■distance corresponds to that of the threads of the screw, every fifth notch being made deeper than the rest to make the work of enume- ration easier. In the original Ramsden eye-piece one filament was stationary, the object being brought into such a position that one of its edges appeared to touch the fixed wire, the other wire being moved by the micrometer screw until it appears to lie in contact Fig. 175.—The micrometer eye-piece -with the other edge of the object; the number of entire divisions on the scale then shows how many complete turns of the screw had been made in the separation of the wires, while the number of index- points on the edge of the milled head shows the value of the fraction •of a turn that may have been made in addition. Usually a screw with 100 threads to the inch is employed, which gives to each divi- sion in the scale in the eye-piece the value of T of an inch, whilst the edge of the milled head is usually divided into 100 parts. Both wires or filaments have since been made to move, a screw and divided head being fixed to the wire that Ramsden made stationary. There is no advantage in this plan, and it involves needless complexity in calculation. The best method, there can be no doubt, is the one employed by Mr. Nelson, which is to have one thread fixed, but not in the centre of the eye-piece, but five notches in the scale from the centre on the side furthest from the screw- head. This not only permits of a much larger object being spanned, but also keeps the average of measurements in the middle of the *■ field.’ This is not only convenient but important, because the magnification is not uniform throughout the field. If the power 228 ACCESSOEY APPAEATUS employed is high, in order to effect the span of the great magnifica- tion, one wire (the fixed centred one) will be in the middle of the field, the other at the margin, and the comparison will not be true on account of the unequal magnification of the eye-piece through- out the field, whereas if the w-ire be placed five notches on one side, both measurements are brought more within the centre of the field. Messrs. Zeiss now make a Ramsden micrometer eyepiece. It is provided with a glass plate w ith crossed lines, which together with the eye-piece are carried across the image formed by the objective Fig. 170. by means of the measuring screw, so that the adjustment always remains in the centre of the held of view. Fig. 176 illustrates this instrument, complete and in longitudinal section. Each division on the edge of the drum corresponds to 0-002 mm.. Whole turns are counted on a numbered scale seen in the visual held, and the image may be measured up to 8 mm. A modification of this instrument, facilitating both accuracy and simplicity, has recently been devised by Mr. Nelson, of which we- think highly, and of which we give an illustration in hg. 177. This. screw micrometer eye-piece differs from those of the old form mainly in two respects : first, the optical part is compensated ; secondly, the micrometer part with both webs can be made to traverse en bloc the held of the eye-piece by screw motion. More particularly speaking, the instrument consists of two parts :: one, a hat rectangular box containing the hxed and movable webs,, the micrometer screw and divided head complete ; the other part may be called an £ eye-piece adapter ’ with an outer case to hold the- above-mentioned rectangular box. NELSON'S SCREW MICROMETER EYE-PIECE 229 The flat inner box has a screw attached to it which engages with a head on the exterior of the outer box. This gives about one inch of screw movement to the inner box, which causes the webs to traverse the field of the microscope. It must be remembered that this in no way affects the movement of the movable web from the fixed, which can alone be accomplished by turning the graduated micrometer head as in the old form. The ‘ eye-piece adapter ’ portion of the instrument is, as its name implies, merely an adapter to take the optical part of positive com- pensating eye-pieces of various powers. Fig. 177 — Nelson’s new form of screw micrometer eye-piece. Immediately below the web is an iris diaphragm. This permits a diaphragm [to be used suitable to the power of the eye-piece ■employed. A guiding line at right angles to the webs has been -•added. Care must be taken to observe that when the movable web coincides precisely with the fixed web, the indicator on the graduated head stands at zero. If this is not the case, the finger screw must be loosed, which will liberate the graduated head, and then it can be placed in its proper position and fixed. This is of universal application to all screw micrometers. Four points are gained by this arrangement:— (1) The compensating eye-piece yields far better definition when measuring with apocliromatic objectives than either the Huyghenian or Ramsden forms. (2) Different-powered eye-pieces can be employed. (3) By means of the screw which moves the micrometer webs across the field it is possible to perform measurements with the webs equidistant from the centre of the field, and thus eliminate errors due to distortion. (4) The pi’eceding advantage is secured without sacrificing the benefit of a fixed zero web. To use the screw micrometer with success it should not be inserted, as the custom has been, like an ordinary eye-piece into the tube of the microscope, but it should hare a firm stand quite independently, preventing actual contact with the body-tube. Plate II. gives the mode of its employment, the illustration being ACCESSORY AP PA RAT l' S 230 made from a photograph by Mr. Nelson. The micrometer eye-piece,, it will be seen, is fitted into a stand wholly independent of the microscope. This consists of a strong upright, fitted into a massive tripod or circular foot. The foot in either case only rests on three points; the upright is capable of telescopic extension by a clamping tube ; a short tube which takes the eye-piece is fixed to this upright by a compass joint. To use it the object to be measured is placed in position, and the microscope inclined in the usual way. The ordinary eye-piece is removed, and the separate stand with the micrometer in its place is put in front of the microscope, the extension tube being raised or lowered until the tube at the top of it,’carrying the micrometer, is made continuous with the tube of the microscope, as seen in the drawing. It is well to leave from ’th to of an inch of space between the body-tube and the micrometer tube. It will be now needful to employ corrections to compensate for the increased length of tube. If the objective be provided with a 1 correction collar ’ the adjustment must be recorrected; but if it is not so provided the tube of the microscope must be shortened exactly as much as the tube carrying the micrometer will have lengthened it. By this arrangement it will be found that manipulation can be effected without the vibration of the microscopical image which is ine- vitably the result of the revolving of the micrometer screw-head when the micrometer eye-piece is placed, as it usually has been, in the body- tube of the microscope. The consequence is that much more minute spaces can be measured, and with much greater accuracy. Mr. Nelson has repeatedly spanned the of an inch by means of a stage micrometer in the focus of the objective: this was replaced by a mounted specimen of Amphvpleura pelJucida, and he has counted ninety-six lines in the TTj(TTyth of an inch by making the movable wire pass successively over them until the fixed wire was reached. By similar means the Editor has measured single objects less than the To oVrooth of an inch. It will have been premised by the careful reader that the stage micrometer must be used in every set of measurements ; at least we would strictly emphasise this as the only accurate and scientific method. It has been advised that a record of comparisons with the various lenses in the possession of the microscopist should be made once for all. We decidedly deprecate this method, unless it be in such utterly valueless work, as is sometimes done, where lenses are uncorrected and accuracy of tube-length forgotten or ignored. The correction of an objective and the tube-length ought "to vary with every object, and therefore a comparison of the stage-micrometer and the screw-micrometer should be made with every set of measure- ments. Moreover, the majority of stage micrometers exhibit very con- siderable discrepancies in the several intervals between the lines ; it is well in the interests of accuracy to take the screw value of each under a high power, find the value of the average, and then note the particular space or spaces that may be in agreement with the average and always use it. An illustration will make this clear. PLATE II. The Arrangement of Microscope with Stand for Micrometer Eye-piece as employed to secure Steadiness and Accuracy in Measurement. TO OBTAIN THE VALUE OF A MICROMETER INTERVAL 231 Zeiss provides a stage micrometer of 1 mm. divided into -1 and •01. The following are the actual values obtained for each of the -05 divisions, viz.— g.^Q 8-37 8-38 8-38 8-36 8-36 8-58 8-33 8-31 8-47 8-33 8-33 8-38 8-44 8-38 8-40 8-37 8-40 8-25 8-38 20) 10-700 8"38 mean value. In this instance it will be seen that the last division, 8-38, agrees with the mean, and is the best for all future use.1 Having thus obtained a screw-micrometer value for a certain known interval, the screw-micrometer value for any other object being known, the size of the object may be found by simple propor- tion ; thus, viz. if 8-38 is the screw-micrometer value for -05 mm. and 6-45 that for a certain object, the size of the object is (i) 8-38 : 6-45 :: -05 : ccmm.; 6-45 x -05 r,QQ“ x = -————— = -038o mm, 8"38 If the answer is required in fractions of an English inch, all that we need remember is that 1 inch = 25-4 mm.; then (ii) 8-38 : 6*45 :: : x inch ; N 7 25*4 * = x-ooiw==,001B19 incIl. 8'38 8-38 If the stage-micrometer is ruled in fractions of English inches, then suppose the screw-micrometer value for yoWth inch = 4’2o7, and that for the object = 0’45 as before. (iii) 4-257 : 6-45 :: -001 : x inch ; x = X = -001515 inch. 4-257 1 In the number given for screw value the whole number stands for a complete revolution or number of revolutions of the screw-head, and the decimal, the portion of a revolution read off beyond this. 232 ACCESSORY APPARATUS If the answer is required in metrical measurement, then as 1 inch = 25-4 mm. (iv) 4-257 : 6-45 :: (-001 x 25-4) : a; mm.; 6-45 x -0254 -1638 x = = - - = -0380 mm. 4-257 4-257 In this connection it will be as well to give two examples of scale comparison which are sometimes required. Thus you have a certain interval on a metrical stage micrometer which you know to be accurate, and you wish to compare an English stage micrometer with this scale in order to find out which particular interval of TCVfr inch agrees with it. Suppose ‘05 mm. = 8‘38 screw value as above, then all that is necessary is to find the point to which the screw micrometer must be set in order that it may accurately span the inch. Take 1 inch = 25-4 mm. as before ; then ‘001 inch = ‘0254. (v) ‘05 mm. : ‘0254 mm. :: 8‘38 ; x screw value ; •0254x8-38 a on? , x = - = 4‘2o7 screw value. •05 Conversely, if a metrical scale is to be compared with an accurate English one where -001 inch = 4'257 screw value, then the screw value for '05 mm. may be found thus : '001 inch = ‘0254 mm. (vi) ‘0254 mm. : ‘05 mm. :: 4‘257 : x screw value ; •05 x 4-257 o QC t e x = „ = 8-38 screw value tor ‘05 mm. ■0254 A cheap substitute for the screw-micrometer has been devised by Mr. G. Jackson. It consists in having a transparent arbitrary scale inserted into an ordinary Huyghenian eye-piece in the focus of the eye-lens, so that it will be in the same plane as the magni- fied image of the object to be mea- sured. It is seen in tig. 178. The method of using it is precisely similar to that of the screw micrometer; the a alue of titow inch or -,V mm., as the case may be, is found in terms of the arbitrary scale. The value of the object in terms of the same scale is also found, and compari- son made accord- ingly. All that need >e done is to substitute the terms of the arbitrary scale for screw values in the preceding examples, and they will meet the case. I lie arbitrary scale should be capable of movement by a screw, Fig. 178.—Jackson’s eye-piece micrometer. ESTIMATING THE EDGES OF MINUTE OEJECTS 233 otherwise the appliance is hardly as accurate as the first method of micrometry by simple drawing described above. Of all the methods of micrometry the most accurate is that performed by photo-micrography. A negative of the object to be measured is taken, and then, without any alteration in tube- or camera-length, the magnified image of the stage micrometer is pro- jected on the ground glass : this is spanned by means of a pair of spring dividers. The negative film is then scratched by these dividers. Then you are in a position to make the most accurate measurement the microscope is capable of yielding. It is exceedingly important, when performing micrometric measurements, to remember that the precise edges of all objects in the microscope are never seen. Consequently it is impossible to ascer- tain from what point to what point the measurement is to be made. This, while hardly affecting large and coarse objects, becomes supremely important with small objects. Instead of a real edge to an object you get diffraction bands. These bands alter with focus, and also to a greater extent with the angle of the illuminating cone as well as with the aperture of the objective. Hence it ensues that the accurate micrometry of delicate objects presents one of the most difficult matters encountered in practical microscopy. At the present time opinions differ greatly as to the treatment of particular cases. The following plan of Mr. Nelson’s is the outcome of a long .series of experiments :— 1. The focus and adjustment to be chosen may be termed that of the ‘ black dot ’ (see Elimination of errors of interpretation, p. 356) ; in other words, if the object were a slender filament it would be represented white with black edges. These black edges are due to diffraction. If the filament is very slender and the illuminating cone small, there may be seen a white diffraction edge outside the black one, and perhaps another faint black one outside that again. 2. Reduce, as far as possible, the extent of these diffraction bands by (a) using an objective with as large an aperture as possible ; (6) by using as large an illuminating cone as possible. 3. Measure from the inner edge of the inner diffraction band to the inner edge of the inner diffraction band on the opposite side. 4. But if the diameter of a hole be required, then the measure- ment must be made from the outer edge of the outer black diffractive band to the outer edge of outer diffraction band on the opposite side. It must not be forgotten, however, that these rules only apply for a particular focus and a particular adjustment. II. The Camera Lucida and its IJses.—There are a large number of contrivances devised for the purpose of enabling the observer to see the image of an object projected on a surface upon which he may trace its outlines, but they resolve themselves practically into two kinds, viz. 1. Those which project the microscopical image on to the surface provided for the drawing. 2. Those which project the pencil and paper into the field of the microscope. * 234 ACCESSOKY APPAKATUS We shall describe what we consider the most practical forms of each. In point of antiquity Wollaston's camera lucida claims the post of honour ; but to use it the microscope must be placed in a hori- zontal position. Its general form is shown in fig. 179. The rays on leaving the eye-piece, above which it is fixed by a collar, enter a prism, and after two internal reflexions pass up- wards to the eye of the observer. It is easy to see a projection of the micro- scopic image with this instrument, but it is when we desire at the same time to see the paper and the fingers holding the pencil that the difficulty begins. The- eye has to be held in such a position that the edge of the prism bisects the pupil, so that one-half of the pupil receives the microscopic image and the other half the images of the paper and the hand enqdoyed in drawing. If this bisection is not equal, too much of one image is seen at the expense of the other. This was in some sense supposed to be compensated by the use of lenses, as seen in the figure ; but the difficulty of keeping the eye precisely in one position has caused this instrument to fall into disuse, several cameras, being now devised free from this defect. It has nevertheless one special point in its favour—it does not invert the image, causing the right to be turned to the left, and vice verm. This is an advantage- the value of which we shall subsequently see. A simple camera was made by Soemmering by means of a small mirror or circular reflector, which is placed in the path of the emergent pencil at an angle of 45° to the optic axis, thus reflecting rays from the image upwards. The instrument is seen in fig. 180 and slides on to the eye-piece. The reflector must be smaller than the pupil of the eye, because it is. through the peripheral portion of the pupil that the rays, not stopped out by the mirror, come from the paper and pencil. Hence, as in the case of Wol- laston’s camera, the pupil of the eye must be kept perfectly centred to the small reflector. As there is but one reflexion, the image is inverted but not transposed. To see the outline of the image as it is in the micro- scope, the drawing must be made upon tracing paper, and inverted, looking at it as a transparency from the wrong side. There is considerable variety in the experience of different microscopists as to the facility with which these two instruments- can be used. The difference in all probability depends on the greater normal diameter of the pupils of the eyes of some observers in comparison with that of others. Fig. 179. Fig. 180— Simple camera. Dr. Lionel Beale devised one of the simplest cameras, which has the advantage of being thoroughly efficient. It consists of a piece of tinted glass placed at an angle of 45° to the optic axis,, in the path of the emergent pencil. The first surface of the glass- reflects the magnified image upwards to the eye, the paper and CAMERA LUCIDiE 235 pencil uenig seta l/iiiuugn Llie glass. ill US Simplest IOVm It IS Seeii in fig. 181. The glass is tinted to render the second reflexion from the internal surface of the glass inoperative. The reflexion of the image is identical with that of Soemmering’s. Fig. 182 shows a fitting adopted by Bausch andLomb forthzneutrcil tint camera. It is made of vulcanite, and the half ring to which the frame holding the neutral tint glass is fixed, fits on the cap of the eye-piece, and with sufficient grip. Amongst the camerce lucidce which 'project the image of the paper and pencil into the microscope faibe is first that devised by Amici, and adapted to the horizontal microscope by Chevalier. The eye looks through the micro- scope at the object (as in Fig. 181.—Beale’s camera. Fig. 182.—Bausch and Lomb’s fitting for Beale’s neutral tint camera lucida. the ordinary view ot it), instead ot looking at its projection upon the paper, the image of the tracing point being projected upon the field—an arrangement which is in many respects more advantageous. This is effected by combining a per- forated silver-on-glass mirror with a reflecting prism ; and its action will be understood by the accompanying diagram (fig. 183). The ray a b proceeding from the object, after emerging from the eye-piece of the microscope, passes through the central per- foration in the oblique mirror M, which is placed in front of it, and so directly onwards to the eye. On the other hand, the ray a', proceeding upwards from the tracing point, enters the prism P, is reflected from its inclined sur- face to the inclined surface of the mirror M, and is by it reflected to the eye at 0 , in sucn parallelism to the ray b proceeding from the object that the two blend into one image. Fig. 183. 236 ACCESSORY APPARATUS The Editor has used with great facility and success a camera devised by Dr. Hugo Schroder and produced by Messrs. Ross. It is figured at 184, and consists of a combination of a right-angled prism (fig. 185) A B Cand a rhomboidal prism D E F G, so arranged that when adjusted very nearly in contact (i.e. separated by only a thin stratum ■of air) the faces B C and D E are parallel, and consequently between D E and B E' they act together as a thick parallel plate of glass through which the drawing paper and pencil can be seen. The rhomboidal prism is so constructed that when the face G F is applied at right angles to the optic axis of the microscope, the axial ray H passes without refraction to I on the internal face E F ; whence it is totally reflected to J in the face D G. At J a part of the ray is reflected to the eye by ordinary reflexion in the direction of J K, and a part transmitted to J' on the face A C of the right-angled prism. Of the latter a portion is also reflected to K by ordinary reflexion at J'. The hypothenuse face A C is cut at such an angle that the reflexion from J' coincides with that from J at the eye point lv, thus utilising the secondary reflexion to strengthen the Fig. 184.—Schroder’s camera lucida. Fig. 185.—Diagram explaining Schroder’s camera lucida. luminosity of the image. The angle G is arranged so that the extreme marginal ray H' from the Held of the B eye-piece strikes upon D G at a point just beyond the angle of total reflexion, the diffraction-bands at the limiting angle being faintly discernible at this edge of the field. This angle gives the greatest amount of light by ordinary reflexion, short of total reflexion. In use, the microscope should be inclined at an angle of 45°, and the image focussed through the eye-piece as usual ; the camera is then placed in position on the eye-piece, and pushed down until the image of the object is fully and well seen. The drawing paper must be fixed upon a table on a level with the stage immediately under the camera. The observer will then see the microscopical image pro- jected on the paper, and the fingers carrying the pencil point will be clearly in view, the whole pupil of the eye being available for both images, the diaphragm on the instrument being considerably larger than the pupil. The eye may be removed as often as required, and if all is allowed to remain without alteration, the drawing may be left and recommenced, without the slightest shifting of the image. If a vertical position of the microscope be needful, this may be ABBE’S CAMERA LUCIDA 237 done by inclining the table and drawing paper to an angle of 45° either in front or at the side of the microscope. For accurate drawing, in all azimuths, the drawing paper should of course coincide with the plane of the optical image. This camera may be used with a hand-magnifier, or with simple lenses used for dissection and other purposes. Professor Abbe has also devised an instrument which we have used with complete success. The accompanying drawing (fig. 186) will at once show the simplicity of its action. The image of the paper and pencil coming, say, in a vertical direction (S.,, fig. 186) is reflected by a large mirror in a horizontal direction, W, to a cube of glass which has a silvered diagonal plane with a small circular hole in it in the visual point of the eye-piece. The microscopic image is seen directly through this aperture in the silvering of the prism, while the silvered plane of the prism trans- mits the image of the paper and the operator’s fingers and pencil. By the concentricity thus obtained of the bundle of rays reaching the eye from both the microscope and the paper, the image and the Fig. 186. pencil with which it is to be drawn are seen coincidentally without any straining of the eyes. This instrument requires the paper to be placed in a plane parallel to that of the object; thus, if the microscope is vertical the paper must be horizontal, and vice versa, and it presents the image precisely as it is seen in the microscope. For the pui-pose of drawing simply, and where the observer has had no experience in the use of a camera lucida, we should be inclined to recommend this one as the instrument presenting to the tiro the greatest facility. But there is a use to be made of the camera lucida to which this one does not so readily lend itself, which is none the less of great importance; that is, the determining of the magnifying power of objectives. It is manifest that the distance between the paper and the eye of the observer cannot be so readily determined in this case as in those forms of the instrument where the image of the paper and pencil is seen direct. With one or other of the foregoing contrivances, everyone may learn to draw an outline of the microscopic image ; and it is ex- tremely desirable for the sake of accuracy that every representation of an object should be based on such a delineation. Some persons will use one instrument more readily, some another, the fact being. 238 ACCESSORY APPARATUS that there is a sort of ‘ knack ’ in the use of each, which is commonly acquired by practice alone, so that a person accustomed to the use ■of any one of them does not at first work well with another. Although some persons at once acquire the power of seeing the image and the tracing point with equal distinctness, the case is more frequently ■otherwise ; and hence no one should allow himself to be baffled by the failure of his first attempt. It will sometimes happen, especially when the Wollaston prism is employed, that the want of power to see the pencil is due to the faulty position of the eye, too large a part of it being over the prism itself. When once a good position has been obtained, the eye should be held there as steadily as pos- sible, until the tracing shall have been completed. It is essential to keep in view that the proportion between the size of the tracing and that of the object is affected by the distance of the eye from the paper ; and hence that if the microscope be placed upon a support of different height, or the eye-piece be elevated or depressed by a slight inclination given to the body, the scale will be altered. This it is, ■of course, peculiarly important to bear in mind when a series of tracings is being made of any set of objects which it is intended to delineate on a uniform scale. A valuable adjunct to a camera lucida is a small paraffin lamp, .seen to the left of Plate III., which illustrates the correct method of using the camera lucida ; this lamp is simple and is capable of being raised or lowered, fitted with a paper shade, for a great deal of the success attendant on the use of the camera depends on the relative illumination of the microscopic image on the one side and of the paper and fingers and pencil of the executant on the other. It is not a matter to be determined by rules ; personal equation, sometimes idiosyncrasy, determines how the light shall be regulated. Many finished micro- draughtsmen use a feeble light in the image, and a strong light on the hand and paper, and others equally successful manipulate in the precisely reverse way. Put upon the adjustment of the respec- tive sources of light to the personal comfort of the draughtsman •will depend his success. Care must be exercised in this work in the case of critical images. These must not be sacrificed either by racking the condenser into or out of focus, or by reducing its angle by a diaphragm. If the in- tensity of the light has to be reduced, it must be done by the inter- position of glass screens, and this is beautifully provided in Abbe’s camera. The illustration of how the various apparatus for the use of the camera lucida should be disposed, given in Plate III., may be pro- fitably studied. Both mirror and bull’s-eye are turned aside, and the hand and pencil are illuminated by the shaded lamp. The lamp illuminating the image is seen, with such a screen of coloured glass as may be found needful, and the lamp illuminating the paper and pencil, and carefully shaded above, is also seen at the eye-piece end of the body-tube. Often, if the image is too bright, we find that bringing the lamp down to illuminate the paper more intensely suffices. If not, use screens ; the illuminating cone must not be tampered with. III. The determination of magnifying power is an important and independent branch of this subject. For this uurpose, and for PLATE III. Abrangement or the Microscope and Accessories for the Employment of the Camera Lucida. MOW TO DETERMINE MAGNIFYING POWER 239 the reason given above, Beale’s neutral tint camera 1 is eminently suitable indeed, is the best. We can easily and accurately measure the path of the ray from the paper to the eye. What is necessary is to project the image of a stage micrometer on to an accurate scale placed ten inches from the eye-lens of the eye-piece. There must be complete accuracy in this matter. We can best show how absolute magnifying power is thus deter mined by an example. Suppose that the magnified image of two TD(TTTths of an inch divisions of the stage micrometer spans Ts0ths of an inch on a rule placed as required ; then (i) -002 inch : ’8 inch :: 1 inqh : x power ; x = X = 400 diameters : ■002 for it is obvious that under these conditions one inch bears the same proportion to the magnifying power that x 0-U ()ths of an inch bears to ygths of an inch. Suppose, now, as it sometimes happens, that the operator is pro- vided with a metrical stage micrometer, but is without a metrical scale to compare it with, there being nothing but an ordinary foot- rule at hand. Let it be assumed that the magnified image of two T ’fTf mm. when projected covers yjy inch ; then, as there are 25-4 mm. in one inch (ii) "02 mm. ' (’8 inch x 25-4):: 1 : x power ; x — ~ X = 1016 diameters. If the reverse is the case, viz. that you have an English stage micrometer and a metrical scale, then, if the magnified image of two inch spans 18 mm., (iii) -002 inch : :: 1 : x ; ' 7 vb'4 x — = 354'3 diameters. The above results indicate the combined magnifying power of the objective and eye-piece taken at a distance of ten inches. The arbi- trary distance of ten inches is selected as being the accommodation distance for normal vision. The magnifying power, however, is very different in the case of a myopic observer. Let us investigate the case of one whose accom- modation distance is five inches. Here he will be obliged, in order to see the object distinctly, to form the virtual image from the eye-piece at a distance of five inches. To do this he must cause the objective conjugate focus to approach the eye-lens; consequently he must shorten his anterior objective focus. In other words, he must focus his objective nearer the object. This will have the effect of causing the posterior conjugate focus to recede from the objective towards the eye-lens, and the fact of bringing the inverted objective image nearer the eye-lens brings also the virtual image of the eye-lens nearer. 1 Page 285. 240 ACCESSORY APPARATUS Shortening the focus of the objective has the effect of increasing its power ; but as this alteration is proportionately very little, the increase in power is very small ; but the shortening of the eye-piece virtual from ten to five inches has the effect of nearly halving its power. Consequently the combined result of the eye-piece and objective in the case of halving the eye-piece virtual is to nearly halve the power of the microscope. The increase of the objective power is practically so small that it may be neglected.1 In practice it is found by us that if the image is projected on a ground glass screen ten inches from the eye-piece, the image is nearly the same size whether focussed by ordinary or myopic sight. This is in harmony with Abbe’s demonstration (pp. 25, 26, fig. 28) that both images are seen under the same visual angle. But, on the other hand, if a myopic sight compares the image with a scale, the magnification will be less than with ordinary vision. To find the precise initial power of any lens, or to find the exact multiplying power of any eye-piece, is not so easy. A laborious calculation, involving the knowledge of the distances, thickness, and refractive indices of the lenses, is required. But a very approximate determination, sufficiently accurate for all practical purposes, may be easily made, especially if one has a photo-micrographic camera at hand. The principle is as follows. Select a lens of medium power—a is very suitable. Now with the microscope in a horizontal position, and with a powerful illumination, project the image of the stage micrometer onto a screen distant five feet, measured from the back lens of the objective. If no photo-micrographic camera is at hand, it will be necessary to perform the experiment in a darkened room shading the illuminating source. Divide the magnifying power thus obtained by 6 ; the quotient will give the initial power of the lens at ten inches to a very near approxi- mation. The reason why the result is not perfectly accurate is that the ten inches must be measured from the posterior principal focus of the lens, and that is a point which is not given. But in the case of a power such as a it is, in practice, found to be very near the back lens of the objective. So by taking a long distance, such as five feet, the error introduced by a small displacement of the posterior prin- cipal focus does not materially amount to much. There is a further error introduced by the approximation of the objective to the stage micrometer in order to focus the conjugate at such a distance, but this is small. We can see, therefore, that this error tends to slightly increase the initial magnifying power. The initial power of the \ being found, and its combined magni- fying power, with a given eye-piece, being known, the combined power divided by the initial power gives the multiplying power of the eye-piece. Care must be of course taken to notice the tube- length 2 when the combined power is measured. The initial power of any other lens may be found by dividing the combined power of 1 English Mechanic, vol. xlvi.No. 1185. Article on measurements of magnifying power of microscope objectives by E. M. Nelson. 2 Ibid. vol. xxxviii. No. 981, ‘ Optical Tube-length,’ by Frank Crisp. ANCIENT ‘ NOSE-PIECES ’ 241 that lens with the eye-piece, whose multiplying power has been •determined, by the multiplying power of that eye-piece.1 Nose-pieces.—The term nose-piece primarily means that part of a microscope into which the objective screws, but the term is also applied to various pieces of apparatus which can be fitted between the nose-piece of the microscope and the objective. There are, for instance, rotating, calotte, centring, changing, and analysing nose-pieces. Nose-pieces, although thought to be so, are not a modern idea ; •our predecessors of a century ago employed similar means. Mr. screw the wheel to the under side of the metal stage. Now, if there are neither washers nor a shoulder to the screw, it is more than probable that when the diaphragm is rotated it will screw up and jamb. The purchaser may easily observe a matter of this kind. Condensers for Sub-stage Illumination.2—This condenser is an absolutely indispensable part of a complete microscope. Its value cannot be over-rated, for the ability of the best lenses to do their best work, even in the most skilful hands, is determined by it. Perfection in the corrections of object-glasses is indispensable ; but those who suppose and affirm that this is all that we need—that the objective is the microscope—cannot understand the nature of modern critical work. The importance of it could not have been realised in the sense in which we know it in the earlier dates of the history of the instrument ; but at as early a period as 1691 we pointed out (p. 135) that a drawing of Bonanni’s horizontal microscope showed the presence of a compound condenser. It is, in fact, of some interest to note how our modern condensers gradually arose. The microscope that amongst the older forms (1694) appears most efficient and suited for the examination of objects by trans- mitted light was that of Hartsoeker, p. 135, fig. 102. It will be re- membered that it not only was furnished with a condenser, but with a focussing arrangement to be used with it, which was not in any way affected by a change of focus in the object. This is a featui’e which, although not then important, is of the utmost importance now. In the correction of dispersion in the lenses employed in the dioptric form of microscope so much difficulty was experienced that several efforts were made to produce catoptric forms of the instru- ment ; the most successful of these was that of Dr. Smith, of Cam- bridge, in 1838 ; but this and all other forms of reflecting microscope had but a brief existence and passed for ever away. To the improve- ment of simple lenses much of the earlier progress of microscopic investigation is attributable; and that known as ‘ Wollaston's doublet,’ devised in 1829, was a decided improvement in all respects. It consisted of two plano-convex lenses • but this was again improved 1 Quekett, Micro. Journ. vol. iv. p. 121 et scq. 1 The word ‘ condenser ’ throughout this work is applied to optical appliances for the sub-stage; what is known as the ‘ bull’s-eye ’ is not called a ‘ condenser.’ EARLY USE OF THE CONDENSER 2 49 by Pritchard, who altered the lens distances and placed a diaphragm between the lenses. When the object was illuminated with a con- denser this formed what was the best dioptric microscope of pre- achromatic times. Good results, within certain limits, may be obtained by means of the best Pritchard doublets. With a inch the surface of a strong Podura scale may be seen as a surface symmetrically scored or engraved, but the Editor has never himself been able to reveal the ‘exclamation ’ marks ; and as this is the experience of other efficient experts, it may be taken that no resolution of these was accomplished in pre-achromatic days; these lenses, in fact, overlapped the discovery of achromatism. But the practical results of the use of achromatic lenses soon led the most experienced men in its theory and practice to perceive that if it were good for the lenses which formed the image, it was also good for the condenser. Thus Sir David Brewster in 1831 ad- vocated an achromatic condenser in these remarkable words, viz. ‘ I have no hesitation in saying that the apparatus for illumination requires to he as perfect as the apparatus for vision, and on this account I would recommend that the illuminating lens should he perfectly free from chromatic and spherical aberration, and that the greatest care be taken to exclude all extraneous light both from the object and from the eye of the observer.’ This is a judgment which every advance in the construction of the optical part of the microscope, as used by the most accomplished and experienced experts, has fully confirmed. We have no knowledge, from an inspection of the piece of apparatus itself, of the construction of the compound sub-stage con- denser of Bonanni (p. 135) ; it does not appear to have attracted much attention, and of course it was quite impossible to secure a critical image by its means. It was focussed on the object merely to obtain as bright an illumination as possible in order that the ob- ject might be seen at all. In the condenser used by Smith in his catoptric microscope (p. 144) we have the earliest (1738) known condenser, by means of which a distinction between a ‘ critical ’ image—that is, an image in which a sharp, clear, bright definition is given throughout, free from all ‘rottenness’ of outline or detail—and an ‘ uncritical ’ or imperfect image could be made. It was not, apparently, at the time it was first used considered to be so important as we now know it to be ; and it is probable that the mode of focussing the light upon the object by its means was to direct the instrument to the sky with one hand and to use the biconvex condenser with the other. In 1837 Sir D. Brewster writes of it with appreciation, saying that ‘ it performs wonderfully well, though both the specula have their polish considerably injured. It shows the lines on some of the test objects with very considerable sharpness.’ No advance was made on this condenser for nearly a century. In 1829 Wollaston recommends the focussing of the image of the diaphragm by means of a plano-convex lens of f of an inch focus upon the object, and Goring in 1832 says concerning it: ‘ There is no 250 ACCESSORY APPARATUS modification of daylight illumination superior to that invented by Dr. Wollaston.’ But Sir D. Brewster objected to this, contending that the source of light itself should be focussed upon the object. He preferred a Herscheleian doublet placed in the optic axis of the microscope. But whilst there is a very clear difference between these authorities we can now see that both were right. Goring, who was also a leader in the microscopy of his day, used diffused daylight, and as the lens he employed was a plano-convex of j of an inch focus, the method of focussing the diaphragm was as good as any other, because the diaphragm was placed at a distance from the lens of at least five times its focus, so that the difference between diaphragm focus, and ‘ white cloud ’ focus, or the focussing of the image of a white cloud upon the object, was not very great. But Brewster was writing of a candle-flame when he insisted on the bringing of the condenser to a focus on the object, and in this lie was, beyond all cavil, right. In 1839 Andrew Ross gave some rules for the illumination of objects in the ‘ Penny Cyclopaedia.’ These were— 1. That the illuminating cone should equal the aperture of the objective, and no more. 2. With daylight, a white cloud being in focus, the object was to be placed nearly at the apex of the cone. The object was seen better sometimes above, and sometimes below the apex of the cone. 3. With lamplight a bull’s-eye is to be used to parallelise the rays, so that they maybe similar to those coming from a white cloud. Of the old forms of condenser, that devised by Mr. Gillett was, there can be no doubt, the best. It was achromatic, and had an aperture of 80°. Fig. 197 illustrates it. It was fitted with a rotating ring of d iapliragms placed close behind the lens combination. This was formed, as the figure shows, by a conical ring with apertures and stops, and on account of the large num- ber of apertures and stops it would admit, which, pro- vided they are carefully ‘ cen- tred,’ are of great value in practical work, as well as from the fact that they are so placed as not to interfere with the stage, makes this arrangement of diaphragms and stops an excellent one, and it is not clear why it has fallen into disuse. It had been the custom to recommend the use of this instrument racked either within or without its focus. Carpenter employed it without and Quekett within, and one or other of these methods was general. But in the use of good achromatic condensers with high- Fig. 197.—Gillett’s condenser, from ‘ Hogg on the Microscope.’ WHAT IS ESSENTIAL IN A CONDENSER power work it soon became manifest to practical workers that it is only when, as Sir David Brewster pointed out, the source of light is focussed by the condenser on the object that a really critical image was to be obtained. And Mr. Nelson readily demonstrated this fact even with the condenser Gillett had devised. The next condenser of any moment is a most valuable one, and constitutes one of the great modern improvements of the microscope. It was an achromatic condenser of 170° devised and manufactured by Messrs. Powell and Lealand. We have used this instrument for twenty-five years on every variety of subject, and we do not hesitate to attirm that for general and ordinary critical work it is still un- surpassed. Pig. 198 illustrates this apparatus. The optical combina- tion is a Ith of an inch power, and it is therefore more suitable for objectives from a of an inch and upwards ; but by removing the front lens it may be used with objectives as low as one inch. Having given to this condenser so high a place amongst even those of our immediate times, it may be well to specify what the requirements are which a condenser employed in critical work with high powers should meet. It is needful that we should be able (1) to obtain at will the largest £ solid ’ cone of light devoid of spherical aberration.1 Directly spherical aberration makes itself apparent the condenser fails ; that is, when, on account of under-correction, the central rays are brought to a longer focus than the marginal rays, or when, because of over-correction, the marginal rays have a longer focus than the central. But (2) it is also an absolute essential that if a condenser is to be of practical service it must have a working distance sufficiently large to enable it to be focussed through ordinary slips. It would be an advantage if all objects mounted for critical high-power work were mounted on slips of a fixed gauge, say ‘06 inch, which would be £ medium,’ ’05 inch being accounted £thin,’ and ‘07 inch ‘thick.’ It is plain, however, that to combine a large aperture with a great working distance the skill of the optician is fully taxed, for this can only be accomplished (a) by keeping .the diameter of the lenses just large enough to transmit rays of the required angle and no more ; (b) by working the convex lenses to their edge ; (c) by making the flint lenses as thin as possible. Now it is due to the eminent Firm whose condenser we have been •considering with such appreciation, to say that the condenser referred to (d) transmits the largest 1 solid ’ cone free from spherical -aberration ; (e) that it has the greatest working distance ; (f) that its chromatic aberrations are perfectly balanced. In the pos- 251 Fig. 198.—Powell and Lealand’s condenser. 1 This is one of the many expressions which are inevitable to the practical use of .apparatus; it is simply convenient, and means a full cone of light, a cone with none •of its rays stopped out. 252 ACCESSORY APPARATUS session of these three essential qualities it has stood unrivalled for upwards of thirty years. The removal of the front lens of this condenser, which may he readily unscrewed, reduces it in power and angle, and therefore makes it suitable for objectives of lower power. This, however, is rather an adaptation involving compromise than an ideal condenser for low powers. When the highest class of work has to be done it is needful to have condensers suited to the 'power of the objective used. A low-power condenser of much merit is made by Swift and Son; it begins, in its relation to low powers, where the condenser of Powell and Lealand leaves off. It consists of two doublets with a single front, and is much lower in both power and aperture than that of the latter makers ; but by sliding off the front cap into which the front lens is burnished both power and aperture may be further reduced. It is achromatic, and is a practical and useful instrument capable of adap- tation to any microscope. Fig. 199 is a general illustration of this appliance. A condenser having con- siderable value, and specially adapted to lenses of low power, and up to those of inch foe us, has just been constructed and placed in our hands by Messrs. Powell and Lealand. It was made in response to the earnest suggestion of several leading microscopists, and in many respects fully answers its purpose. It is achromatic, has a numerical aperture of '83, with an aplanatic aperture of '5, and for dark-ground illumination is possessed of the highest qualities. Its power is a inch, and will prove a most useful adjunct to the photo-micrograplier, since it will enable him to get a large image of the source of light on the object; but its aberrations are not so perfectly balanced as we could desire. It is possessed of a new feature so far as the condenser of these makers is concerned, having permanently placed beneath the optical arrangement an iris diaphragm, and in addition the con- denser mount is supplied with a series of diaphragms and stops which are placed in a turn-out-arm carrier ; this provides the worker with facility as well as accuracy of method, since both of these can be used under the same adjustment. The aperture of the cone trans- mitted by the condenser with each diaphragm is engraved upon that diaphragm, and with the stops for dark ground; the aperture of the ob- jective with which the stop will yield a dark ground is also engraved on it. This embodies the recommendations we have made below. We give an illustration which is self-explanatory of this appa- ratus, fig. 200. Before the introduction of the homogeneous system, and the production of such great apertures by Powell and Lealand as Fig. 199.—Swift’s condenser. ACHROMATIC CONDENSER OF LARGE APERTURE 253 a 1'5 in a Jth, a T\,th, and a of an inch focus, the cone transmitted by Powell’s dry achromatic condenser was as large as could be utilised. But with apertures such as these, and the subsequent introduction of the apocliromatic system of lenses, much larger cones were required. To meet this necessity Powell and Lealand, at the urgent suggestion of English experts, made first a chromatic condenser on the homogeneous system; but this was subsequently succeeded by an achromatic instrument of great value on the same system. This combination consists of a duplex front with two doublet backs ; it is nearly of the same power as their dry achromatic condenser, but is of much greater aperture. It has been brouLdit still more recently to a very hik, where the object on the stage is supposed to be situated. By moving the slit by the screw S1 the spectrum is caused to pass over the object, the different colours following in succession. The instrument may be used for low powers with ordinary daylight, Fig. 224. but foi’ high powers sunlight must be employed. Moreover in practice it is needful to use a bull’s-eye to focus the light on the slit &p. But this instrument lacks the higher qualities required of it on account of the low angle of the combination O which acts as a condenser. We really want a pencil of monochromatic light, either parallel or slightly divergent, of such a size as toJill the hack lens of a high-class condenser. If such a condenser as Powell s dry achromatic or later homogeneous achromatic of large aperture could be adapted to this instrument (which is no doubt possible) it might be of service to microscopists. Sorby-Browning Micro-spectroscope.1—When the solar ray is decomposed into a coloured spectrum by a prism of sufficient disper- 1 For general information on the spectroscope and its uses the student is referred to Professor Roscoe’s Lectures on Spectrum Analysis, or the translation of Dr. Schellen’s Spectrum Analysis, and How to TJsc the Spectroscope, by Mr. John Brownin''. NATURE OF THE MICRO-SPECTROSCOPE 273 sive power, to which the light is admitted by a narrow slit, a multitude of dark lines make their appearance. The existence of these was originally noticed by Wollaston ; but as Fraunhofer first subjected them to a thorough investigation and mapped them out, they are known as Fraunhofer lines. The greater the dispersion given by the multiplication of prisms in the spectroscope, the more of these lines are seen ; and they bear considerable magnification. They result from the interruption or absorption of certain rays in the solar atmosphere, according to the law, first stated by Angstrom, that ‘ rays which a substance absorbs are precisely those which it emits when made self-luminous.’ Kirchhoff showed that while the incandescent vapours of sodium, potassium, lithium, plates of glass, of the proper size, of any desired thickness, kept apart by half a ring of vulcanised indiarubber, the whole screwed tightly enough together by three milled heads to prevent leakage. But leakage or the fracture of glasses is not uncommon witli this otherwise convenient form. An excellent, though shallow, trough was made by Mr. C. G. Dunning, which we illustrate in fig. 262. The lower plate or trough proper is made of metal, 3 inches long by wide and about TL thick, with an oval or oblong perforation in the centre, and the under side is recessed, as shown in fig. 262, B. In this recess is fixed by means of Canada balsam or shellac a piece of stout covering glass, forming the bottom of the cell, the recess being sufficiently deep to pre- vent the thin glass bottom from coming into actual contact with the stage of the microscope or with the table when it is not in use. Two- pieces are provided near the bottom edge of the cell: the cover (fig. 262,C) is formed of a piece of thin brass, rather shorter than the trough, but about the same width it has an opening formed in it to correspond with that in the trough, and under this opening is cemented a piece of cover-glass. The cover plate is notched at the two bottom corners, and at the two top corners are formed a couple of projecting ears. In order to use this apparatus it must be laid flat upon the table, and filled quite full of Avater. The object 1 Watch-spring or other elastic metal should not be used on account of oxidation. Fig. 202. EXAMINING INFUSORIA 299 to be examined is then placed in the cell, and may be properly arranged therein ; the cover is then lowered gently down, the two- notches at the bottom edges being first placed against the pins ; in this, way the superfluous water will be driven out and the whole apparatus may be wiped dry. The capillary attraction, assisted by the weight of the cover, will be found sufficient to prevent any leakage ; and the pins at the bottom prevent the cover from sliding down when the microscope is inclined. This zoophyte trough possesses two im- portant qualities : first, it does not leak ; second, it is not readily broken without gross carelessness. The shallowness may be over- come by placing an ebonite plate with the required aperture between the two mounted glasses. Infusoria, minute Algae, &c. however, can be well seen by placing a drop of the water containing them on an ordinary slide, and laying a thin piece of covering glass on the top ; and objects, of somewhat greater thickness can be examined by placing a loop or ring of fine cotton-thread upon an ordinary slide, to keep the covering glass at a small distance from it; and the object to be ex- amined being placed on the slide with a drop of water, the covering glass is gently pressed down till it touches the ring. Still thicker objects may be viewed in the various forms of ‘ cells ’ hereafter to be described, and as, when the cells are filled with fluid, their glass- covers will adhere by capillary attraction, provided the superfluous moisture that surrounds their edges be removed by blotting-paper, they will remain in place when the microscope is inclined. An annular cell, that may be used either as a 1 live-box ’ or as a ‘ grow- ing slide,’ has lately been devised by Mr. AVeber (U.S.A.). It is a slip of plate-glass, of the usual size and ordinary thickness, out of which a circular ‘ cell ’ of |-inch diameter is ground, in such a manner that its bottom is convex instead of concave, its shallowest part being in the centre and the deepest round the margin. A small drop of the fluid to be examined being placed upon the central convexity (the highest part of which should be almost flush with the general surface of the plate), and the thin glass cover being placed upon it, the drop spreads itself out in a thin film, without finding- its way into the deep furrow around it ; and thus it holds-on the covering glass by capillary attraction, while the furrow serves as an air-chamber. If the cover be cemented down by a ring of gold size or dammar, so that the evaporation of the fluid is prevented, either animal or vegetable life may thus be maintained for some days, or, if the two should be balanced (as in an aquarium), for some weeks. Dipping Tubes.— In every operation in which small quantities- of liquid, or small objects contained in liquid, have to be dealt with by the microscopist, he will find it a very great convenience to be provided with a set of tubes of the forms represented in fig. 263, but of somewhat larger dimensions. These were formerly designated as ‘ fishing tubes,’ the purpose for which they were originally de- vised having been the fishing out of water-fleas, aquatic insect- larvae, the larger animalcules, or other living objects distinguishable- either by the unaided eye or by the assistance of a magnifying glass from the vessels that may contain them. But they are equally ACCESSORY APPARATUS 300 •applicable, of course, to the selection of nnnute plants ; and they may be turned to many other no less useful purposes, some of which will be specified hereafter. When it is desired to secure an object which can be seen either with the eye alone or with a magnifying +>*.} «! 310 OBJECTIVES, EYE-PIECES, THE APERTOMETER in extending the aperture of a objective to 85°, or ‘68 N.A. ; and a TVinch objective to 135°, or -93 N.A. Of this latter it was affirmed that it was ‘ the largest angular pencil that could be passed through a microscope object-glass.’ In 1850 object-glasses were made with a tripleback combination ; these were attributed to Lister ; but it is also affirmed that they were the previous device of Amici. It may well be a disputed point, for it is quite certain that this device brought the dry achromatic objective potentially to its highest perfection. The combination is illustrated in tig. 272, and under the conditions of its construction it may be well doubted if anything will ever surpass the results obtained by English opticians in achromatic objectives constructed with this triple front, double middle, and triple back combinations. It may be noticed that Tully’s objective had a triple back, but it was not the result of intended construction; it was a fortunate combination the real value of which was neither understood nor appreciated, and as a consequence its existence was evanescent. In this same year Wenham produced another modification of the achromatic objective of considerable value, but more to the manu- facturer than the user of the microscope. It consisted of a single front; the combination is seen in fig. 273, which it will be seen is a simpler construction, but this did not affect in the least the price of the objectives produced. Subsequently, how- ever, the form was adopted on the Continent for low-priced objectives, which led to a reduction of the cost of English objectives of the same con- struction. Manifestly, the single front lessened the risk of technical errors, but we have never been able yet to find a single front dry achromatic objective which has shown any superiority over a similar one possessing a triple front. The single front employed with two combina- tions at the back was the form in which the celebrated water- immersion objectives of Powell and Lealand were made. It was by one of these that the striae on Amphipleura pellacida were first resolved. Indeed, what is known as the water-immersion system of objectives, devised by Professor Amici, was the next advance upon the old form ; but it was an advance the optical principles of which were certainly not at the time understood. In Paris, Prazmowski and Hartnack brought these objectives to great perfection, and were enabled to take the premier place against all competitors at the exhibition of 1867. The next year, however, Powell and Lealand adopted the system, and in turn they distanced' the Paris opticians and produced some of the finest objectives ever made. Their ‘New Formula’ water-immersions were made after the fine model of Tolies referred to below, and had a duplex front, a double middle, and a triple back. In 1877, when the water- Fig. 272.—A triple- back combina- tion by Lister (or Amici ?). Fig. 273.—A single- front combination by Wenham. THE INFLUENCE OF THE DIFFRACTION THEORY immersion system touched its highest point, apertures as great as 1’23 were reached; and in America, Spencer, Tolies, and Wales produced some extremely fine lenses of large aperture. During the year 1869 Wenham experimented with and sug- gested 1 the employment of a duplex front; that is so say, a front combination made up of two uncorrected lenses in contradistinction to an achromatised pair. An illustration of the plan suggested is given in fig. 274, which hardly appears to us as a practicable form, and which certainly wras never brought to perfection or put into practice. But in the month of August 1873 Tolies actually made, on wholly independent lines, a duplex front formula for a 4-glycerine im- mersion of 110° balsam angle, which passed into the possession of the Army Medical Museum at Washington. There can be little doubt but this objective would have produced ;a much deeper impression but for the fact that it was in advance of its immediate time. Tolies, as we have hinted above, used the duplex front in the construction of some of his immersion objectives, and was followed in this by the best English makers, and in the case of a celebrated 4-inch, purchased by Mr. Crisp, Tolies was able to reach a balsam angle of 96°.. . At the time that the water-immersion lenses were being con- structed by rival opticians with increasing perfection the great theory of Professor Abbe concerning microscopic vision, the import- ance of diffraction spectra, and the relation of aperture to power 1 was entirely unknown. In the absence of this knowledge wholly mistaken value was attached to power per se in the objective. With a focus as short as the gC-inch, it was not uncommon to find apertures less than 1*2, while objectives of -fy, and even higher powers, were made with extremely reduced apertures. This was done in the interests of the common belief that ‘ power ’— devoid of its suitable concurrent aperture—could do what was so keenly wanted. This impression, however, was far from universally relied on; there were several earnest workers, who, without being able to explain, as Abbe subsequently did, why it was so, still urged the opticians, in the manufacture of every new power, especially the higher ones, to produce the largest possible amount of aperture ; and the evidence of this is still to be found in the objectives they then succeeded in obtaining. But there can be no doubt that a reckless desire for magnifying power, all other considerations apart, greatly obtained ; and the opticians were able to encourage it, for it is far easier to construct an objective of high power and low aperture than it is to make a low power with a large aperture. Thus a i-inch of 065 N.A. will be far more expensive and prob- ably not as well corrected as of 07 N.A. The objective, Fig. 274.—A suggested combination by Wen- ham, 1869. 1 Monthly Micro. Journ. vol. i. p. 172. 312 OBJECTIVES, EYE-PIECES, THE APERTOMETER even if a good one, is sure to exhibit spherical aberration, while- the | of low aperture will show many minute objects with con- siderable clearness, especially if a comparatively narrow illuminating cone be used. This difference becomes still more conspicuous as the difference between aperture and power grows relatively greater, until we obtain ultimately an amplification more than useless from its utter inability,, on account of deficiency of aperture, to grasp details.1 Up to 1874, however, there was an entire absence of knowledge, even on the part of the leaders in microscopic theory, art, and practice, as to the real optical principles that enabled us to see a microscopic image, and consequently to understand the essential requirements to be aimed at in the best form of microscope. But in 1877 Abbe’s great diffraction theory of microscopic vision appeared, which has led to changes of incomparable value in the principles of construction of objectives and eye-pieces, and, as a consequence, has to some considerable extent given a new character to the entire in- strument. Its promulgation has indeed inaugurated an entirely new epoch in the construction and use of the microscope. The general character and the details of Abbe’s theory are given in the second chapter of this treatise ; but its practical bearing upon the theory and application of the optical part of the instrument were soon manifest; for in 1878 the homogeneous system of immersion objectives2 was introduced as a logical outcome of the diffraction theory of microscopic vision. A formula for a objective on this system was prepared by Abbe, to whom, we learn from himself, it had been suggested by Mr. J. W. Stephenson, of the Royal Microscopical Society.3 It has been already shown 4 that the homo- geneous system was so called because it employed the oil of cedar wood to unite the front lens of the objective to the cover-glass of the object, in the same way as water had been employed in the ordinary immersion system ; but as there was a practical identity between the refractive and dispersive indices of the oil, and those of the crown glass of the front lens, the rays of light passed through what was essentially a homogeneous substance in their path across from the balsam-mounted object to the front lens ; and a homogeneous system of objectives took the place of the previous water immersions. ; This was the first great step in advance in optical construction qnd application following the theory of Abbe. , As often happens in matters of this kind, there had been an. apparent anticipation of this system of lenses by Amici as far back as 1844 ; but it is very apparent that Amici employed the oil of aniseed without any clear knowledge of the principles involved im the homogeneous system; being wholly unaware of either the increase of aperture involved or the cause of it. But this cannot be said of Tolies, of New York. We have pointed out that as early as 1873 lie made a and subsequently in the same year a i-inch objective, each with a duplex front to work in soft balsam, and with a N.A. of l-’27i These objectives were examined by the late Dr- 4- 1 Vide Chapter'll. ■ - - • ■> > * Ibid. 5 P. 27; also Journ, Jtoy. Micros?, Soc. tvql, il. 1879, p. 257. 4 Chapter I. THE EXCLUSION OF THE SECONDARY SPECTRUM Woodward, of the Army Medical Department, Nsw Ytrrk, and with that examination were allowed to drop. For Tolies as an original deviser of a practical homogeneous system this was unfortunate; for the actual introduction of the system in a form capable of universal application, and worked out in all its details in an entirely inde- pendent manner, we are wholly indebted to Abbe. The principle was not, nevertheless, so readily and warmly adopted in England on its first introduction as might have been anticipated. This arose partly, however, from the fact that water im- mersions had been brought to so high a point of excellence by Messrs. Powell and Lealand that the early homogeneous objectives were not pos- sessed of more aperture, and were not sensibly superior to the best immersions made in England. The homogeneous objectives were made with duplex fronts and two double backs. A general diagram of their mode of construction is given in fig. 275. So long as crown glass was employed in their manufacture, and the anterior front lens was a hemisphere, it appeared that N.A. 1*25 to l-27 was the aperture limit they could be made to reach. Messrs. Powell and Lealand however, by making the anterior front lens greater than a hemisphere increased the aperture of a objective to 1*43 N.A. This front, from being greater than a hemisphere, presented difficulty in mounting ; this was at first overcome by cementing its plane surface to a thin piece of glass, which was then fixed in the metal. Eventually, however, this form of construction was changec by these makers in a very ingenious manner; so to, speak, the} entirely inverted the combination, and accomplished the end b) making the front of flint. By this means they obtained apertures which have not as yet been equalled by any other makers, reaching in a a jb, and a a N.A. of 1 ‘50, out of a theoretically possible aperture of P52. Professor Abbe has since, it is true, made an objective with a numerical aperture of 1*63, but this requires the objects to be mounted and studied in a medium of corresponding refractive index, and consequently, in the present state of our know- ledge of the subject of media, not applicable to the investigation of ordinary organic structures—certainly not of living things. These objectives fully occupied the microscopist until 1886, when the most important epoch since the discovery and application of achromatism was inaugurated. , ' o , ’ ;; 1 We have already pointed out in detail1 that it was the great defect of the ordinary crown and flint achromatics that two colours only could be combined and that the other colours caused out-of focus images, which appeared as fringes round the object. This was, what was known as the residuary secondary spectrum. In like manner, it has been shown that it was not possible in the: flint and crown achromatic to combine two colours in all the zones* Fig. 275.—Combina- tion for ‘ homoge- neous ’ immersion’ by Abbe. 1 Chapter I. OBJECTIVES, EYE-PIECES, THE APERTOMETER of the objective, so that if two given colours are combined in the in- termediate zone, they will not be combined in the peripheral and the central portions of the objective. These phenomena, it has been pointed out,1 arise from what is known as the irrationality of the spectrum. To correct this we have seen that Drs. Abbe, Schott, and Zeiss directed their attention to the devising of vitreous compounds which should have their dispersive powers proportional to their refractive indices for the various parts of the spectrum. Only by these means could the outstanding errors of achromatism be corrected. It is therefore a fact that the old flint and crown objectives, whether for the microscope, the telescope, or the photographic camera, are, strictly speaking, neither achromatic nor aplanatic. Glass whose properties far more nearly approximated the theo- retical requirements than any previously attainable having been manufactured by the Jena opticians,2 Abbe was able to produce objectives entirely cleansed of the secondary spectrum. From calcu- lations of a most elaborate and exhaustive kind made by Dr. Abbe, objectives are made by Zeiss which not only combine three parts of the spectrum instead of two, as formerly, but which are also aplanatic for two colours instead of for one. This higher stage of achromatism Abbe has called apochromatism. A general plan of the construction of an apochromatic objective as made by Zeiss is shown in fig. 276, which, it will be understood, is diagrammatic, but sufficiently illustrates the elaborate corrections by which the perfect results given by these objectives are accomplished. But, in addition to their form of construction and the special optical glass of which they are composed, it is now known that they owe much of their high quality to the use of fluorite lenses amongst the combination. Fluorite is a mineral which has lower refractive and dispersive indices than any glass that has yet been composed, and therefore, by its introduction, the optician can reduce the spherical and chro- matic aberrations greatly below that reached by achromatic combinations of the known type. It is a somewhat depressing fact that fluorite is very difficult to procure in the clear condition needful for the optician, but from what we have seen the optician can do in the manufacture of glass, we may hope that an equivalent of this mineral in all optical qualities may be discovered. The medium for mounting and immersion contact has, of course, to be of a corresponding refractive and dispersive index in all objectives of great aperture, and it is insisted by Abbe that the glass of which the mount is made, both slip and cover, must, when the limit of refraction by crown glass is passed by the objective, be of flint glass. This he presents as a sine qua non in the case of the new objective just made by the house of Zeiss, and a specimen of which has been generously given by the Firm to the Royal Microscopical Society. This glass has a numerical Fig. 276.—Diagram of apochromatic com- bination. 1 Chapter I. 2 Chapter II. COMPARATIVE ACTION OF APOCHROMATIC & OTHER LENSES aperture of 1 ‘63 ; it is too early to criticise its qualities, but in a subsequent chapter on the present state of our knowledge as to the ultimate structure of diatoms, we are enabled to present the results of some of the photo-micrographs produced by its means. But it may be noted that very much will depend upon the N.A. of the illuminating cone which can be employed with it—not theoreti- cally, but practically. On the whole, and for the purposes of practical biological inves- tigation, it is to the dry apochromatics that we are most indebted, and from their use we shall derive the largest benefit. As no subject is really of more importance than a clear under- standing of the difference of action of chromatic, achromatic, and apochromatic lenses, we venture to present a diagrammatic illustra- /. .2. 3. Fig. 277. tion, which, while not strictly accurate, will carry with it no error, as a popular illustration of this important subject. In fig. 277, 1, 2, 3, we have representations, as truly as they can be drawn, of zones of equal light ; that is to say, the peripheral zone will transmit an amount of light equal to that given either by the intermediate zone or the central circle. Let them therefore be called equilucent zones. l If we assign a numerical value for the visual intensity of the whole spectrum, say 100, made up of the following parts, viz. Red 15 Orange yellow . 40 Yellow green ....... 30 Blue 15 then if in any one of the equilucent zones the whole spectrum is brought to a focus, we shall have for that zone 100 as its effective value. But the entire object-glass is divided, as in the diagram, into three equilucent zones ; consequently 300 will represent the value of the whole lens, provided the whole of the spectrum is brought to the same focus. By referring to the diagrams we see that in a non-achromatic lens (fig. 277, 3) we shall get only 40, because only one part of the spectrum is brought to the focus in its intermediate zone ; and as spherical aberration causes the light which passes through the other 316 OBJECTIVES, EYE-PIECES, THE APERTOMETER zones to be brought to other foci, they for all practical purposes might be stopped out. In the achromatic lens we have (fig. 277, 1) in the intermediate zone two parts of the spectrum combined, as 40 + 30 = 70, and one in each of the other zones is also brought to the same focus, say 30 in the outer zone, and 40 in the centre circle. The result is that the whole achromatic lens gives a total of light on the principle stated above of 30 + 70 + 40 = 140. In the apochromatic system, how- ever (fig. 277, 2) we find in the intermediate zone three parts of the spectrum united; that is to say, 40 + 30 + 15 = 85; and two in each of the others, say, 40 + 30 = 70. Thus an apochromatic objective will give 70 + 85 + 70 = 225. Recalling the suppositions we have made for the purpose of this- graphic presentation of a difficult subject, it will be seen that a non- achromatic objective would give 40, an achromatic 140, and an apochromatic 225, out of a possible total of 300. This illustration might be exceeded in severe accuracy, but scarcely in simplicity, and it sufficiently explains from this point of view alone the vast gain of the apochromatic system. It is interesting to note that, while the microscope in its earlier form took its powerful position by borrowing achromatism from the- telescope, it has now led the way to the apochromatised state, which without doubt it will be the work of the optician in constructing- the telescope of the immediate future to follow. We would beg the reader to bear in mind in the purchase of objectives that, whilst the vitreous compounds with which Abbe’s- beautiful objectives are constructed are now accessible to all opticians, and whilst without these Abbe’s objectives could never have been constructed, yet it does not by any means follow that because an objective is made with the Abbe-Schott glass it is therefore apo- chromatic ; the secondary spectrum 'must be removed, and the spherico- chromatic aberration balanced, or it is ‘ apochromatic ’ only by mis- nomer. It is another feature of these objectives, which it is import- ant to note, that they are so constructed that the upper focal points of all the objectives lie in one plane. Now as the lower focal points, of the eye-pieces are also in one plane, it follows that, whatever eye- piece or whatever objective is used, the optical tube-length will remain the same. Professor Abbe has found 1 that in the wide-aperture objective of high power .there is an outstanding error, which there are no- means of removing in the objective alone, but, as we have already explained, this is left to be balanced by an over-corrected eye-piece. As this peculiarity pertains only to the higher powers, a correspond- ing error had to be intentionally introduced into the lower powers in order that the same over-corrected eye-pieces, might be available for use with them. It appears worthy of note in this relation that one of the best forms for the combination of three lenses is that known as Steinheil’s. formula, which consists of a bi-convex lens encased in two concavo- convex lenses. It will be observed by reference to the figure illustrat- 1 Chapter II. THE CHARACTER OF ZEISS’S APOCHROMATICS 317 ing the apochromatic lens construction (tig. 276) that this is largely made use of. In some instances the encasing lenses possess sufficient -density with regard to the central bi-con vex lens to altogether over- power it, the result being a bi-con vex triple with a negative focus. It is another distinctive feature of the 3 mm. objective that it has a triplex front ; thus Zeiss’s 3 mm. (= focus) has the ■errors from three uncorrected lenses balanced by two triple backs, i.e. nine lenses taken together. The foci of the set of apochromatic lenses now made by Zeiss are integral divisions of what may be termed a unit lens of 24 mm.; 24 he chooses as a means of avoiding the inconveniences inseparable from the use of the decimal system.1 The unit lens is therefore a little higher than 1 inch in power. In the series of dry lenses there are two powers of the same aperture. Thus 24 mm. and 16 mm., ■corresponding to English 1 inch and -f inch, each has an aperture of '3 ; a 12 mm. and 8 mm. = English inch, and inch, have •each an aperture of -65 ; while a 6 mm. and a 4 mm. = 1 inch and inch, have both an aperture of ’95. There are also water-immersions ; a 2-5 mm. = inch, with N.A., 1*25, and two oil-immersions respectively 3 mm. and 2 mm. = inch and rV inch, both being made either with 1*3 or 1-4 N.A. Apart from these, intended to be used for photographic pur- poses without an eye-piece, is a 70 mm. = a 3-inch, also a 35 mm. or 14-inch objective. With the exception of the 6 mm., 4 mm., and 2-5 mm. objectives which have the screw-collar adjustment, this series have rigid mounts, •correction being secured by alteration of the tube-length. The performance of these lenses, as they are now made, is of the very highest order. They present to the most experienced eye unsur- passed images. They are corrected with a delicate perfection which only this system, coupled with technical execution of the first order, can possibly be made to produce. The optical polish, the centring, the setting, and the brasswork certainly have never been surpassed. It is a matter also worthy of note that Zeiss’s apochromatic ■series of objectives are true to their designations as powers. The is such, and not a designated This was equally true of the early acliromatics. A. Ross produced a under that name. One now before us, made fifty years ago, has an initial power of 41 ; and that of a -J-inch has an initial power of 21. But modern achromatics of fair aperture are always greatly in excess of their designated power ; -f are nearly -J-inch. A 4-inch •over 40° has an initial power of 25, and is a ; objectives are in reality ; and 4'inch objectives of 90° and upwards have initial powers of 50 instead of 40, which they should have, so that they are in reality -J ths ; some in fact—by no means uncommon—have an initial power of 60, and are actually -|th-inch objectives. 1 Although the foci of the lenses are expressed in integers, with the single excep- tion of the water immersion 2'5 mm., there are inconvenient decimal fractions in the initial magnifying power of all the series except those of 2'5 and 2 mm. focus. 318 OBJECTIVES, EYE-PIECES, THE APERTOMETER This is explicable enough from the maker’s point of view ; it is far easier to put power into an object-glass than aperture. It is easier to make a -J-inch of 100° than a with 100° ; the result is that low powers with suitably wide apertures are costly. In the Zeiss apochromatic series of objectives the 24 mm. of -3 N.A. and 12 mm. of '65 N.A. may be considered as lenses of the very highest order ; the relation of their aperture to their power is such that everything which a keen and trained eye is capable of taking cognisance of is resolved when the objective is yielding a magnification equal to twelve times its initial power ; for this purpose an objective must have 026 N.A. for each hundred diameters of combined magnification. Under these conditions an object is seen in the most perfect manner possible. It may be well for the student to prove this, which may be readily done. Take a suitable object, such as a well-prepared proboscis of a blow-fly, and examine it under critical illumination with the 24 mm. ‘3 N.A. (= 1 inch) objective, and a 12 compensating eye- piece. Note with close attention every particular of the image : the resolution of the points of the minute hairs, the form of the edges of the cut suctorial tubes, the extent of the surface taken into the ‘ field/ and the relation of all the parts to the whole. Now change the objective for the 16 mm. *3 N.A. (= but with the same aperture). Nothing more is to be seen ; the most dexterous manipulation cannot bring out a single fresh detail; the resolution is in no sense carried farther ; the cut suctorial tubes were in fact, in our judgment, better seen with a lower power, while with it all of course a smaller extent of the object occupies the ‘ field.’ It can in fact be scarcely doubted that the picture presented by the is a distinct retrogression in every sense compared with that presented by the 1 inch when both are equally well made and have equal apertures, viz. -3. But beyond all this, whatever may be done by the 16 mm. -3 N.A. can be accomplished in an equally satisfactory manner by removing the 12 eye-piece and re- placing it with practically no other alteration by an 18 eye-piece ; and still higher results can be obtained without the slightest detri- ment to the image by using an eye-piece of 27. Not less interesting and convincing will it be to examine the same object with a 12 mm. -65 N.A. (= 3-inch), and an A Zeiss achromatic of -20 N.A. (= -frds inch) using a 12 eye-piece. Those who may still retain some conviction as to the value of ‘ low-angled glasses to secure penetration ’ can want no further evidence than such a simple experiment affords of its entire fallacy. For those who prefer it a true histological object may be selected. We choose a portion of a frog’s bladder treated wTith nitrate of silver, in which are some convoluted vessels, enclosed in a muscular sheath which had contracted. This object is presented by photo-micrograpli in figs. 7 and 8 of the frontispiece. In fig. 7 the vessel in the frog’s bladder is seen by a Zeiss A -2 N.A., magnified 140 diameters. The object of the photograph is to expose the fallacy which underlies the generally HISTOLOGICAL ADVANTAGE OF LARGE APERTURE 319 accepted statement that low-angled glasses are the most suitable for histological purposes. The assumption is founded on the fact that the penetration of a lens varies inversely as its aperture, and it is taken for granted that ‘ depth of focus ’ will be obtained, not to be secured by large apertures, and therefore it is taken for granted that we are enabled to see into the structure of tissues. In examining the illustration (which will with advantage permit the use of a lens) it will be seen that scarcely an endothelium cell can be clearly seen. A sharp outline is nowhere manifest, because the image of one cell is confused with the outlines of others upon which it is superposed. We have seen that there is no perspective proper in a microscopic image ; therefore it is better to use high apertures in objectives, and obtain a clear view of one plane at one time and train the mind to appreciate perspective by means of focal adjustment. It will be admitted that no clear idea of what an endothelium cell is can be obtained from fig. 7. But fig. 8 (frontispiece) represents the same structure slightly less magnified (x 138) by means of an apochromatic N.A. ’65. Here only the upper surface of the tube is seen ; but the endothe- lium cells can be clearly traced, and a sharp definition is given to every cell. The circular elastic tissue is also displayed, while the whole image has an increased sharpness and perfection. Thus, with the objective (A ‘20 N.A. = frds inch) of lower aperture the endothelium cells can be seen ; but when the image is compared with that of the objective of wider aperture (-65 N.A.), the former image is found to be dim and ill-defined. The muscular sheath is so ill-defined that it would not be noticed at all if it had not been clearly revealed by the objective of wider aperture. But, on the other hand, the objective of greater aperture not only shows the muscular sheath, but it also shows the elongated nuclei of the muscle cells ; and at the same time brings out the convoluted vessels lying in the muscular sheath as plainly as if it were an object of sufficient dimensions to lie upon the table appealing to the unaided eye. We have pointed out in the proper place,1 that although ‘pene- trating power ’ varies inversely as the numerical aperture, it also varies inversely as the square of the power. Now, from what we know of histological teaching in this country, we do not hesitate to say that a histologist would not have attempted to examine the above object with even a Zeiss A objective. He would have advised the use of ‘ the of perhaps '65 aperture ; but by so doing he would have secured only one-tliird of the pene- trating power qud aperture and one-seventh of the penetrating power qud power. It is manifest, then, that pursuing this course in the histological laboratory defeats the end sought, and which it is so desirable to attain. It is absolutely unwise to use a higher power than is needful. A -inch where a would answer involves loss in many ways, 1 Chapter L OBJECTIVES, EYE-PIECES, THE APERTOMETER and would never jbe resorted to if the aperture of the lenses employed were as great as the power used legitimately permitted.1 A given structure to be seen at all must have a given aperture ; to obtain this, as objectives now made for laboratory purposes run, they are obliged to use too high a power. The result is that in seek- ing to avoid what is accounted the loss of ‘ penetrating power ’ at an inverse ratio to the aperture, it is forgotten that we are losing it inversely as the square of the power ! Moreover, the two apochromatic objectives we have already referred to as test lenses are equally able to show the value of apochromatism, not so much on account of the removal of the secondary spectrum as for the reduction of the aberrations depend- ent on the irrationality of the spectrum in ordinary achromatics. Use the 12 mm. '65 N.A. objective. Place a diatom in balsam in the focus of it on a dark ground ; the diatom will shine with a silvery whiteness, and the image will be wholly free from fog. Now take one of the best achromatics obtainable of 1-inch focus of 80° (almost certainly a in power) and examine the same diatom in the same circumstances; it will be bathed in fog. If, however, the achromatic objective is an exceptionally good one, and we reduce its aperture to 60°, we shall get a fair picture of the diatom—one indeed that was considered critical until that with the apochromatic was seen. But in comparison it is dull and yellowish. Prom which it follows that an exceptionally fine achromatic r4n- inch of 60° or -5 N.A. will not suffer comparison of the image it yields with that of an apochromatic 1-inch of '65 N.A. Speaking generally on the whole question, then, it would be the utmost folly for histologists or opticians to shut their eyes to the magnificent character of the series of dry apochromatics of Zeiss, ranging from 1 inch (24 mm.) to I-inch (4 mm. '95 N.A.), and more recently J-inch. They are the most perfect and efficient series of objectives ever placed in the hands of the worker ; and unless English lenses on a truly apochromatic principle and equal quality are produced, it must be to the detriment of either the opticians or the workers of this country. Nor need it be supposed that the production of objectives approximate to these must be costly ; great steps have been taken lately in the reduction of their cost. A remarkable instance of this is provided by the production of two objectives by E. Leitz, of Germany ; they have lately come into our hands ; they are but •semi-apochromatic. The one is low, having an initial power of 14, with an aperture of 30° ; the other is practically a of -88 N.A. The low power has surpassed every achromatic of its kind we have met with, and the higher power can, without hesitancy, be spoken of as an ■exceedingly good glass ; nevertheless the price of these two objectives is together less than the price charged for the lower power, if made in England on achromatic principles, would certainly be ! Yet Reichert has even surpassed this, and we feel that we shall be doing ■a great service to students of small means in calling their attention to the following remarkable and low-priced objectives : Leitz No. 2 1 Chapter II. THE EYE-PIECE 321 (1|) N.A. -2 ; Leitz No. 3 (less than frds) N.A. -26 ; Reichert No. 6 (fith) N.A. -81 ; Reichert 18b ( N.A. P24. The Reichert No. 6 is a lens whose low price is astonishing, when the perfection of the performance of all that we have seen is taken into account, for it is the rival of even true apochromatics. This fact is of im- portance to the medical student and to the opticians generally. By apochromatised objectives of the highest order the work of present and future microscopy will be done—that is inevitable. To thoroughly understand what its very best results, theoretically and practically, must be becomes the imperative aim of the optician who would be abreast of the direct wants of his time ; and to produce the nearest to these in objectives and eye-pieces at the lowest possible price is, apart from all other issues, to be a direct benefactor of true science. The Eye-piece.—The eye-piece, sometimes called the ocular, is an optical combination, the purpose of which is so to refract the diver- ging pencils of rays which form the real object-image that they may all arrive at the pupil of the observer’s eye. They have also to form a virtual image of the real image which is presented to them as the object. For this purpose a combination is indispensable, but this may be varied. There are ordinary and special eye-pieces. Those in ordinary use separate into two divisions : (1) positive eye-pieces and (2) negative eye-pieces. These are easily distinguished : with a positive eye-piece we can obtain a virtual image of an object by using it as a simple microscope, because its focus is exterior to itself. This is not easy with the negative eye-piece, because its focus is within itself. The eye-piece in common use is negative, and is generally known as Huyghens’, and sometimes as Campani’s. Hooke appears to have been the first (1665) to have applied the field-lens to the eye-lens of the microscope, although there is a high probability that such a lens was first used by Monconys ; but how far he was indebted for this to the compound eye-piece attributed to Huyghens cannot now be determined. This instrument as commonly used consists of an eye-lens and a field-lens, each being plano-convex, having their convex sides towards the object, their foci being in the ratio of 3 : 1, and the distance between them being equal to half the sum of their focal lengths, a diaphragm being placed in the focus of the eye-lens. The general form of a Huyghenian eye-piece is shown in longitudinal section in fig. 278. This makes a very convenient form of eye-piece of 5 and 10 magnifying power ; but when the power much exceeds this last amount, the eye-lens becomes of deep curvature and short focus, so that the eye must be placed uncomfortably near the eye-lens. This, however, is its chief defect, and it may fairly be considered the best ordinary eye-piece. Perhaps the best form of the Huyghenian eye-piece is that de- vised by Sir G. B. Airy : its field-glass is a meniscus with radii 11 : 4 the convexity towards the object-glass. The eye-lens is a crossed convex 6 : 1 the flatter side to the eye, the distance be- tween them being twice the focus of the eye-lens, focus of field- 322 OBJECTIVES, EYE-PIECES, THE APERTOMETER lens being three times that of the eye-lens, the diaphragm being in focus.1 Another negative eye-piece is that known as the Kellner, or orthoscopic. This consists of a bi-convex field-glass, and an achromatic doublet meniscus (bi-convex and bi-concave) eye-lens. A vertical section of one so constructed is seen in fig. 279. These eye-pieces usually magnify ten times, and the advantage they are supposed to give consists in a large field of view ; but they are not good in practice for this very reason, they take in a field of view greater than the Fig. 278.—Huyghenian eye-piece. objective can stand, and as a rule even the centre of the field will not bear comparison in sharpness with the Huyghenian form. It is a suggestion of Mr. Nelson’s that a crossed convex 6 : 1 field-lens and a meniscus and concave-convex doublet eye-lens might work well for this form of eye-piece. Fig. 279.—Kellner eye-piece. Positive Eye-pieces.—In the early compound microscopes the eye-pieces were all positive ; that is to say, they consisted of a single bi-convex eye-lens and no field-glass. The definition with this must have been most imperfect ; the addition of a field-lens, though it were a bi-convex, not in the correct ratio of focus, nor the theoretically best distance, must have been considered a- great advance. In this way matters rested, however, until the theoretically perfect Huyghenian. form was devised. Nothing has yet displaced this com- bination or successfully altered its formula. Object-glasses have been used as eye-pieces and all forms of loups oi simple microscopic lenses have been employed for the same purpose. Solid eye-pieces have also been used both in England and America, but with no results that surpassed a well- made Huyghenian combination; but the best form of all of the combinations which have been tried by us as positive single eye- pieces are the Steinheil triple loups; a section of one of these is- Fig. 280. 1 It is a curious fact that in practice the usual formula for the Huyghenian eye- piece is radius of field-lens twice that of eye-lens, and the distance between them, equal to half the sum oi their foci. COMPENSATING EYE-PIECES 323 seen in fig. 280. But a positive eye-piece was devised by Ramsden, consisting of two plano-convex lenses of equal foci ; the distance is to be equal to two-thirds the focal length of one. The diaphragm will of course be exterior. Abbe’s Compensating Eye-pieces.—We have already given a general description of the nature and action in connection with the apochromatic objectives of this form of eye-piece.1 In the section above on objectives we have referred to the fact that these eye-pieces are over-corrected; this may be easily seen by observing the colour at the edge of the diaphragm, which is an orange yellow. If we compare this with the colour in the same position with a Huyghenian eye-piece, this will be blue, being seen through the simple uncorrected eye-lens. There are three kinds of compensating eye-piece as designed by Abbe. These are 1. Searcher eye-pieces 2. Working ,, 3. Projection „ 1. The searcher forms are negatives of very low power, intended only for the purpose of finding an object; they consist of a single field-lens and a doublet eye-lens. The working forms are both positive and negative. The 4 power is a negative form resembling the searcher in that for the short tube ; the eye-piece for the long tube has a triplet eye-lens; but the remainder, viz. 8, 12, 18, and 27, when first introduced, were all positives ; they were subsequently, however, changed for negatives. Having used both, we are glad to learn that the positives are being again introduced. It may be convenient to have the 8 a negative like the 4, but with regard to the 12, 18, 27, it is important that they should be positives. These positive forms are on a totally new plan, being composed of a triple with a single plano-convex over it; the diaphragm is, of course, exterior to the lens. With these the definition is of the finest quality throughout the field ; they present the admirable condition that with the deeper powers the proper position of the eye is further from the eye-lens, which makes it as easy to use an eye- piece of as great a power as 18 or 27 as one of 4 or 8. The field of these eye-pieces has, as we believe, been very wisely limited to five or six inches. The attempt on the part of English opti- cians to give to our eye-pieces fields reaching eighteen inches is an error. A microscopic objective with the lowest aperture has the field greatly in excess of any other optical instrument; and to deal with such eccentric pencils as must be engaged by an eye-piece with a field of eighteen inches is a strain not justified by what is gained. The powers of the working eye-pieces are also arranged in a new way. The multiplying powers for the long tube are 4, 8,. 12, 18, 27 ; it will be seen at once, therefore, that they bear no definite ratio to one another, and if we seek to simplify the focal lengths, we are by the employment of the metrical system confronted with decimal 1 Chapter I., p. 33. OBJECTIVES, EYE-PIECES, THE APERTOMETER 324 fractions. But without further elaboration it may be well to say that 12 is the most generally useful eye-piece, and if only one com- pensating eye-piece is to be selected, there can be no question, from a practical point of view, but this is the best to employ. The 4 is too low, and the 27 is too high for general purposes, and the 8 and 18 are sufficiently near the 12 to give the latter the advantage in general work. We cannot, however, refrain from the expression of the opinion that a series of 5, 10, 20, or 6, 12, 24 powers would be in many senses more useful, and would offer facilities in application not secured by the series of Abbe now in use. It may be well to give further emphasis to the fact that this con- struction of eye piece is not only essential to the proper work of apochromatic objectives, but they greatly enhance the images given by ordinary achromatic lenses ; and it may be noted that the 8, 12, and 18 eye-pieces for the short tube are identical with 12, 18, 27 for the long tube. The projection eye-piece is mainly intended for photo-micro- graphy, but it is also useful for drawing and exhibition purposes. It is a negative, with a single field-lens and a triple projection-lens. The projection-lens is fitted with a spiral focussing arrangement in order that the diaphragm which limits the field may be focussed on to the screen or paper. The field of this eye-piece is small, but its definition is exquisitely sharp. It may not be generally known that good photo-micrographs can be obtained by projection with the ordinary compensating working oye-pieces, but this is a fact worthy of note. It will perhaps be of practical utility if we append a table indi- cating the focus of the compensating eye-pieces when used with the long and the short body. Focus of Eye-pieces for long Body. Power. 2 4 8 12 18 27 Focus in mm. 135 67’5 337 22-5 15 10 „ inches . 5-3 2-6 1-3 •88 •59 •39 Power. 1 2 4 6 8 12 18 Focus in mm. 180 90 45 30 22-5 15 10 „ inches . 7-08 3-54 1-77 1-18 •88 •59 •39 Focus of Eye-pieces for short Body. Projection Eye-pieces.—2 for short and 3 for long bodies = 90 mm. or 3-54 inches ; 4 for short and 6 for long bodies = 45 mm. or 1-77 in. Special Eye-pieces.—The most important of these, the micrometer eye-piece, we have already considered, so far as its application to micrometry is concerned.1 Its optical character may be properly considered here. If it is a negative eye-piece the micrometer is placed in the focus of the eye-lens, but if a positive combination it is placed 1 Chapter IV. HOW TO TEST OBJECTIVES 325 in the focus of the eye-piece itself. The Ramsden form described above is thoroughly suited for this purpose, but a negative form is generally employed, the micrometer being placed inside the eye-piece in the diaphragm, i.e. the focus of the eye-lens. In order that the micrometer may be susceptible of focus for various sights, it is necessary that the eye-lens in the case of a negative eye-piece, and the whole eye-piece in the case of a positive one, should be mounted in a sliding tube ; and one with a spiral slot will be preferable, since it makes the work of focussing both facile and accurate. If only one micrometer eye-piece is used it should be of medium power, such as l),-inch focus ; but it is an inexpensive and a useful plan to have an additional set of lenses to screw on to the same mount, so as to make the eye-piece, say, a §-inch focus. Spectroscopic, polarising, goniometer, and binocular eye-pieces are each treated under their respective subjects. The index eye-piece is one which has a pointer placed at the dia- phragm, so constructed that it can be turned in or out of the field, and is used to point to the position of an object. It is of limited utility, for with low powers it is scarcely needed ; we can readily indicate by description, to another, the position of an object in a field every object of which is in view. With a high power, on the other hand, it is very difficult to use such a £ finger.’ A better plan when the magnification is great is to have a series of diaphragms of different apertures to drop into the eye-piece and diminish the field of view. This not only makes the object to be pointed out more easily accessible to the eye, but—as we have by many years of observation proved—it aids in close observation upon minute objects by cutting off a large area of light without altering the intensity of what remains, and so makes close observation more easy. As it is directly associated with the eye-piece, we shall find no better place to note the curious and hitherto unexplained fact, that when resolving strife or lines with oblique light the effect is much strengthened by placing a NicoVs analysing prism over the eye- jnece. Testing Object-glasses.—It will have been noted by the attentive reader that many of the more important qualities of objectives are determined by the principles of their construction, and become in fact questions simply of the quality of the workmanship involved in producing the optical and mechanical parts of the object-glass. The quality of the workmanship may be tested by technical means described below, and by that subtil power which comes with experience. This can only be imparted through the paths of labour and experiment, by which in every case it is reached. But, granted that an object has been illuminated in an intelligent and satisfactory manner, the first complete view of the image (which must of course be a thoroughly familiar one) will enable the expert to come to a conclusion as to the quality of a given objective. The character of the image to the expert determines at once the character of the lens. This is the more absolute if a series of eye-pieces ('up to the 326 OBJECTIVES, EYE-PIECES, THE APERTOMETER most powerful that can be obtained) are at hand. Nothing tests the quality of an objective so uncompromisingly as a deep eye-piece. For brilliancy of image a moderate power of eye-piece is of course best ; but the capacity of the object-glass is clearly commensurate with its ability to endure high eye-pieces without loss of character, and even sharpness in the image. Unless the objective be of high quality, the sharpness of the image gradually disappears as the more powerful eye-pieces are used, until at last either all or part of the image breaks up into the ‘rotten’ details of a coarse lithograph. A lens finely corrected (with large aperture) will bear the deepest eye-piecing with no detriment. The 24 mm. and the 12 mm. of Zeiss will suffer any eye-piecing accessible to the microscopist without the smallest surrender of the sharpness of the image. We have in fact tried in vain to ‘ break the image5 yielded by these objectives ‘ down.’ This mode of testing is of course to a large extent subjective, oi* at least is controlled by incommunicable judgments. It is most important therefore to have a mode of judgment that shall be acces- sible to the beginner and the interested amateur. Dr. Abbe has proposed a method which is at least accessible to all. In ordinary practice microscope objectives, if tested at all by their possessors, are simply subjected to a comparison of perform- ance with other lenses tried upon the same ‘ test objects.’ The relative excellence of the image seen through each lens may, however, depend in a great part upon fortunate illumination, and not a little upon the experience and manipulative skill of the ob- server ; besides which any trustworthy estimate of the performance of the lens under examination involves the consideration of a suitable test-object, as well as the magnifying power and aperture of the objective. It is knowing what is meant by a ‘ critical image,’ and being able to discover whether or not a given objective will yield it. Clearly all tests of optical instruments, which are not capable of numerical expression, must be comparative. Magnifying power can be measured numerically ; it is not comparative. In the same way resolving power is mathematically measurable ; so is penetrating power. But definition and brilliancy of image, and evidence of centring, can have no numerical expression ; they are consequently comparative. The structure of the test-object should be well known, and the value of its ‘ markings ’—if intended to indicate microscopical dimen- sions—should be accurately ascertained, care being taken that the minuteness of dimensions and general delicacy and perfection of the test-object should be adapted to the power of the lens. A fairly correct estimate of the relative performance of lenses of modei’ate magnifying power may doubtless be thus made by a competent observer; but it is not possible from any comparisons of this kind to determine what may or ought to be the ultimate limit of optical performance, or whether any particular lens under examination has actually reached this limit. Assuming the manipulation of the instrument and the illumination of the object tobeasperfect as possible, and further that the test object has been selected with due appreciation of the requirements of perfect TESTING OBJECTIVES 327 optical delineation, a fair comparison can only be drawn between objectives of the same magnifying power and aperture. Which of two or more objectives gives the better image may be readily enough ascertained by such comparison, but the values thus ascer- tained hold good only for the particular class of objects examined. The best performance realised with a given magnifying power may possibly exceed expectation, yet still be below what might, and there- fore ought to be obtained. On the other hand, extravagant expectations may induce a belief in performances which cannot be realised. The employment of the test-objects most in use is moreover calculated to lead to an entirely ■one-sided estimation of the actual working power of an objective— as, for example, when ‘ resolving power ’ is estimated by its extreme limits rather than by its general efficiency, or ‘ defining power ’ by extent of amplification rather than by clearness of outline. So that an observer is tempted to affirm that he can discern through his pet lens what no eye can see or lens show. This happens chiefly with ■the inexperienced beginner, but not unfrequently also with the more experienced worker who advocates the use of great amplification, in whose mind separation of detail means analysis of structure, and ■optically void interspaces prove the non-existence of anything which lie does not see. As much time is often lost by frequent repetition of these com- petitive examinations (which, after all, lead to no better result than that the observer finds or fancies that one lens performs in his hands ■more or less satisfactorily than some other lens) it seems worth while to consider the value of a mode of testing which can be readily applied whatever its value may be. A short and easy method of testing an objective—not by comparison with others only, but by itself and on its own merits—affords not only the most direct and positive evidence of its qualities to those who are more concerned in proving these instruments than using them, but also yields to the genuine worker the satisfying conviction that his labour is not frustrated by faulty construction and performance of his instru- ment. It is, however, to be borne in mind that the microscopist, in any scrutiny of the quality of his lenses which he may attempt, has no other object in view than to acquire such insight into the optical conditions of good performance as will enable him to make the best use of his intrument, and acquire confidence in his interpretation of what he sees as well as manipulative skill in examining micro- scopical objects. To the constructor and expert of optical science are left the severer investigations of optical effects and causes, the difficulties of technical construction, the invention of new lens- -combinations, and the numerous methods of testing their labours by delicate and exhaustive processes which require special aptitude, and lie entirely outside the sphere of the microscopist’s usual work. Professor Abbe’s mode of testing objectives is explained in his 4 Beitrage zur Theorie des Mikroskops.’ The process, in our judgment, requires large experience and much skill to be of practical service ; but it is based on the following principle :— 328 OBJECTIVES, EYE-PIECES, THE APRRTOMETER In any combination of lenses of which an objective is composed the geometrical delineations of the image of any object will be more or less complete and accurate according as the pencils of light coming from the object are more or less perfectly focussed on the conjugate focal plane of the objective. On this depend fine definition and exact distribution of light and shade. The accuracy of this focussing function will be best ascertained by analysing the course of isolated pencils directed upon different parts or zones of the aperture, and observing the union of the several images in the focal plane. For this purpose it is necessary to bring under view the collective action of each part of the aperture, central or peripheral, while at the same time the image which each part singly and separately forms must be distinguishable and capable of comparison with the other images. 1. The illumination must therefore be so regulated that each zone of the aperture shall be represented by an image formed in the upper focal plane of the objective (i.e. close behind or above its back lens), so that only one narrow track of light be allowed to pass for each zone, the tracts representing the several zones being kept as far as possible apart from each other. Thus, supposing the working surface of the front lens of an objective to be -j- inch in diameter, the image of the pencil of light let in should not occupy a larger space than inch. When two pencils are employed, one of these should fall so as to extend from the centre of the field to T](T inch outside of it, and the other should fall on the opposite side of the axis in the outer periphery of the field, leaving thus a space of inch clear between its own inner margin and the centre of the field, as in fig. 281, where the objective images of the pencils occupy each a quarter of the diameter of the whole field. If three pencils of light be employed, the first should fall so as to extend from the centre of the field to inch outside of it; the second should occupy a zone on the opposite side of it, between the and inch (measured from the centre); and the third the peripheral zone on the same side as the first in fig. 282. This arrangement places the pencils of light in their most sensi- tive position and exposes most vividly any existing defect in correc- tion, since the course of the rays is such that the pencils meet in the focal plane of the image at the widest possible angle. As many distinct images will be perceived as there may be zones or portions of the front face of the objective put in operation by separate pencils of light. If the objective be perfect all these images should blend with one setting of focus into a single clear, colourless picture. Such a fusion of images into one is, however, prevented by faults of the- image-forming process, which (so far as they arise from spherical aberration) do not allow this coincidence of several images from different parts of the field to take place at the same time, and (so far as they arise from dispersion of colour) produce coloured fringes on the edges bordering the dark and light lines of the test-object and the edges of each separate image, as also of the corresponding co- Fig. 281 Fig. 282. AEEE’S 3I0DE OF TESTING incident images in other parts of the field. It is to be borne in mind that the errors which are apparent with two or three such pencils of light must necessarily be multiplied when the whole area of an objective of faulty construction is in action. This would appear to us to be the strongest reason for utilising the whole area, because what we are seeking is the defects—the errors of the objec- tive-—and to make these as plain as possible is a sine qud non. Dr. Abbe proceeds, however, to consider— 2. The means by which such isolated pencils can be obtained. As a special illuminating apparatus, the condenser of Professor Abbe is recommended, or even a hemispherical lens. But we are convinced that the illuminating apparatus should be as nearly apla- natic as it can be. This is certainly not true of Abbe’s chromatic condenser or a hemispherical lens. The reason is obvious : the spherical aberration wholly prevents the rays passing through the holes in the diaphragm from being focussed on the object—the silvered plate of lines—at the same time. In the lower focal plane of the illuminating lens must be fitted diaphragms (easily made of blackened cardboard) pierced with two or three openings of such a size that the images, as formed by the objective, may occupy a fourth or sixth part of the diameter of the whole aperture (i.e. of the field seen when looking down the tube of the instrument, after re- moving the ocular, upon the objective image). The required size of these holes, which depends, first, on the focal length of the illumi- nating lens, and, secondly, on the aperture of the objective, may be thus found. A test object being first sharply focussed, card dia- phragms having holes of various sizes (two or three of the same size in each card) must be tried until one size is found, the image of which in the posterior focal plane of the objective shall be about a fourth to a sixth part of the diameter of the field of the objective. Holes having the dimensions thus experimentally found to give the required size of image must then be pierced in a card, in such a position as will produce images situate in the field, as shown by figs. 281, 282 ; the card is then fixed in its place below the condenser. We are strongly, however, inclined to believe, partly from experiment, that better results would be obtained by putting sections of annular slits at the back of the objective. If the condenser be fitted so as to revolve round the axis of the instrument, and also carry with it the ring or tube to which the card diaphragm is fixed, the pencils of light admitted through the holes will, by simply turning the con- denser round, sweep the face of the lens in as many zones as there are holes. Supposing the condenser to be carried on a rotating sub-stage, no additional arrangement is required besides the diaphragm-carrier. Thus, for example, if a Collins condenser fitting- in a rotating sub-stage be used, all that is required is to substitute for the diaphragm which carries the stops and apertures as arranged by the maker, a diaphragm pierced with, say, three openings of f-inch diameter, in which circles of card may be dropped, the card being- pierced with holes of different sizes according to the directions given above. We doubt, however, if any sub-stage will revolve with sufficient accuracy for so delicate a test. 330 OBJECTIVES, EYE-PIECES, THE APERTOMETER Another plan adopted by Dr. Fripp, and found very convenient in practice, is to mount a condensing lens (Professor Abbe’s in this case) upon a short piece of tube, which fits in the rotating sub-stage. On opposite sides of this tube, and at a distance from the lower lens equal to the focal distance of the combinations, slits are cut out through which a slip of stout cardboard can be passed across and below the lens. In the cardboard, holes of various sizes, and at various distances from each other, may be pierced according to pleasure. By simply passing the slip through the tube, the pencils of light admitted through the holes (which form images of these holes in the upper focal plane of the objective) are made to traverse the field of view, and by rotating the sub-stage the whole face of the lens is swept, and thus searched in any direction required. But here, again, the spherical aberration of an uncorrected condenser would, with an objective of large aperture, cause the oblique pencils under some conditions to pass under the object; and alteration of focus will not properly alter this—at least without a disturbance of the focus of the objective. When an instrument is not provided with a rotating sub-stage, it is sufficient to mount the condenser on a piece of tubing, which may slide in the setting always provided for the diaphragm on the under side of the stage. Card diaphragms for experiment may be placed upon the top of & thin piece of tube (open at both ends) made to slide inside that which carries the condenser, and removable at will. By rotating this inner tube the pencils of light will be made to sweep round in the field, and thus permit each part of the central or peripheral zones to be bi’ouglit into play. Against the accurate value of this, again, the spherical aberration of an uncorrected condenser would strongly operate. Abbe’s Test-plate.—This test-plate is intended for the examina- tion of objectives with reference to their corrections for spherical and chromatic aberration, and for estimating the thickness of the cover-glass for which the spherical aberration is best corrected. Fig. 283. The test-plate consists of a series of cover-glasses, ranging in thickness from 0'09 mm. to 024 mm., silvered on the under surface, and cemented side by side on a slide, the thickness of each being marked on the silver film. Groups of parallel lines are cut through the films, and these are so coarsely ruled that they are easily resolved by the lowest powers; yet from the extreme thinness of the silver they also form a very delicate test for objectives of even the highest power and widest aperture. The test-plate in its natural size is seen in fig. 283, and one of the circles enlarged is seen in fig. 284. To examine an objective of large aperture, the discs must be focussed in succession, observing in each case the quality of the image in the centre of the field, and the variation produced by using alter- nately central and very oblique illumination. When the objective is perfectly corrected for spherical aberration for the particular thick- ness of cover-glass under examination, the out- lines of the lines in the centre of the field will be perfectly sharp by oblique illumination, and without any nebulous doubling or indistinctness of the minute irregularities of the edges. If, after exactly adjusting the objective for oblique light, central illumination is used, no altera- tion of the focus should be necessary to show the outlines with equal sharpness. If an objective fulfils these conditions with any one of the discs it is free from spherical aberration when used with cover-glasses of that thickness. On the other hand, if every disc shows nebulous doubling, or an indistinct appearance of the edges of the lines with oblique illumination, or if the objective requires a different focal ad- justment to get equal sharpness with central as with oblique light, then the spherical correction of the objective is more or less im- perfect. Nebulous doubling with oblique illumination indicates over-cor- rection of the marginal zone ; indistinctness of the edges without marked nebulosity indicates under-correction of this zone; an alteration of the focus for oblique and central illumination (that is, a difference of plane between the image in the peripheral and central portions of the objective) points to an absence of concurrent action of the separate zones, which may be due to either an average under- or over-correction, or to irregularity in the convergence of the rays. The test of chromatic correction is based on the character of the colour-bands which are visible by oblique illumination. With good correction the edges of the lines in the centre of the field should show only narrow colour-bands in the complementary colours of the secondary spectrum, namely, on one side yellow-green to apple-green, and on the other, violet to rose. The more perfect the correction of the spherical aberration, the clearer this colour-band appears. To obtain obliquity of illumination extending to the marginal zone of the objective, and a rapid interchange from oblique to central light, Abbe’s illuminating apparatus is manifestly defective -on account of its spherical aberration. We want at least his achromatic condenser. For the examination of ordinary immersion objectives, the apertures of which are, as a rule, greater than 180° in arc (l'OO A.N.), and those homogeneous immersion objectives which considerably exceed this, it will be necessary to bring the under surface of the test-plate into contact with the upper lens of •the illuminator by means of cedar oil, even if water-immersion ob- ieetives arc used. Wa mav add as a matter of exnerience. that THE TEST-PLATE Fig. 284. 332 OBJECTIVES, EYE-PIECES, THE APERTOMETER having once centred the light and the condenser, we hold, with deference to Dr. Abbe, that the light should on no account be touched, which to obtain obliquity he advises by mirror changes. We believe that this should be secured solely by the movement of the diaphragm. For the examination of objectives of smaller aperture (less than 40° to 50°) we may obtain all the necessary data for the estimation of the spherical and chromatic corrections by placing the concave mirror so far laterally that its edge is nearly in the line of the optic axis, the incident cone of rays then only filling one-half of the aper- ture of the objective, by which means the sharpness of the outlines and the character of the colour-bands can be easily estimated. It is of fundamental importance in employing the test-plate to have brilliant illumination and to use an eye-piece of high power. With oblique illumination the light must always be thrown perpen- dicularly to the direction of the lines. When from practice the eye has learnt to recognise the finer differences in the quality of the outlines of the image, this method of investigation gives very trustworthy results. Differences in the thickness of cover-glasses of OOl or 002 mm. can be recognised with objectives of 2 or 3 mm. focus. The quality of the image outside the axis is not dependent on spherical and chromatic correction in the strict sense of the term. Indistinctness of the outlines towards the borders of the field of view arises, as a rule, from unequal magnification of the different zones of the objective ; colour-bands in the peripheral portion (with good colour-correction in the middle) are always caused by unequal magnification of the different coloured images. Imperfections of this kind, improperly called ‘ curvature of the field,’ are shown to a greater or less extent in the best objectives, when their aperture is considerable. Testing an objective does not mean seeing the most delicate points in an object ; it rather means the manner in which an object of some size is defined. A test for low powers up to } of 80° or N.A. ’65 is an object on a dark ground. Nothing is so sensitive. One of the Polycistince because it takes light well, is good. For higher powers a coarse diatom, a Triceratium fmbriatum, is excellent; for unless an objective is well corrected the image will be fringed and surrounded with scattered light, and the aberration produced by the cover-glass is plainly manifest, and by accurate correction can be done away. Error of centring is one of the special defects of objectives which the Abbe method of testing does not cover. But if we place a sensitive object in a certain direction, and when the best adjust- ments have given the best image rotate that object through an angle of 90°, only a well-centred objective will give an unaltered image throughout. If not well-centred it will at certain parts grow fainter or sharper. The most useful image for this purpose with medium powers is a hair of PoJyxenus lagurus mounted in balsam (frontispiece, fig. 6). For higher powers nothing surpasses a podura scale. In this particular it has always been of great value to opticians. It should OBJECTS FOR LENS-TESTINO. APERTOMETER 333 be strongly marked, and must be in optical contact with the cover- glass ; this may be tested by means of an oil-immersion and the ‘ vertical illuminator ’ (p. 284)., The objectives of widest aperture are not readily tested because there is no condenser sufficiently aplanatic to do it exhaustively. The best that can be done is to take a diatom, such as a Coscinodis- cus, in balsam with strong ‘ secondaries ’ (Plate I. figs. 3 and 4), with the largest aplanatic cone that can be obtained, which at present can be best accomplished with Powell and Lealand’s achromatic condenser of P4 N.A. It must be a good objective indeed that does not show signs of breaking down under this strain ; and there is extreme susceptibility to cover correction to which close attention must be paid. An illuminating cone of N.A. 1 -0 is probably just below the point of over-strain with the best lenses at present at our disposal. Testing lenses, therefore, resolves itself into two methods, viz. 1. For low and medium powers, dark ground with a Polycistina, or a diatom according to the power. 2. Centring for medium powers (an ordeal not needful for very low powers) should be by means of a hair of Polyxenus lagurus. 3. Centring for high powers by means of podura scale. 4. Definition Coscinodiscus asteromphalus with wide-angled cone obtaining sharp, brilliant, and clear view of ‘ secondaries.’ The apertometer, as its name implies, is an instrument for mea- suring the aperture of a microscopic objective. As correct ideas of aperture have only obtained during the past few years, it may be inferred that apertometers constructed before the definition of aper- ture was given and accepted were crude and practically useless. The controversy on the ‘ aperture question,’ which was in full operation some eighteen years since, is not an altogether satisfactory page in the history of the modern microscope, and for many reasons it is well to pass it unobservantly by. It will suffice to state that during its progress an apertometer was devised by R. B. Tolies, of America, which accurately measured the true aperture of an objec- tive. About the same time Professor Abbe gave his attention to the subject, and with the result, as we have seen, that he has given a definite and permanent meaning to numerical aperture, making it, as we have seen, the equivalent of the mathematical expression n sine u, n being the refractive index of the medium and u half the angle of aperture.1 The application of this formula to, and its general bearing upon, the diffraction theory of microscopic vision has been given in its proper place ; but as the aim of this manual is thoroughly practical we shall be pardoned for even a small measure of repetition in endeavouring to explain the use of this formula in such a manner that only a knowledge of simple arithmetic will be required to enable the student to work out any of the problems which are likely to arise in his practical work. 1 A knowledge of the meaning of the trigonometrical expression ‘ sine ’ is not necessary in solving any of the following questions. As the values are all found in tables it is only necessary to caution those who are unacquainted with the use of mathematical tables to see that they have the ‘ natural sine ’ and not the ‘ log sine.’ 334 OBJECTIVES, EVE-PIECES, THE APERTOMETER We can best accomplish this by illustration. i. If a certain dry objective has an angular aperture of 60°, what is its N.A. 1 (i.e. numerical aperture). All that is needful is to find the value of n sine u ; in this case n = the refractive index of the medium, which is air, is 1 ; and uy which is half of 60°, = 30° opposite 30° in a table of natural sines,1 is •5 ; sine u, therefore, = -5, which multiplied by 1 gives -5 as the N.A. of a dry objective having 60° of angular aperture. ii. What is the N.A. of a water-immersion whose angular aperture = 44° ? n here = 1 '33, the refractive index of water ; and u, or half 44°, is 22°. Sine 22° from tables = ‘375, which multiplied by 1-33 = *5 (nearly), which is the N.A. required. iii. What is the N.A. of an oil-immersion objective having of angular aperture ? n the refractive index of oil, which is equal to that of crown glass, is 1'52 ; u — and sine it from tables = -329, which multi- plied by T52 = ’5. Thus it is seen that a dry objective of 60°, a water-innnersion of 44°, and an oil-immersion of all have the same N.A. of ‘5. It will be well, perhaps, to give the converse of this method. iv. If a dry objective is ’5 N.A., what is its angular aperture ? Here because n sine it = ‘5, sine u — — ; the objective being dry n — 1, therefore sine u = 5. Opposite -5 in the table of natural sines is 30° ; hence u = 30°. But as u is half the angular aperture of the objective, 2it or 60° = the angular aperture required. v. What is the angular aperture of a water-immersion objective whose N.A. = *5 ? •5 -5 Here n = 1'33, n sine it — • 5 ; sine u = — = = ’376 ; n 1-33 it = 22° (nearly) from tables of sines ; .*. 2u = 44°, the angle re- quired. vi. What is the angular aperture of an oil-immersion objective of -5 N.A. ? - • .5 .K Here n = l-52, n sine u — -5, sine u = 1- = ' = -329 * n 1 -52 n = 191° (by tables of sines) ; and 2u = 38?7, the angle required. We may yet further by a simple illustration explain the use of n sine u. In the accompanying diagram, fig. 285, let n' represent a vessel of glass ; let the line A be perpendicular to the surface of the water C I) ; suppose now that a pencil of light impinges on the surface of the water at the point where the perpendicular meets it, making an angle of 30° with the perpendicular. This pencil in penetrating the water will be refracted or bent towards the perpendicular. The problem is to find the angle this pencil of light will make with the perpendicular in the water. To do this we must remember that n sine u on the air side is 1 Vide Appendix A to this volume, SIMPLE ILLUSTRATION OF THE USE OF N SINE U squal to n sme w on the water side. Thus on the air side n = 1, a = 30°, and by the tables of sines sine 30° = "5 ; consequently on the air side we have n sine u = • 5. On the water side n' = T33, and u' is to be found. But as , - / • , n sine u ‘5 or.~ ,■ , / n sine u = n sine u, sine u ■= = = -3<6 ; which (as n 1*33 the tables show) is the natural sine of an angle of 22° ( nearly) ; con- sequently u' — 22° ; so the pencil of light in passing out of air into water has been bent 8° from its original direction. Conversely a pencil in water making an angle of 22° with the perpendicular would on emerging from the water be bent in air 8° further away from the perpendicular, and so make an angle of 30° with it. Now if we suppose that these pencils of light revolve roxtnd the ;perpendicular, cones would he de- scribed, and we can readily see that a solid cone of 60° in air is the exact equivalent of a solid cone of 44° in water. If we further suppose that the water in the vessel is replaced by cedar oil, the pencil in air remain- ing the same as before, will, when it enters the oil, be bent more than it was in the water, because the oil has a higher refractive index than water ; n in this case is equal to T52. The exact position of the pencil can be determined in the same manner as in the previous case. On the air side, as before, n sine u = • 5 ; on the oil side n' sine u' = n sine u; sine u' = 'Yt S1I16 tt *) non i • r /i = 1-52 = 329, which (by the tables) is the natural sine of It follows that the pencil has been bent in the cedar oil out of its original course, and a cone of 60° in air becomes a cone of in cedar oil or crown glass. Finally, it is instructive to note the result when an incident pencil in air makes an angle of 90° with the perpendicular; wsine u becomes unity, and u in water 48|°, in oil 41° (nearly) ; conse- quently a cone of either in water, or in oil or crown glass,, is the exact equivalent of the whole hemispherical radiant in air. In Fig. 285. 336 OBJECTIVES, EYE-PIECES, TIIE APERTOMETEK •other words, and to vary the mode in which this great truth has been before stated, the theoretical maximum aperture for a dry lens is equivalent to a water-immersion of 97-1°, and an oil-immersion of angular aperture. The last problem that need occupy us is to find the angular aperture of an oil-immersion which shall be equivalent to a water- immersion of 180° angular aperture. vii. On the water side n = 1’33, u = 90°, sine 90° = 1, n sine u = 1‘33. On the oil side n' = 1‘52 and u' has to be found. ■ , . , . . , n sinew l-33 , ~,0 As n sine u — »sme u, sine u = —— = —— ='8/o; u =ol ’ n' 1-52 ’ (nearly) by the tables; 2u' = 122° (nearly), the angle required. It thus appears (1) that dry and immersion objectives having different angular apertures, if of the same equivalent aperture, are designated by the same term. Thus objectives of 60° in air, or 44° in water, or 381° in oil, have identically the same aperture, and are known by the same designation of -5 N.A. (2) The penetrating power of any objective is proportional to ——, and its illuminating power to (N.A.)2 Therefore, if we double JN .-A,. the N.A. we halve the penetrating power, and increase the illumi- nating power four times. In comparing the penetrating and illuminating powers of objec- tives, however, care must be taken to avoid a popular error, by making them between objectives of different foci. It cannot, for example, be said that a 1-inch objective of -8 N.A. has half the penetrating power of a i-inch of -4 N.A. Neither can it be said that it has four times the illuminating power. What is meant is that a -j-inch of '8 N.A. has half the penetrating and four times the illuminating power of a j-inch objective of -4 N.A. But because penetrating and illuminating powers diminish as the square of the foci, a objective of -6 N.A. has four times the illuminating and nearly four times the penetrating power of a jjf-inch of ‘6 N.A. The old nomenclature, in use before numerical aperture was so happily introduced, did not of course admit of comparisons of pene- trating and illuminating powers by inspection ; which, however, is a manifest advantage, contributing to accuracy and precision in important directions. (3) It may be well, for tlie sake of completeness, to repeat1 here that the resolving power of an objective is directly proportional to its numerical aperture. If we double the N. A. we also double the resolving power ; and this not simply with objectives of the same foci, as in the case of penetrating and illuminating powers. Thus it is not only true that a rrinch objective of -6 N.A. resolves twice as many lines to the inch as a 1,-incli of -3 N.A., but so also does a J-inch of 1*4 N.A. resolve twice, and only twice, as many as a of • 7 N.A. Within certain limits, then, the advantage lies with long foci of 1 Chapter I. THE USE OF THE APERTOMETER 337 wide angle, because we thus secure the greatest resol ving power with the greatest penetrating and illuminating powers. I rom what has here been shown, then, it becomes evident that dhe employment of the microscope as an instrument of precision is largely due to Abbe’s work, and that the introduction of numerical aperture, with its strictly accurate meaning, has been a practical gain of untold value. But this has been greatly enriched by his having introduced a thoroughly simple and useful apertometer. This involves the same principle as that of Tolies, but it is carried out in a simpler manner. Abbe’s instrument is presented in fig. 286. It will be seen that it consists of a flat cylinder of glass, about three inches in diameter and half an inch thick, with a large chord cut off so that the portion left is somewhat more than a semicircle ; the part where the segment is cut is bevelled from above downwards to an angle of 45°, and it will be seen that there is a small disc with an aperture in it denoting the centre of the semicircle. This instrument is used as follows. The microscope is placed in a vertical position, and the aperto- meter is placed upon the stage with its circular part to the front and the chord to the back. Diffused light, either from sun or lamp, is assumed to be in front and on both sides. Suppose the lens to be measured is a dry ; then with a 1-inch eye-piece having a large field, the centre disc with its aperture on the apertometer is into focus. The eye-piece and the draw-tube are now removed, leaving the focal arrangement undisturbed, and a lens supplied with the apertometer is screwed into the end of the draw- tube. This lens with the eye-piece in the draw-tube forms a low- power compound microscope. This is now inserted into the body- tube, and the back lens of the objective whose aperture we desire to measure is brought into focus. In the image of the back lens will be seen stretched across, as it were, the image of the circular part of the apertometer. It will appear as a bright band, because the light which enters normally at the surface is reflected by the bevelled part of the chord in a vertical direction, so that in reality • a fan of 180° in air is formed. There are two sliding screens seen Pig. 286.—Abbe’s apertometer. 338 OBJECTIVES, EYE-PIECES, THE APERTOMETER on either side of the figure of the apertometer ; they slide on the vertical circular portion of the instrument. The images of these screens can be seen in the image of the bright band. These screens should now he moved so that their edges just touch the -periphery of the hack lens. They act, as it were, as a diaphragm to cut the fan and reduce it, so that its angle just equals the aperture of the objec- tive and no more. This angle is now determined by the arc of glass between the screens ; thus we get an angle in glass the exact equivalent of the aperture of the objective. As the numerical apertures of these arcs are engra ved on the apertometer they can be read off by inspection. Nevertheless a difficulty is experienced, from the fact that it is not easy to determine the exact point at which the edge of the screen touches the periphery of the back lens, or, as we prefer to designate it, the limit of aperture, for curious as this expression may appear we have found at times that the back lens of an objective is larger than the aperture of the objective requires. In that case the edges of the screen refuse to touch the periphery. On the whole we have found that a far better way of employing this instrument is to use it in connection with a graduated rotary stage, the edge of the flame of a paraffin lamp being the illumi- nator. Thus : Set the lamp in a direction at right angles to the chord of the apertometer, and suppose that the index of the stage is at 0°. The edge of the flame will be seen in the centre of the bright band. The sliding screens being dispensed with, rotation of the stage will cause the image of the flame to travel towards the edge of the aperture ; rotation is continued until the image of the flame is half extinguished by the edge of the aperture, the arc is then read, and the same thing is repeated on the other side, and the mean of the readings is taken. If the stage rotates truly, and if the instrument is properly set up, the reading on the one side ought to be identical with that on the other. Suppose that the sum of the readings on both sides = 60°, the mean reading is consequently 30°, which is the semi-angle of aperture of the lens in glass. From this datum we have to determine the N.A. of the dry j-inch as well as its angular aperture in air.1 (i) As before, N.A. = n sine u, and n sine u = n' sine u' ; which means that the aperture on the air side is equal to the aperture on the glass side ; n = 1 for air ; n = 1-615, the refractive index of the apertometer ; u' is the mean angle measured, which in this case is 30° ; and n sine u has to be found. Now sine 30° = -5 (by the tables); n' sine u' — 1-615 x sine 30° = 1-615 x -5 = -8 = n sine w=the N.A. required. (ii) Again, to find the angular aperture or 2 u. As before, n sinew / • i j • n' sine u' 1-615 x *5 ~ =n sine u and sine u — = =S ■ u = 53 n 1 nearly (by the tables) ; 2u = 106°, which is the angle required. 1 Vide p. ‘2 et seq. THE USE OF THE APERTOMETER 339 (iii) If it be a water-immersion we have to deal with, suppose the mean angle = 45° = u' ; sine 45° = *707 (by the tables) ; n = 1*33 ; and n' = 1*615. n sine u = n' sine u' = 1*615 x *707 = 1*14, the N.A. required. /• \ a • • w' sine m' 1*615 x *707 (iv) Again, sineu =.— = —— = -86 ; u — 594° n 1*33 • 4 (by the tables); and 2u — 118|°, the angle required. (v) In the case of an oil-immersion, suppose the mean angle = 60° = u; sine 60° — *866 (by the tables) ; n — 1*52 ; n' = 1*615 ; n sine u = n' sine u' = 1*615 x *866 = 1*4, which is the N.A. required. / -x » . n' sine u' 1*615 x‘866 (vi) Again, sine u = = = *92. n 1*52 u = 67° (by the tables), 2u — 134°, the angle required. It is manifest that if the refractive index of the apertometer equals that of the oil of cedar, the mean angle measured is the semi- angle of aperture of the objective, and its sine multiplied by that refractive index is the numerical aperture. This will be found the more accurate and universally applicable method of measuring the apertures of objectives, as the extinction of the light shows precisely when the limit of aperture is reached. Powell and Lealand’s stands lend themselves admirably for use with the apertometer. The body being removable the lens can be placed in the upper part of the nose-piece, and any measurement can be accurately made. We would advise every microscopist to master the use of this admirable instrument, and to demonstrate for himself the aperture capacity of his lenses that he may know with precision their true resolving powers. 340 CHAPTER VI PRACTICAL MICROSCOPY: MANIPULATION AND PRESERVATION OF THE MICROSCOPE Without attempting to occupy space with a discussion of the ques- tion of the right of ‘ microscopy ’ to be considered a science, we may venture to affirm that it will be but a recognition of practical facts if we claim as a definition of microscopy that it expresses and is in- tended to carry with it all that belongs to the science and art of the microscope as a scientific instrument, having regard equally to its theoretical principles and its practical working. Hence ‘ practical microscopy ’ will mean a discourse on, or discussion of, the methods of employing the microscope and all its simplest and more complex appliances in the most perfect manner, based alike and equally upon theoretical knowledge and practical experience. On this condition a ‘ microscopist ’ means (or at least implies) one who, understanding ‘ microscopy,’ applies his theoretical and practical knowledge, either to the further improvement and perfec- tion of the instrument, or to such branches of scientific research as he may profitably employ his ‘ microscopy ’ in prosecuting. He is, in fact, a man employing specialised theoretical knowledge and prac- tical skill to a particular scientific end. But a ‘ microscopical society ’ has a noble raison d'etre, because it is established, on the one hand, to promote—without consideration of nationality or origin—improvements in the theory and practical construction of both the optical and mechanical parts of the micro- scope, and to endeavour to widen its application as a scientific in- strument to every department of human knowledge, recording, in- vestigating, and discussing every refinement and extension of its application to every department of science, whether old or new. In this sense no more practical definition of a ‘ microscopical society ’ can be given than is contained in the invaluable pages of the ‘ Journal of the Royal Microscopical Society ’ from the end of 1880 to the present day; and no better justification for the existence of such a society can be needed than is afforded by the work done directly or indirectly by it, in inciting to and promoting the theo- retical and practical progression of the instrument and its ever- widening applications to the expanding areas of natural knowledge. In this chapter we propose to discuss the best practical methods cf using the instrument and its appliances, the theory concerning which has already been discussed, while the mode of applying this MICEOSCOPISTS’ WOEIv-TABLES 341 knowledge to biological and other investigations is entered upon in the subsequent chapters of the book. To begin his work with success—if his object be genuine work—• the student must be provided with some room, or portion of a room, which he can hold sacred to his purpose. Unless special investiga- tions are undertaken, it is not a large area that is required, but a space commanding, if possible, a north aspect, and which can be arranged to readily exclude the daylight and command complete darkness. The first requirement will be a suitable table. This should be thoroughly firm, and it should be rectangular in shape. A round table, if small especially, is most undesirable, as it offers no support for the arms on either side of the instrument; and with prolonged work this is not only a serious, but an absolutely fatal defect. In a rectangular table the centre may be kept clear for micro- scopical work, while there are two corners at the back, one on the left and the other on the right hand. The former may be used for the locked case or glass shade for protecting the instrument when not in use ; and when it is in use it in no way interferes with the usefulness of the table. In the same way the right-hand corner may be used for the cabinet of objects which is being worked, or the apparatus needful for use. The most important part of the table—that is, the middle, from front to back—should be kept quite clear for the purposes of mani- pulation, and a sufficient space should be kept clear on either side of the instrument for resting the arms, and no loose pieces of apparatus should ever be deposited within those spaces. This soon becomes a habit in practice, for experience teaches—sometimes painfully by the unwitting destruction of a more or less valuable appliance. The spaces to the right beyond that left for the arm of the operator may be used for the work immediately in hand—especially for a second and simpler microscope. An instrument with only a coarse adjustment and a 1-inch or a -|-inch objective will suffice, or a good dissecting-stand will answer every purpose. Those who do much practical work will find such a plan more rapid and more efficient than the cumbrous method of a rotary nose-piece, especially where critical work has to be done. When work is being done in a darkened room there should be on the extreme right a small lamp with a paper shade. (Special shades for this purpose can be obtained from Baker, of Holborn.) This light may be kept low or used for general illumination when required—it is never obtrusive, and always at hand. A similar space on the left hand should be reserved for a small round stand fitted with a fiat cylindrical glass shade with a knob on the top. The stand should be suitably arranged to hold two eye- pieces, three objectives, one condenser, a bottle of cedar-oil (fitted with a suitable pointed dipper), and a box containing the condenser- stops. This is a most useful arrangement for such a table ; and it need not have a diameter greater than nine inches. The size for the top of such a table should be 4i x 3 feet, and as 342 PRACTICAL MICROSCOPY no work, such as mounting or dissecting, maybe supposed to be done at this table, it is well to cover the surface with morocco, that being very pleasant and suitable to work upon. It should be remembered that for a full-sized microscope a depth of three feet is required for comfortable work. When the micro- scope is set up for drawing,1 the lamp being used direct, 2 ft. 5 in. is the narrowest limit in which this can be accomplished. Another point of much importance is the height of the table. Ordinary tables, being about 2 ft. 4 in. high, are too low even for large microscopes. Two or three inches higher than this will be found to greatly facilitate all the work to be done. It is best to have the table made completely, on thoroughly solid square legs, to the height of 2 ft. 7 in. ; but we may employ the glass blocks employed underneath piano feet as an expedient. It is further im- portant to have the table quite open underneath, and not with nests of drawers on either side, because with this par- ticular table it will be frequently required that two persons may sit side by side, which is only possible with a clear space beneath. The accompanying il- lustration (tig. 287), with the appended references, will make quite clear the character of the table which we recommend, as well as the mode of using it. The table above de- scribed is supposed to be employed wholly for general purposes of observation or research on wholly or partially mounted objects. But the microscopist who aims at more than this will require an arrangement for dissecting, mounting, and arranging histological and other preparations, and in some cases a special table for general purposes of microscopical biology. These are certainly not essen- tials, especially if the work done is a mere occasional occupation; but where anything like continuity or periodical regularity of occupation with such work is intended, it will be of great service. A dissecting and mounting table is indeed of inestimable value to those who affect complete order and cleanliness in the accomplishment of such work. We have found in practice that a table firmly made, with a height of 2 ft. 6 in., semicircular in form, and a little more than half the circle in area on the outside, with the arc of another circle cut out from it to receive the person sitting at work—much after the fashion of the jeweller’s bench—serves admirably. A rough surges- Fig. 287. —Microscopist’s table. (Scale, h inch to 1 foot.) 1. Case for microscope; 2. Cabinet for objects; 8. Microscope lamp; 4. Lamp with shade; 5. Stand of apparatus ; 6. Book; 7. Large micro- scope ; 8. Second microscope; 9. Writing pad; 10. Bull’s-eye stand ; 11. Light-modifier. 1 Chapter IV. p. 238. LABORATORY TABLE FOR MICROSCOPIST 343 vtion of this is given in fig. 288, which presents the plan of the top •of the table. The whole area beneath should be unoccupied, but at A and B drawers may be put, not extending more than four inches below the under surface of the top of the table : On the side B a couple of shallow drawers, with everything required in the form of scalpels, needles, scissors, forceps, pipettes, life-slides, itc. in the upper one; atid pliers, cutting pliers, small shears, files of various coarse- nesses and finenesses, itc. in the other ; on the A side a single drawer containing slips, covers of various thicknesses, bone, tin, ylass, and other cells of all (assorted) sizes, watch-glasses, staining clips or slabs, lifters (if used), saw with fine teeth, hones of various shapes, peivter plate for grinding and polishing glass, etc., platinum capsule, camera lucidci, three ‘No. 2 ’ sable brushes (water- -colour), &c. Fig. 288.—Dissecting and mounting table. In this way all that is needed for dissection or mounting will be within reach without moving from the chair; and if by an arrange- ment which most moderately ingenious manipulators could accom- plish each of the articles in the drawers has a fixed place, there will be no difficulty in finding by touch what is wanted. The table top may be of pitch pine stained black, or, still better, some very hard wood finished smoothly, but ‘ grey.’ The space in the figure immediately in front of the operator may be cut out to a convenient size and thickness, a thick plate- glass slab whose edges on the right and left sides shall be slightly levelled, so that it may slide firmly into a prepared space cut into the surface of the table and occupy this space, the surface being ■exactly level with the surface of the table. This plate of glass should be made black on its under side, so as to present a uniform black surface. This is often of great value in certain kinds of work. Equally useful is a purely white unabsorbent surface, and a slab of white porcelain may be easily obtained of the same size and be made to fit exactly into the same place. In using this table for dissection the arms have complete rest, 344 PRACTICAL MICROSCOPY and 1 in tlie figure would represent the position of the dissecting microscope. 2 is a suitable position for a small easily managed microtome for general (chiefly botanical) purposes. We find that of Ryder 1 to answer this purpose admirably. 3 is a small vessel of spirit (dilute) for use with the section knife. 4 is a stand of mounting media, in suitable bottles, as Canada balsam in paraffin, or xylol, glycerine, &c. as well as small bottles, of reagents for botanical or zoological histology etc. 5 is a nest of apertures in which to place partly mounted objects* to protect them from dust, while the balsam, dammar, etc. may be hardening on the cover so as to be in a suitable state for final mount- ing. A slide may go over the sloping front of this and wholly ex- clude dust. 6 is a stand of cements, varnishes, etc. such as are needful; and 7 is a turn-table. For the work of dissection, when the subject requires reflected light, one of the desiderata is a mode of illumination at once conve- nient and intense. Mr. Frank R. Cheshire, F.L.S. ut the employment of the mirror at all, the light being sent directly dirough the condenser from the lamp flame. The mode of arrange- ment for this kind of manipulation is presented in Plate V., where t will be observed that the microscope is inclined more towards the lorizontal to suit the observer ; the lamp is directly in front of the sub-stage, the mirror is turned aside, and a frame (fixed upon abull’s- ?ye stand) carrying screens of coloured rda.ss is nlaned between the lamp flame and the con- denser (sub-stage). By this means the light is sent into the condense 1 ind upon the object, and is then treated as is the case (for centring) when the mirror is used. The first step in the direction of efficiency in the use of the microscope is to understand the principles of illumination, and a knowledge of the various effects produced by the bull’s-eye lies on the threshold of this. Having given details as to the forms of lamp which are of most service, we assume that a paraffin lamp with wick is used. If we place the edge of this flame (E, fig. 296) in the centre and exact focus of the bull’s-eye B, A shows the effect of doing so. If a piece of card were held in the path of the rays proceeding from B, the pictui’e as shown at A would not be seen—instead oi it an enlarged and inverted image of the flame. The image at A is obtained by placing the eye in the rays and by looking directly at the bull’s-eye. The light is so intense that it is more pleasant to take the field lens of a 2-inch eye-piece and place it in the path of the rays focussing the image of the bull’s- eye on a card. It should be noticed with care that the diameter of the disc A depends upon the diameter of the bull’s- eye B ; but the intensity of the light in A depends on the focal length of B. The shorter the focus, the more intense will be the light. We are here assuming throughout that the field lens is at a fixed distance from the bull’s-eve B. Fig. 296.—Edge of lamp flame in centre and focus of bull’s-eye. Fig. 297.—Altered relations between lamp flame and bull’s-eye. 352 PRACTICAL MICROSCOPY But if we move the flame, E—still central—within the focus of B, we get the result shown in D, fig. 297. But by moving E without the focus of B we get the picture H, while K is the picture when E is focussed but not centred. A common error, one repeatedly met with, is that of placing a concave mirror C (fig. 298) so that the flame E is in its 'principal focus. The result of this is that parallel rays are sent to B. These rays are brought to a focus at a distance from B about equal to twice the radius of the curvature of B and then scattered; a totally result from what is aimed at. If the concave mirror, C, is to be of any use in illumi- nation, it must be placed so that E is not at its principal focus, but at its centre of curvature. The bull’s-eye gives an illustration of what is of wider application. The method of obtaining a critical image with a condenser by means of transmitted light is shown in fig. 299. E is the edge of the flame, £ represents the sub-stage condenser, and F the object. F is thus the focal conjugate of E, and F and E are in the principal axis of S ; that is to say, these are the re- lations which exist when a condenser is focussed on and centred to an object. Let this be understood as the law, and there can be but little difficulty remain- ing in getting the best results from a condenser. Fig. 300 illustrates another method of getting the same result. We may illuminate a condenser with light direct from the flame, as in fig. 299, or we may interpose the mirror, as in fig. 300. M is the plane mirror and, properly used, exactly the same result may be obtained as in the former case. It is, however, slightly more difficult to set up, but will, on the whole, be prefer- able. Nothing can be of more moment to the beginner than to under- stand the practical use of the condenser, We must direct the student Fig. 298.—Result of placing flame in principal focus of concave mirror. Fig. 299.—Mode of obtaining critical image. Fig. 300.—Another method of getting critical image. PLATE V. Mode or obtaining Transmitted Light direct without the Aid of the Mirror. HOW TO OBTAIN A CRITICAL IMAGE 353 to what has been stated concerning it m Chapter IV. But the following should be carefully considered. Fi, and low-angled £).—Minute hairs on proboscis of blow-fly ; hair of pencil-tail (Polyxenus lagurus) ; diatoms on a dark ground. This last is a most sensitive test; unless the objective is good there is sure to be false light. Medium powers (with wide aperture).—Pleurosigma formosum; Navicula lyra in balsam or sty rax; Pleurosigma angulatum dry on cover; bacteria and micrococci stained. High powers (wide apertures and oil-immersion ~ and TV.—The secondary structure of diatoms, especially the fracture through the perforations. Navicula rhomhoides from cherry field in balsam or styrax; bacteria and micrococci stained. Testwitli a 10 or 12 eye-piece, and take into account the general whiteness and brilliancy of the picture. The podura scale is not mentioned as a test, as it may be very misleading in unskilled hands. One great point in testing objectives is to know your object. Care must be exercised to ascertain by means of vertical illuminator if objects such as diatoms dry on the cover are in optical contact with the cover-glass. Testing objectives is an art which can only be acquired in time and with experience gained by seeing large numbers of objectives. In the manipulation of the microscope it is not uncommon to observe the operator rolling the milled head of the fine adjustment instead of firmly grasping it between the finger and thumb and governing, to the minutest fraction of arc, the amount of alteration lie desires. It is undesirable and an entirely inexpert pi’ocedure to roll the milled head, and cannot yield the tine results which a deli- cate mastery of this part of the instrument necessitates and implies. To use aright the fine adjustment of a first-class microscope is not the first and easiest thing mastered by the tiro. Beyond the correct and judicious use of the microscope and all its appliances, there is the matter of the elimination of errors of in- terpretation to be carefully considered. The correctness of the conclusions which the microscopist will draw regarding the nature of any object from the visual appearances ERRORS OF INTERPRETATION 369 which it presents to him when examined in the various modes now specified will necessarily depend in a great degree upon his previous experience in microscopic observation and upon his knowledge of the class of bodies to which the particular specimen may belong. Not only are observations of any kind liable to certain fallacies arising out of the previous notions which the observer may entertain in regard to the constitution of the objects or the nature of the actions to which his attention is directed, but even the most practised ob- server is apt to take no note of such phenomena as his mind is not prepared to appreciate. Errors and imperfections of this kind can only be corrected, it is obvious, by general advance in scientific knowledge; but the history of them affords a useful warning against hasty conclusions drawn from a too cursory examination. If the history of almost any scientific investigation were fully made known it would generally appear that the stability and completeness of the conclusions finally arrived at had only been attained after many modifications, or even entire alterations, of doctrine. And it is therefore of such great importance as to be almost essential to the correctness of our conclusions that they should not be finally formed and announced until they have been tested in every conceivable mode. It is due to science that it should be burdened with as few false facts and false doctrines as possible. It is due to other truth- seekers that they should not be misled, to the great waste of their time and pains, by our errors. And it is due to ourselves that we should not commit our reputation to the chance of impairment by the premature formation and publication of conclusions which may be at once reversed by other observers better infonned than our- selves, or may be proved to be fallacious at some future time, per- haps even by our own more extended and careful researches. The suspension of the judgment whenever there seems room for doubt is a lesson inculcated by all those philosophers who have gained the highest repute for practical wisdom ; and it is one which the micro- scopist cannot too soon learn or too constantly practise. Besides these general warnings, however, certain special cautions should be given to the young microscopist with regard to errors into which he is liable to be led even when the very best instruments are employed. Errors of interpretation arising from the imperfection of the focal adjustment are not at all uncommon amongst microscopists, and some of the most serious arise from the use of small cones of illumination. With lenses of high power, and especially with those of large numerical aperture, it very seldom happens that all the parts of an object, however minute and fiat it may be, can be in focus together; and hence, when the focal adjustment is exactly made for one part, everything that is not in exact focus is not only more or less indistinct, but is often wrongly represented. The in- distinctness of outline will sometimes present the appearance of a pellucid border, which like the diffraction-band may be mistaken for actual substance. But the most common error is that which is produced by the reversal of the lights and shadows resulting from the refractive powers of the object itself ; thus, the bi-concavity of the blood-discs of human (and other mammalian) blood occasions their 370 PRACTICAL MICROSCOPY centres to appear dark when in the focus of the microscope, through the divergence of the rays which it occasions ; but when they are brought a little within the focus by a slight approximation of the object-glass the centres appear brighter than the peripheral parts of the discs. The student should be warned against supposing that in all cases the most positive and striking appearance is the truest, for this is often not the case. Mr. Slack’s optical illusion, or silica-crack slide,1 illustrates an error of this description. A drop of water holding colloid silica in solution is allowed to evaporate on a glass slide, and when quite dry is covered with thin glass to keep it clean. The silica deposited in this way is curiously cracked, and the finest of these cracks can be made to present a very positive and deceptive appearance of being raised bodies like glass threads. It is also easy to obtain diffraction-lines at their edges, giving an appearance of duplicity to that which is really single. A very important and very frequent source of error, which sometimes operates even on experienced microscopists, lies in the refractive influence exerted by certain peculiarities in the internal structure of objects upon the rays of light transmitted through them, this influence being of a nature to give rise to appearances in the image, which suggest to the observer an idea of their cause that may be altogether different from the reality. Of this fallacy we have a ‘pregnant instance’ in the misinterpretation of the nature of the lacunae and canaliculi of bone, which were long supposed to be solid corpuscles with radiating filaments of peculiar opacity, instead of being, as is now universally admitted, minute chambers with diverging passages excavated in the solid osseous substance. When Canada balsam fills up the excavations, being nearly of the same refractive power with the bone itself, it obliterates them altogether. So, again, if a person who is unaccustomed to the use of the microscope should have his attention directed to a preparation mounted in liquid or in balsam that might chance to contain air- bubbles, he will be almost certain to be so much more strongly impressed by the appearances of these than by that of the object, that his first remark will be upon the number of strange-looking black rings which he sees, and his first inquiry will be in regard to their meaning. Although no experienced microscopist could now be led astray by such obvious fallacies as those alluded to, it is necessary to notice them as warnings to those who have still to go through the same education. The best method of learning to appreciate the- class of appearances in question is the comparison of the aspect of globules of oil in water with that of globules of water in oil, or of bubbles of air in water or Canada balsam. This comparison may be very readily made by shaking up some oil with water to which a little gum has been added, so as to form an emulsion, or by simply placing a drop of oil of turpentine (coloured with magenta or carmine) and a drop of water together upon a slide, laying a thin glass cover over them, and then moving the cover backwards and 1 Monthly Microscojncal Journal, vol. v. 1872, p. 14. STUDIES IN INTERPRETATION 371 forwards several times on the slide. Equally instructive aie the appearances of an air-bubble in water and Canada balsam. The figures which illustrate the appearance at various points of the focus of an air-bubble in water and Canada balsam, and of a fat-globule in water, may be thus illustrated, viz. a diaphragm of about f of a mm. being placed at a distance of 5 mm. beneath the stage, and the concave mirror exactly centred. Air-bubbles in ivater.—No. 1 (fig. 318) represents the different appearances of an air-bubble in water. On focussing the objective to the middle of the bubble (B), the centre of the image is seen to be very bright—brighter than the rest of the field. It is surrounded by a greyish zone, and a somewhat broad black ring interrupted by one or more brighter circles. Round the black ring are again one or more concentric circles (of diffraction), brighter than the field. On focussing to the bottom of the bubble (A) the central white circle diminishes and becomes brighter : its margin is sharper, and it is surrounded by a very broad black ring, which has on its periphery one or more diffraction circles. When the objective is focussed to the upper surface of the Fig. 318.—Air-bubbles in (1) water, (2) Canada balsam; (3) fat-globules in water. 372 PRACTICAL MICROSCOPY bubble (C) the central circle increases in size, and is surrounded by a greater or less number of rings of various shades of grey, around which is again found a black ring, but narrower than those in the previous positions of the objective (A and B). The outer circles of diffraction are also much more numerous. Air-bubbles in Canada Balsam.—Canada balsam being of a higher refractive index than water, the limiting angle instead of being 48° 35' is 41° only, so that the rays which are incident much less obliquely on the surface of separation undergo total reflexion, and it will be only those rays which face very close to the lower pole of the bubble that will reach the eye, and the black marginal zone will therefore be much larger. This is shown in fig. 319, No 2. When the objective is focussed to the bottom of the bubble (A'), we have a small central circle, brighter than the rest of the field, all the rest of the bubble being black, with the exception of some peripheral diffraction rings. On focussing to the centre (B') or upper part (C') of the bubble, we have substantially the same appearances as in B and C, with the exception of the smaller size of the central circle. Fat-globules in water (fig. 318, No. 3).—These illustrate the case of a highly refracting body in a medium of less refractive power. When the objective is adjusted to the bottom of the globule A", it appears as a grey disc a little darker than the field, and separated from the rest of the field by a darkish ring. Focussing to the middle of the bubble (B"), the central disc becomes somewhat brighter, and is surrounded by a narrow black ring, bordered within and without by diffraction circles. On further removing the objective the dark ring increases in size, and when the upper part of the objective is in focus, we have (C") a small white central disc, brighter than the rest of the field, and sharply limited by a broad, dark ring which is blacker towards the centre. These appearances are the converse of those presented by the air-bubble. That, as we saw, has a black ring and a white centre, which are the sharper as the objective is approached to the lower pole of the bubble. The fat-globule has, however, a dark ring which is the broader, and a centre which is the sharper, according as the objective is brought nearer to the upper pole. These considerations, apart from their enabling us to distinguish between air-bubbles and fat-globules, and preventing their being confounded with the histological elements, enable two general principles to be established, viz. bodies which are of greater re- fractive power than the surrounding medium have a white centre which is sharper and smaller, and a black ring which is larger when the objective is withdrawn ; whilst those which are of less refractive power have a centre which is whiter and smaller, and a black ring which is broader and darker when the objective is lowered. Monochromatic Light.—The same phenomena are observed by yellow monochromatic light, except that the diffraction fringes are more distinct, further apart, and in greater numbers than with ordinary light. INTERPKETINCt movement 373 A fat-globule, indeed, seems to be composed of a series of con- centric layers like a grain of starch. With blue light these fringes are also multiplilied, but are closer together and finer, so that they are not so easily visible. Yellow monochromatic light, therefore, constitutes a good means for determining whether the stripe seen on an object are peculiar to it, or are only diffraction lines. In the former case they are not exaggerated by monochromatic light; but if, on the contrary, they are found to be doubled, or quadrupled, with this light, we may be certain that they are diffraction fringes. But there is no source of fallacy to a certain class of workers so much to be guarded against as that arising from errors in the inter- pretation concerning movement as such, and especially concerning the movement exhibited by certain very minute particles of matter in a state of suspension in fluids. The movement was first observed in the fine granular particles which exist in great abundance in the contents of pollen grains of plants known as the fovilla, and which ai'e set free by crushing the pollen. It was first supposed that they indicated some special vital movement analogous to the motion of the spermatozoa of animals. But it was discovered in 1827, by Dr. Robert Brown, that inorganic substances in a state of fine trituration would give the same result; and it is now known that all substances in a sufficiently fine state of powder are affected in the same manner, one of the most remarkable being the movement visible in the contents of the fluid cavities in quartz in the oldest rocks. These have probably retained their dancing motion for peons. A good illustra- tion is gamboge, which can be easily rubbed from a water-colour cake upon a glass slip and covered, and will at once show the characteristic movement; so will carmine, indigo, and other similarly light bodies. But the metals which are from seven to twenty times as heavy as water require to be reduced to a state of minuteness many times greater; but, triturated finely enough, these also show the movement for a long time known, from the name of its discoverer, as Brownian movement, but now more generally called pedesis. The movement is chiefly of an oscillatory nature, but the particles also rotate backwards and forwards on their axes, and gradually (if persistently watched) change their places in the field of view. It is an extremely characteristic movement, and could not be mistaken for any vital motion by an observer acquainted with both; but the student must familiarise himself with this kind of motion or he will be utterly unable to distinguish certain kinds of motion in minute living forms in certain stages of their life from this movement, and will make erroneous inferences. The movement of the smallest particles in pedesis is always the most active, while in the majority of cases particles greater than the TToVo'th of an inch are wholly inactive. A drop of common ink which has been exposed to the air for some weeks, or a drop of fine clay (such as the prepared kaolin used by photographers) shaken up with water, is recommended by Professor Jevons,1 who has recently studied this subject, as showing the movement (which he designates 1 Quarterly Journal of Micro. Science, N.S. vol. viii. 1878, p. 172. PRACTICAL MICROSCOPY 374 pedesis) extremely well. But none of the particles he has examined are so active as those of pumice-stone that has been ground up in an agate mortar; for these are seen under the microscope to leap and swarm with an incessant quivering movement, so rapid that it is impossible to follow the course of a particle which probably changes its direction of motion fifteen or twenty times in a second. The distance through which a particle moves at anyone bound is usually less than ■5-An,th of an inch. This ‘Brownian movement’ (as it is commonly termed) is not due to evaporation of the liquid, for it continues without the least abatement of energy in a drop of aqueous fluid that is completely surrounded by oil, and is therefore cut off from all possibility of evaporation; and it has been known to con- tinue for many years in a small quantity of fluid enclosed between two glasses in an air-tight case ; and for the same reason it can scarcely be connected with the chemical change. But the observa- tions of Professor Jevons (loc. cit.) show that it is greatly affected by the admixture of various substances with water, being, for example, increased by a small admixture of gum, while it is checked by an extremely minute admixture of sulphuric acid or of various saline compounds, these (as Professor Jevons points out) being all such as increase the conducting power of water for electricity. The rate of subsidence of finely divided clays or other particles suspended in water thus greatly depends upon the activity of their ‘ Brownian movement,’ for when this is brought to a stand the particles aggregate and sink, so that the liquid clears itself.1 Pedetic motion depends on, that is, is affected by— 1. The size of the particles. 2. The specific gravity of the particles. Metals, or particles of vermilion, of similar size to particles of silica or gamboge, move much more slowly and less frequently. 3. The nature of the liquid. No liquid stops pedesis, but liquids which have a chemical action on the substance do. This action may be very slow, still it tends to agglomerate the particles. For in- stance, barium sulphate, when precipitated from the cold solution, takes a long time to settle ; whereas, when warm and in presence of hydrochloric acid, agglomeration soon occurs. Iron precipitated as hydrate in presence of salts of ammonium, and mud in salt water, are other instances. The motion does not cease, but the particles adhere together and move very slowly. But besides the right appreciation of the nature of pedesis, there is the utmost caution required in the interpretation of the rapidity of movement, and kind of movement which living and motile forms effect. The observation of the phenomena of motion under the microscope 2 lias led to many false views as to the nature of these movements. If, for instance, swarm-spores are seen to traverse the field of view in one second, it might he thought that they race through the water at the speed of an arrow, whereas they in reality traverse in that time only a third part of a millimetre, which is somewhat more than 1 See also the Rev. -T. Delsaulx ‘On the Thermo-dynamic Origin of the Brownian Motions’ in Monthly Journ. of Microsc. Sci. vol. xviii. 1877. 2 jDas Mikroskop, Naegeli and Scliwendener, p. 258 (Eng. edit.). MOVEMENT IN MICROSCOPIC OEJECTS 375 a metre in a hour. It must not, therefore, be forgotten that the rapidity of motion of microscopical objects is only an apparent one, and that its accurate estimation is only possible by taking as our standard the actual ratio between time and space. If we wish, for the sake of exact comparison, to estimate the magnitude of the mov- ing bodies, we may always do so ; the ascertainment of the real rapidity remains, however, with each successive motion, the princi- pal matter. If a screw-shaped spiral object, of slight thickness, revolves on its axis in the focal plane, at the same time moving forward, it presents the deceptive appearance of a serpentine motion. Thus it is that the horizontal projections of an object of this kind, corre- sponding to the successive moments of time, appear exactly as if the movement were a true serpentine one. As an example of an appear- ance of this nature, we may mention the alleged serpentine motion of Spirillum and Vibrio. Similar illusions are also produced by swarm-spores and sperma- tozoa ; they appear to describe serpentine lines while in reality they move in a spiral. It was formerly thought that a number of differ- ent appearances of motion must be distinguished, whereas modern observers have recognised most of them as consisting of a forward movement combined with rotation, where the revolution takes place sometimes round a central, and sometimes round an eccentric, axis. To this category belong, for instance, the supposed oscillations of the oscillaricce, whose changes of level, when thus in motion, were formerly unnoticed. In addition to these characteristics of a spiral motion it must, of course, be ascertained whether it is right- or left-handed. To dis- tinguish this in spherical or cylindrical bodies, which revolve round a central axis, is by no means easy, and in many cases, if the object is very small and the contents homogeneous, it it quite impossible. The slight variations from cylin- drical or spherical form, as they occur in each cell are therefore just sufficient to admit of our perceiv- ing whether any rotation does take place. Tie discovery of the direction of the rotation is only possible when fixed points whose position to the axis of the spiral is known can be followed in their motion round the axis. The same holds good also, mutatis mutandis, of spirally wound threads, spiral vessels, &c. ; we must be able to distinguish clearly which are the sides of the windings turned towards or tux*ned away from us. If the course of the windings is very irregular, as in fig. 319, a little practice and care are needed to distinguish a spiral line as such in small ob- jects. The microscopical image might easily lead us to the conclusion that we were examining a cylindrical body composed of bells or funnels inserted one in another. The spirally thickened threads, for instance, as they originate from the epidermis cells of many seeds, were thus inter- Fig. 319.—A spiral in motion. 376 PRACTICAL MICROSCOPY preted, although here and there by the side of the irregular spirals quite regular ones are also observed. Moreover, it must not be forgotten that in the microscopical image a spiral line always appears wound in the same manner as when seen with the naked eye, while in a mirror (the inversion being only a half one) a right-handed screw is obviously represented as left-handed, and conversely. If, therefore, the microscopical image is observed in a mirror, as in drawing with the Sbmmering mirror, or if the image-forming pencils are anywhere turned aside by a single reflexion, a similar inversion takes place from right-handed to left-handed, and this inversion is again cancelled by a second reflex- ion, in some microscopes. All this is, of course, well known, and to the practised observer self-evident ; nevertheless many microscopists have shown that they are still entirely in the dark about matters of this kind. One of Professor Abbe’s experiments on diffraction ’phenomena proves that when the diffraction spectra of the first order are stopped out, while those of the second are admitted, the appearance of the structure will be double the fineness of the actual structure which is causing the interference.1 Upon this law there appears to depend a number of possible fal- lacies, errors which may arise from either its misapprehension or misin- terpretation. At least these appear to us, from a practical point of view, to be of sufficient importance to need either caution or a fuller exposition of the great law of Abbe in regard to them. If, for example, figs. 320,321, and 322 may be taken to represent a square grating having 25,000 holes per linear inch at the focus of an objective at P, P D the dioptric beam, P1 P1 diffraction spectra of the first order, and P2 P2 those of the second order, then if the objective is aplanatic all those spectra will be brought to an identical focal conjugate ; and the image of the grating will be a counterpart of the structure, characteristic of such a group of spectra. Let us suppose our objective to be over-corrected, as in fig. 321, then when the grat- ing is focussed at P the spectra of the first order only will be brought to the focal conjugate ; the image, however, will not be materially affected on that account, as the diffraction elements of the first order are alone sufficient to give a truthful representation of the 25,000 per inch grating. If, however, the objective be raised so that the grating lies at P' the diffraction elements of the second order only are brought to the focal conjugate ; consequently by the hypothesis the image will have 50,000 holes per linear inch, or double that of the original. In other words, placing a grating at the longer focus of an over-corrected objective is apparently tantamount to cutting out the diffraction spectra of the first order by a stop at the back of the- objective. Fig. 320. Fig. 321. 1 See Chapter II. INTERPRETATION AND THE N.A. TABLE The effect of this is to give an impression that there is a strong grating with 25,000 holes per linear inch ; and over it another grat- ing with 50,000 holes per linear inch. The raising the focus so as- to bring P to P7 necessarily gives the idea of the fine structure being superimposed on the coarse. Therefore the microscopist should beware whenever he notices a structure of double fineness over another one lest he has acondition of things similar to fig. 321. The following is a test which may be applied to confirm the genuineness of any such structure. First measure by means of the divided head of the fine-adjustment screw as accurately as possible, the movement required to bring P to P7 in fig. 321 : next by means of the draw-tube increase the dis- tance between the eye-piece and the objective : this will have the effect of increasing the over-correction of the objective, and a state of things will be ob- tained as in fig. 322. Hence it will require a larger movement of the fine-adjustment screw to bring P to P7. This will make the distance between the 50,000 grating and the 25,000 grating appear greater than it was before. If this takes place the 50,000 grating is a mere diffraction ghost. A precisely similar condition of things exists with an under- corrected objective, only in that case the false finer grating will appear below the original coarse grating, and to increase the distance between them the draw-tube must be shortened.1 It may therefore be of service to give an example of the use of the numerical aperture table as a check in the interpretation of structure. Fig. 323 gives six illustrations of the back of an objective (the eye-piece being removed) of -83 N.A., or 112° in air : D stands for 377 Fig. 322. Fig. 323. dioptric beam ; 1 for diffraction spectrum of the first order; 2 for diffraction spectrum of the second order. 1 It is well to note here that we have seen a photo-micrograph by Mr. Comber of a diatom surface which is uneven. In those parts in correct focus the structure is single, but in the parts where the focus is withdrawn it is doubled. 378 PRACTICAL OilCROSCOPY When the back of an objective of -83 N.A. shows an arrange- ment as in No. 1 then, although the structure will be invisible, it cannot be coarser than . . . 40,000 per inch. No. 2 „ „ „ 80,000 No. 3 then the structure does not differ greatly from 40,000 „ No. 4 „ „ „ 80,000 No. 5 „ „ „ 20,000 No. (> „ „ „ 40,000 It will be understood by the student that the preservation of the microscope and its apparatus is a matter that must largely depend upon his own action. The stand should be kept from dust, generally wiped with a soft chamois leather after use, and when needful a minute quantity of watchmaker’s oil may be put to a joint working stiffly. There is no better way to preserve this stand than to keep it either under a bell-glass or in a cabinet which is easily accessible. All objectives should be examined after use, and all oils or other fluids carefully wiped away from them with okl cambric which has been thoroughly washed with soda, well rinsed and not ‘ ironed ’ or finished in any way, but simply dried. If chemical reagents are employed the cessation of their use should become the moment for wiping the lenses employed with care ; and all processes involving the use of the vapours of volatile acids, or which develop sulphuretted hydrogen, chlorine, &c. must never take place in a room in which a microscope of any value is placed. Dry elder-pith and Japanese paper are by some workers sug- gested for cleaning the front lenses of homogeneous objectives ; but while these are excellent, especially the former, we rind nothing better than the simple cambric we suggest. Two or three good chamois leathers should be kept by the worker for specific purposes and not interchanged. Cleanliness, care, delicacy of touch, and a purpose to be accurate in all that lie does or seeks to do, are essentials of the successful microscopist. It may be noted that dust on the eye-piece can be detected in a dim light, and can be discovered by closing the iris diaphragm. The lens of the eye-piece on which the dust appears may be localised by rotation ; and this should be done before wiping. In reference to dust on the back of the objective, it should be observed that if the eye- piece be removed, dust sometimes appears to be upon it which comes really from the focus of the sub-stage condenser, and is, in fact, not on the back of the objective at all. To rind this condition, remove the light modifier (if in use), for the dust may be on it, and rotate the condenser; else there will be needless and injurious rubbing of the back-lens of the objective. With oil-immersion objectives dust or air-bubbles in the oil must be carefully avoided. If chamois leather be used for cleaning the lenses, it should be previously well beaten and shaken, and then kept constantly in a well-made box. 379 CHAPTER VII FBEPABAT 10N, MOUNTING, AND COLLECTION OF OBJECTS Under this head it is intended to give an account of those materials, instruments, and appliances of various kinds, which have been found most serviceable to microscopists engaged in general biological re- search, and to describe the most approved methods of employing them in the preparation and mounting of objects for the display of the minute structures thus brought to our knowledge. Not only is it of the greatest advantage that the discoveries made by microscopic research should—as far as possible—be embodied (so to speak) in 1 preparations,’ which shall enable them to be studied by everyone who may desire to do so, but it is now universally admitted that such ‘ preparations ’ often show so much more than can be seen in the fresh organism that no examination of it can be considered as complete in which the methods most suitable to each particular case have not been put in practice. It must be obvious that in a comprehensive treatise like the present such a general treatment of this subject is all that can be attempted, excepting in a few instances of peculiar interest ; and as the histological student can And all the guidance he needs in the numerous manuals now prepared for his instruction, the Author will not feel it requisite to furnish him with the special directions that are readily accessible to him else- where. Materials, Instruments, and Appliances. Glass Slides.—The kind of glass best suited for mounting objects is that which is known as £ patent plate,’ and it is now almost in- variably cut, by the common consent of microscopists in this country, into slips measuring 3 in. by 1 inch. For objects too large to be mounted on these the size of 3 in. by 1J, in. may be adopted. Such slips may be purchased, accurately cut to size, and ground at the edges, for so little more than the cost of the glass that few persons to whom time is an object would trouble themselves to prepare them ; it being only when glass slides of some unusual dimensions are required, or when it is desired to construct 1 built-up cells,’ that a facility in cutting glass with a glazier’s diamond becomes useful. The glass slides prepared for use should be free from veins, air-bubbles, or other flaws, at least in the central part on which the object is placed ; and any whose defects render them unsuitable for ordinary purposes should be selected and laid aside for uses to which the 380 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS working microscopist will find no difficulty in putting them. As the slips vary considerably in thickness, it will be advantageous to determine on a gauge for thin, thick, and medium glass. The first may be employed for mounting delicate objects to be viewed by the high powers with which the apochromatic and achromatic condensers are to be used, so as to allow plenty of room for the focal point of an optical combination with great aperture to be fixed readily upon the plane of the object ; the second should be set aside for the attach- ment of objects which are to be ground down, and for which, there- fore, a stronger mounting than usual is desirable ; and the third are to be used for mounting ordinary objects. Great care should be taken in washing the slides, and in removing from them every trace of greasiness by the use of a little soda or potass solution. If this should not suffice they may be immersed in the solution recommended by Dr. Seiler, composed of 2 oz. of bichromate of potass, 3 fl. oz. of sulphuric acid, and 25 oz. of water, and afterwards thoroughly rinsed. (The same solution may be advantageously used for cleansing cover- glasses.) Before they are put away the slides should be wiped perfectly dry, first with an ordinary ‘glass cloth,’ and afterwards with an old cambric handkerchief ; and before being used, each slide should be washed in methylated spirit to ensure freedom from greasiness. Where slides that have been already employed for mounting preparations are again brought into use, great care should be taken in completely removing all trace of adherent varnish or cement—first by scraping (care being taken not to scratch the glass), then by using an appropriate solvent, and then by rubbing the slide with a mixture of equal parts of alcohol, benzole, and liquor soda?, finishing with clean water. Thin Glass.—The older microscopists were obliged to employ thin laminae of talc for covering objects to be viewed with lenses of short focus ; but this material, which was in many respects objectionable, is entirely superseded by the thin glass manufactured by Messrs. Chance, of Birmingham, which may be obtained of various degrees of thickness, down to the of an inch. This glass, being unannealed, is very hard and brittle, and much care and some dexterity are re- quired in cutting it; hence covers should be purchased, as required, from the dealers, who usually keep them in several sizes and supply any others to order. Save the fact that 1 cover-glass ’ is made by Messrs. Chance, there is no definite information as to the mode of its manufacture and the conditions upon which it is most satisfactorily produced. It would be an advantage to the microscopist to possess information on this point. The different thicknesses are usually ranked as 1, 2, and 3 ; the first, which should not exceed in thickness the -006 in., being used for covering objects to be viewed with low powers ; the second, which should not exceed -005 in. in thickness, for objects to be viewed with medium powers ; and the third, which ought never to exceed -004 in. in thickness, for objects which either require or may be capable of being used with high powers. It must, however, be remembered that the achromatic objectives of great power and great aperture (1-5) will require much thinner covers than even this. The thinnest glass is of course most difficult to LEVEE OF CONTACT 381 handle safely, and is most liable to fracture from accidents of various kinds ; and hence it should only be employed for the purpose for which it is absolutely needed. The thickest pieces, again, may be most advantageously employed as covers for large cells, in which objects are mounted in fluid to be viewed by the low powers whose performance is not sensibly affected by the aberration thus produced. The working microscopist will find it desirable to provide himself with some means of measuring the thickness of his cover-glass ; and this is especially needed if he is in the habit of employing objectives without adjustment, which are corrected to a particular standard. A small screw-gauge of steel, made for measuring the thickness of rolled plates of brass, and sold at the tool-shops, answers this purpose very well; but Ross’s lever of contact (fig. 324), devised for this express purpose, is in many respects preferable. This consists of a small horizontal table of brass, mounted upon a stand, and having at one end an arc graduated into twenty divisions, each of which re- presents the j-gLyth of an inch, so that the entire arc measures the 0-th of an inch ; at the other end is a pivot on which moves a long and Fig. 324.—Eoss’s lever of contact. delicate lever of steel, whose extremity points to the graduated arc, whilst it has very near its pivot a sort of projecting tooth, which bears it against a vertical plate of steel that is screwed to the horizontal table. The piece of thin glass to be measured being in- serted between the vertical plate and the projecting tooth of the lever, its thickness in thousandths of an inch is given by the number on the graduated arc to which the extremity of the lever points. Thus, if the number be 8, the thickness of the glass is ‘008, or the of an inch. It will be found convenient to sort the covers according to their thicknesses, and to keep the sortings apart, so that there may be a suitable thickness of cover for each object. But it is well to remember that, with the exception of objects to which from their size or nature it is impossible to apply high powers, it is better to mount the object so that if it be required or desirable high powers may be used upon it. Another simple and very efficient cover-glass tester is made by Zeiss, of Jena, and illustrated in fig. 325. It will be seen that the measurement is effected by a clip projecting from a box, between the laws of which the cover to be measured is placed; the reading is given by an indicator moving over a divided circle on the upper face •of the box. The divisions show hundredths of a millimetre, and the instrument measures to upwards of 5 mm. 382 PREPARATION, MOUNTING, ANI) COLLECTION OF OBJECTS It is well to keep assorted measured and cleaned cover-glasses ir small separate wide-stoppered bottles of methylated spirit, eacl bottle being labellec with the gauge of thick- ness of the covers it contains. What is then required is a simple ap paratus for cleaning the delicate covers with the least risk of breakage. This can be well accom - plished by having two blocks of boxwood, shaped so as to be easily held one in each hand, turned with perfect true- ness on the faces opposite to the respective handles, so that when the surfaces so flattened are laid upon and pressed towards each other they are everywhere in perfect contact. They should be from two to four inches in diameter, and these flattened surfaces should each have very tightly stretched upon them a firm, even-textured, moderately thick piece of chamois leather. If covers be slightly moistened—even breathed upon—and laid on one of these blocks and pressed down with the other, breath, or moisture applied by a small camel-hair brush to the upper surface of the cover, may be applied, and a few twists of these blocks upon each other when firmly pressed together will effectually clean without breaking the thinner covers. It will be often needful to treat both sides of the covers thus, as one side generally adheres while the other is subject to the friction. For cleaning slips and covers by hand, finishing should be done with old fine cambric handkerchiefs. These should not be washed with soap, but with common soda and hot water, plenty of the latter being subsequently employed to get rid of every trace of the alkali. But when dry these cloths must not be ‘ ironed ’ or smoothed in any way, the ‘ rough-dry ’ surface acting admirably for wiping delicate glass. Fig. 325.—Zeiss’s cover-glass tester. Varnishes and Cements.—There are three very distinct purposes for which cements which possess the power of holding firmly to glass,, and of resisting not merely water but other preservative liquids,, are required by the microscopist, these being (1) the attachment of the glass covers to the slides or cells containing the object, (2) the formation of thin ‘ cells ’ of cement only, and (3) the attachment of the ‘ glass plate ’ or ‘ tube-cells ’ to the slides. The two former of these purposes are answered by liquid cements or varnishes, which, may be applied without heat ; the last requires a solid cement of greater tenacity, which can only be used in the melted state. Among the many such cements that have been recommended by different workers, two or three will be selected by the worker for general purposes, and perhaps three or four for special purposes, and the re- mainder will be in practice neglected. We do not hesitate to say VARNISHES AND CEMENTS 383 that the two cements on which the most complete trust may be re- posed are japanner’s gold size and Bell’s cement. This opinion is the result of over twenty years of special observation. A good varnish may easily, in a general way, be tested : when it is throughly hard and old, if scraped oft' it comes away in shreds ■ unsafe varnishes break under the scraper in flakes and dust. To those who put up valuable preparations and objects of value the risk should never be run of using a new and unknown varnish or cement. Neither appearance nor facility nor cheapness in use should for one moment weigh against a varnish or cement of known and tested worth. Japanned s gold-size may be obtained from the colour shops. It may be used for closing-in mounted objects of almost any description. It takes a peculiarly firm hold of glass, and when dry it becomes extremely tough without brittleness. When new it is very liquid and ‘ runs ’ rather too freely ; so that it is often advantageous to leave open for a time the bottle containing it until the varnish is some- what thickened. By keeping it still longer, with occasional exposure to air, it is rendered much more viscid, and though such £ old ’ gold- size is not fit for ordinary use, yet one or two coats of it may be ad- vantageously laid over the films of newer varnish, for securing the thicker covers of large cells. Whenever any other varnish or cement is used, either in making a cell or in closing it in, the rings of these should be covered with one or two layers of gold-size extending beyond it on either side, so as to form a continuous film extending from the marginal ring of the cover to the adjacent portion of the glass slide. Asphalte Varnish.—This is a black varnish made by dissolving- half a drachm of caoutchouc in mineral naphtha, and then adding 4 oz. of asplialtum, using heat if necessary for its solution. It is very important that the asplialtum should be genuine, and the other materials of the best quality. Some use asphalte as a substitute for gold-size ; but the Author’s experience leads him to recommend that it should only be employed either for making shallow £ cement cells ’ or for finishing off preparations already secured with gold-size. For the former purpose it may advantageously be slightly thickened by evaporation. Hell’s cement is sold by J. Bell and Co. chemists, Oxford Street, London ; they are the sole makers and retain the secret of its com- position. It is of great service for glycerin mounts ; but the edge of the cover should be ringed with glycerin jelly before this cement is applied. It is an extremely useful and reliable varnish, which is extremely easy of manipulation. It can be readily dissolved in either ether or chloroform. Canada balsam is the oleo-resin from Abies balsamect and Finns canadensis ; it is so brittle when hardened by time that it cannot be safely used as a cement, except for the special purpose of attaching hard specimens to glass, in order that they may be reduced by grinding &c. Although fresh, soft balsam may be hardened by heating it on the slide to which the object is to be attached, yet it may be preferably hardened en masse by exposing it in a shallow vessel to 384 PREPARATION, MOUNTING, AND COLLECTION OF OEJECTS the prolonged but moderate heat of an oven, until so much of its volatile oil has been driven of! that it becomes almost (but not quite) resinous on cooling. If, when a drop is spread out on a glass and allowed to become quite cold, it is found to be so hard as not to be readily indented by the thumb-nail, and yet not so hard as to ‘ chip,’ it is in the best condition to be used for cementing. If too soft, it will require a little more hardening on the slide, to which it should be transferred in the liquid state, being brought to it by the heat of a water-bath ; if too hard it may be dissolved in chloroform or ben- zole for use as a mounting ‘ medium ’; we do not recommend its use for mounts with glycerin. Brunswick black is a very useful cement, obtainable at the op- tician’s as prepared for the use of microscopists. It is one of the best cements for the purpose of ringing mounts, and it may be recom- mended for turning cells. We have already stated that we do not, as a rule, recommend opaque or black ground mounting; but if this is desired or needful no better * ground ’ can be obtained than by putting on the centre of the slide a disc of Brunswick black the size of the outside of the cell or cover-glass, and while it is wet putting a thin cover-glass upon it. The cover-glass becomes quickly fixed and a pleasant surface is formed to receive the object which it is intended to mount. Should it be desirable to have the floor of the opaque cell dead instead of bright, this can be quickly accomplished -with a little emery-powder and water applied to the surface by a flattened block of tin fixed in boxwood. Brunswick black is soluble in oil of turpentine, and it dries quickly. Glue and honey mixed in equal parts is very valuable for special purposes and softens with heat. Shellac cement is made by keeping small pieces of picked shel- lac in a bottle of rectified spirit, and shaking it from time to time. It cannot be recommended as a substitute for any of the preceding, but it may be employed to put a thin film upon the edge of all mounts—however closed and finished—that are to be used with homo- geneous lenses. It is a sure protection against the otherwise in- jurious action of the cedar oil. Hollis’s liquid glue may also be employed with confidence for this purpose. Sealing-wax varnish, which is made by digesting powdered sealing-wax at a gentle heat in alcohol, should never be used as a cement ; it is serviceable only as a varnish, and resists cedar oil. Venice turpentine is the liquid resinous exudation of Abies larix. It must be dissolved in enough alcohol to filter readily, and after filtering must be placed in an evaporating dish, and by means of a sand-bath must be reduced by evaporation one-fourth. This cement is used for closing glycerin mounts. Square covers are used, and we find it best to edge the cover with glycerin jelly. A piece of copper wire of No. 10 to No. 12 gauge is taken, and one end of it is bent just the length of one of the sides of the cover at right angles to the length of the wire. This end is now heated in a spirit lamp, plunged into the cement, which adheres in fair quantity, and is instantly brought down upon the slide and the margin of the COLOUKED VARNISHES—DRY MOUNTING- 385 cover. The fluid turpentine distributes itself evenly along the cover and slide and hardens at once. We have no long experience of it, but from some of its characteristics we are inclined to believe it will prove a useful cement for this purpose. Marine glue, which is composed of shellac, caoutchouc, and naphtha, is distinguished by its extraordinary tenacity, and by its power of resisting solvents of almost every kind. Different qualities of this substance are made for the several purposes to which it is ap- plied, and the one most suitable to the wants of the microscopist is known in commerce as G K 4. The special value of this cement, which can only be applied hot, is in attaching to glass slides the glass or metal rings which thus form ‘ cells’ for the reception of objects to be mounted in fluid, no other cement being comparable to it either for tenacity or for durability. The manner of so using it will be presently described. Various coloured varnishes are used to give a finish to mounted preparations, or to mark on the covering glasses of large preparations the parts containing special kinds of noteworthy structure. A very good black varnish of this kind is made by working up very finely powdered lamp-black with gold-size. Dor red, sealing-wax varnish may be used ; but it is very liable to chip and leave the glass when hardened by time. The red varnish specially prepared for microscopic purposes by Messrs. Thompson and Capper, of Liverpool, seems likely to stand better. For white, ‘zinc cement ’ answers well, which is made of benzole, gum dammar, oxide of zinc, and turpentine. But it is inexpensive, and either in Cole’s or Ziegler’s formula may be obtained at the optician’s. Blue or green pigments may be worked up with this if cements of those colours be desired. For attaching labels to slides either of glass or wood, and for fixing down small objects to be mounted ‘ dry ’ (such as foraminifera, parts of insects, &c.), the Author has found nothing preferable to a rather thick mucilage of gum arabic, to which enough glycerin has been added to prevent it from drying hard, with a few drops of some essential oil to prevent the development of mould. The following formula has also been recommended : Dissolve 2 oz. of gum arabie in 2 oz. of water, and then add oz. of soaked gelatin (for the solution of which the action of heat will be required), 30 drops of glycerin, and a lump of camphor. The further advantage is gained by the addition of a slightly increased proportion of glycerin to either of the foregoing, that the gum can be very readily softened by water, so that covers may be easily removed (to be cleansed if necessary) and the arrangement of objects (where many are mounted together) altered. Cells for Dry-mounting.—Where the object to be mounted ‘ dry * (i.e. not immersed either in fluid or in any ‘ medium ’) is so thin as to require that the cover should be but little raised above the slide, a ‘ cement cell ’ answers this purpose very well ; and if the ap- plication of a gentle warmth be not injurious, the pressing down of the cover on the softened cement will help both to fix it and to prevent the varnish applied round its border from running in. Where a somewhat deeper cell is required, Prof. H. L. Smith 386 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS •(U.S.A.) suggests the following specially for the mounting of diatoms. A sheet of thin writing paper dipped into thick shellac varnish is hung up to dry ; and rings are then cut out from it by punches of two different sizes. One of these rings being laid on a glass slide, and the cover, with the object dried upon it, laid on the ring, it is to be held in its place by the forceps or spring-clip, and the slide gently warmed so as to cause a slight adhesion of the cover to the ring, and of the ring to the slide ; and this adhesion may then be rendered complete by laying another glass slide on the cover and pressing the two slides together, with the aid of a continued gentle heat. Still deeper cells may be made with rings punched out of tinfoil of various thicknesses and cemented with shellac varnish on either side. And if yet deeper cells are needed, they may be made of turned rings of vulcanite or ebonite, cemented in the same manner. There is, however, a tendency in shellac-formed cells to throw off a cloudiness inside the cell, usually called ‘ sweating,’ which is very undesirable. It has been found that a ring of solid' paraffin to which the cover is attached, if first ‘ ringed ’ with the same material, and afterwards with a finishing varnish, makes a useful and permanently clean dry shallow cell ; or paper may be saturated with paraffin and treated as described for shellac. • Cement-cells.—Cells for mounting thin objects in any watery medium may be readily made with asphalte or Brunswick black varnish by the use of Mr. Shadbolt’s £ turn-table ’ or one of its modi- fications (p. 391). The glass slide being placed under its spring, in •such a manner that its two edges shall be equidistant from the centre (a guide to which positionis afforded by the circles traced on the brass), and its four corners equally projecting beyond the circular margin of the plate, a camel’s-hair pencil dipped in the varnish is held in the right hand, so that its point comes into contact with the glass over whichever of the circles may be selected as the guide to the size of the ring. The turn-table being made to rotate by the application of the left forefinger to the milled head beneath, a ring of varnish, of a suitable breadth, is made upon the glass ; and if this be set aside in a horizontal position, it will be found, when hard, to present a very level surface. If a gi’eater thickness be desired than a single appli- cation will conveniently make, a second layer may be afterwards laid on. It will be found convenient to make a considerable number of such cells at once, and to keep a stock of them ready prepared for use. If the surface of any ring should not be sufficiently level for a covering glass to lie flat upon it, a slight rubbing upon a piece of fine emery paper laid upon a flat table (the ring being held down- wards) will make it so. ' Ring-cells.—For mounting objects of greater thickness it is desirable to use cells made by cementing rings, either of glass or metal to the glass slides, with marine glue. Glass rings of any size, dia- meter, thickness,- and breadth are made by cutting transverse sections of thick-walled tubes, the surfaces of these sections being ground flat and parallel. Not only may round cells (fig. 326, A, B) of vari- ous sizes be made by this simple method, but, by flattening the tube (when hot) from which they are cut, the sections may be made quad MOUNTING IN CELLS 387 rangular, or square, or oblong (C, D). For intermediate thicknesses between cement-cells and glass ring-cells, the Editor has found nc kind more convenient than the rings stamped out of tin, of various thicknesses. These, after being cemented to the slides, should have their surfaces made perfectly flat by rubbing on a piece of fine grit or a corundum-file, and then smoothed on a Water-of-Ayr stone : to such surfaces the glass covers will be found to adhere with great tenacity. The ebonite and bone-cells are cheap, and also easy of manipulation. They are specially useful for dry mounts. The glass slides and cells which are to be attached to each other must first be heated on the mounting plate ; and some small cuttings of marine glue are then to be placed either upon that surface of the cell which is to be attached, or upon that portion of the slide on which it is to lie, the former being perhaps preferable. When they begin to melt, they may be worked over the surface of attachment by means of a needle point ; and in this manner the melted glue may be uniformly spread, care being taken to pick out any of the small gritty particles which this cement sometimes contains. When the surface of attachment is thus com- pletely covered with liquefied glue, the cell is to be taken up with a pair of forceps, turned over, and deposited in its proper place on the slide ; and it is then to be firmly pressed down with a stick (such as the handle of the needle), or with a piece of flat wood, so as to squeeze out any superfluous glue from beneath. If any air-bubbles should be seen between the cell and the slide, these should if possible be got rid of by pressure, or by slightly moving the cell from side to side ; but if their presence results, as is some- times the case, from deficiency of cement at that point, the cell must be lifted off again, and more glue applied at the required spot. Sometimes, in spite of care, the glue becomes hardened and blackened by overheating ; and as it will not then stick well to the glass, it is preferable not to attempt to proceed, but to lift off the cell from the slide, to let it cool, scrape off the overheated glue, and then repeat the process. When the cementing has been satis- factorily accomplished, the slides should be allowed to cool gradually in order to secure the firm adhesion of the glue : and this is readily accomplished, in the first instance, by pushing each, as it is finished, Fig. 326.—Glass ring-cells. 388 towards one of the extremities of the plate. If two plates are in> use, the heated plate may then be readily moved away upon the ring which supports it, the other being brought down in its place ; and as the heated plate will be some little time in cooling, the firm attach- ment of the cells will be secured. If, on the other hand, there be- only a single plate, and the operator desire to proceed at once in mounting more cells, the slides already completed should be carefully removed from it, and laid upon a wooden surface, the slow conduc- tion of which will prevent them from cooling too fast. Before they are quite cold, the superfluous glue should be scraped from the glass with a small chisel or awl, and the surface should then be carefully cleansed with a solution of potash, which may be rubbed upon it with a piece of rag covering a stick shaped like a chisel. The cells, should next be washed with a hard brush and soap and water, and may be finally cleansed by rubbing with a little weak spirit and a soft cloth. In cases in which appearance is not of much consequence, and especially in those in which the cell is to be used for mounting large opaque objects, it is decidedly preferable not to scrape off the glue too closely round the edges of attachment, as the ‘hold’ is much firmer, and the probability of the penetration of air or fluid much less, if the immediate margin of glue be left both outside and inside the cell. To those to whom time is of value, it is recommended that all cells which require marine glue cementing be purchased from the dealers in microscopic apparatus, and it is well to note that all cells cemented with marine <_due should be well ‘ payed,’ as the nautical expression is, or well surrounded with) shellac varnish, or gold- size as indicated by the nature of the enclosed fluid. Many media, saline fluids especially, work their way between the cell and the slide, and at length destroy the marine- glue. Plate-glass Cells.— Where large shallow cells with flat bottoms are re- quired (as for mounting zoophytes, small medusae, &c.), they may be made by drilling holes in pieces of plate-glass of various, sizes, shapes, and thick- nesses (fig. 327, A), which are then cemented to glass slides with marine glue. By drilling two holes at a suitable distance, and cutting out the piece between them, any required elongation of the cavity may be obtained (B, C, D). Sunk-cells.—This name is given to round or oval hollows, exca- vated by grinding in the substance of glass slides, which for this PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS Fig. 327.—Plate-glass cells. purpose, should be thicker than ordinary. They are shown in lig. 328, A, B, C. Such cells have the advantage not only of comparative cheapness, but also of durability, as they are not liable to injury by a sudden jar, such as sometimes causes the detachment of a cemented plate or ring. For ob- jects whose shape adapts them to the form and depth of the cavity, such cells will be found very convenient. It naturally suggests itself as an ob- jection to the use of such cells that the concavity of their bottom must so deflect the light-rays as to distort or obscure the image ; but as the cavity is tilled either with water or some other liquid of higher refractive power, the deflection is so slight *as to be practically in- operative. Before mount- ing objects in such cells the microscopist should see that their concave surfaces are free from scratches or roughnesses. Built-up Cells.—When cells are required of forms or dimensions, not otherwise procurable, they may be built up of separate pieces of glass cemented together. Large shallow cells, suitable for mounting zoophytes or similar flat objects, may be easily constructed after the following method : A piece of plate-glass, of a thickness that shall give the desired depth to the cell, is to be cut to the dimensions of its outside wall; and a strip is then to be cut off with the diamond from each of its edges, of .such breadth as shall leave the interior piece equal in its dimensions to the cavity of the cell that is desired. This piece being rejected, the four strips are then to be cemented upon the glass slide in their original posi- tion, so that the diamond- cuts shall tit together with the most exact precision ; and the upper surface is then to be ground flat with emery upon a pewter plate and left rough. The perfect construc- tion of large deep cells of this kind, as shown in fig. 329, A, B, how- GROUND-OUT AND BUILT-UP CELLS 38 9 Fig. 328.—Plate-glass sunk-cells. Fig. 329.—Built-up cells. 390 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS' over, requires a nicety of workmanship which few amateurs possess,, and the expenditure of more time than microscopists generally have- to spare ; and as it is consequently preferable to obtain them ready- made, directions for making them need not be here given. Wooden Slides for Opaque Objects. Such ‘dry’objects as fora- mini/era, the capsules of mosses, parts of insects, and the like, may he conveniently mounted in a very simple form of wooden slide (first devised by the Author and now come into general use), which also- serves as a protective ‘ cell.’ Let a number of slips of mahogany or cedar be provided, each of the 3-inch by 1-inch size, and of any thickness that may be found convenient, with a corresponding- number of slips of card of the same dimensions, and of pieces of turns round an axis on the column It ; to it is joined the arm L, which carries in front the fine tube r (connected with tt'), and also the rod 2) • the latter is movable perpendicularly, and to its lower end a bridge or grip with two small rollers i and i' is fastened. The rod p is so placed that on each side of the metal strip b, screwed on to the knife-support, there is one of the rollers. By the adjusting screws Ht, the whole apparatus is so arranged that, when the knife- carrier is in motion, no other friction occurs than that of the rollers on the strip b b b. The vessel is filled by screwing off the head Z. As the tube r acts as a siphon, it is necessary when the cock is turned on to blow down the tube. The stream of spirit should be directed at a right angle to the knife, and about the middle of the object. This done, the object Ob, by means of the screw K, is firmly grasped in the fangs of the object-carrier; the correct direction for the position of the knife is given to its surface by the screws at f and J\, and then the axes of the fangs are tightened up by the levers q and q'. If the height of the object is not quite correct, adjustment is made by the screw m. By turning the screws S, S, the holder is fixed. V is a wheel with cranked axle Ew, and this by means of a cat- gut band moves the knife. For the rapid production of ribbons of sections, however, the instrument par excellence is the Cambridge rocking microtome. It is illustrated in fig. 353. The principle is the employment of a rotary instead of a sliding movement of the parts. Two uprights are cast on the base-plate, and are provided with slots at the top, into which the razor is placed and clamped by two screws with milled heads. The inner face of the slot is so made as to give the razor that inclination which has in practice been found most advantageous. The razor is thus clamped between a flat surface and a screw acting in the middle of the blade, and the edge of the razor is consequently in no way injured. The imbedded object is cemented with paraffin into a brass tube which fits tightly on to the end of a cast-iron lever. This tube can be made to slide backwards or forwards, so as to bring the imbedded object near to the razor ready for adjusting. The cast-iron lever is pivoted at about 3 in. from the end of the tube. To the other end of this lever is attached a cord by which the motion is given, and the object to be cut brought across the edge of the razor. The bear- ings of the pivot are V-shaped grooves, which themselves form part of another pivoted system. Immediately under the first pair of V’s is another pair of inverted V’s, which rest on a rod fixed to two uprights cast on the base-plate. A horizontal arm projects at right angles to the plane of the two sets of V’s, the whole being parts of the same casting. On the end of the horizontal arm is a boss with a hole in it, through which a screw passes freely. The bottom of the boss is turned out spheri- cally, and into it fits a spherical nut working on the screw. The nut is prevented from turning by a pin passing loosely through a slot THE ROCKING MICROTOME 409 in the boss. The bottom of the screw rests on a pin fixed in the base-plate. It will be seen that the effect of turning the screw is to raise or Fig. 853.—The Cambridge rocking microtome. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS lower the end of the horizontal arm, and therefore to move backwards or forwards the upper pair of V’s, and with them the lever and object to be cut. The top of the screw is provided with a milled head, which may be used to adjust the object to the cutting distance. The distance between the centres of the two pivoted systems is 1 in. and the distance of the screw from the fixed rod is 6j- in. The thread of the screw is 25 to the inch ; thus, if the screw is turned once round,. the object to be cut will be moved forward of ~ , or in. The turning of the screw is effected automatically as follows : A wheel with a milling on the edge is fixed to the bottom of the screw : an arm to which a pawl is attached rotates about the pin which supports the screw. This arm is moved backwards and for- wards by hand or by a cord attached to any convenient motor. When the arm is moved forward the pawl engages in the milling and turns the wheel; when the arm is moved back the pawl slips over the milling without turning the wheel. A stop acting against the pawl itself prevents any possibility of the wheel turning, by its own momentum, more than the required amount. The ai-m is always moved backwards and forwards, between two stops, a definite amount, but the amount the wheel is turned is varied by an adjustable sector, which engages a pin fixed to the pawl and prevents the pawl from engaging the milling of the wheel. By adjusting the position of this sector, the feed can be varied from nothing to about -.-/V of a turn; and hence, since the screw has 25 threads to the inch, the thickness of the sections cut can be varied from a minimum, depending on the perfection with which the razor is sharpened, to a 5 11 1 maximum of — of — of • . or ——• of a turn. The practical mini- 32 25 6-]- 1000 mum thickness obtainable with a good razor is approximately inch. The value of the teeth on the milled wheel are as follows :— 1 tooth of the milled wheel = i5i65 in. = -000025 mm. 2 teeth „ „ =£oiroo in- = ’001250 mm. 4 » » »» =10560 in. = -0025 mm. 16 v j. >, =251oo in. = 01 mm. The movement of the lever which carries the imbedded object is effected by a string attached to one end of the level-. This string passes under a pulley and is fastened to the arm carrying the pawl. Attached to the other end of the lever is a spring pulling downwards. When the arm is moved forward the feed takes place, the string is pulled, the imbedded object is raised past the razor, and the spring is stretched. When the arm is allowed to move back, the spring draws the imbedded object across the edge of the razor, and the sec- tion is cut. The string is attached to the lever by a screw which allows the position of the imbedded object to be adjusted, so that at the end of the forward stroke it is only just past the edge of the razor. This is an important adjustment, as it causes the razor to com- mence the cut when the object is travelling slowly, and produces the most favourable conditions for the sections to adhere to each other. The following are perhaps the most prominent advantages of this ETHER FREEZING MICROTOMES 411 instrument : (1) the price is low, one-sixth that of the original form. (2) Less skill is required from the operator, for the endless silk band is superseded, and the troublesome and difficult operation of lifting the first sections from the razor on to the silk band is entirely avoided. The ribbon of sections now falls of its own weight direct from the- razor on to a piece of paper or glass slide placed to receive them, and by occasionally moving the paper forward any length of ribbon can be obtained. (3) The razor is fixed at what has in practice been found the most advantageous inclination and angle for cutting ; and thus an unnecessary adjustment and waste of time are avoided. (4) The imbedded object is with great ease and quickness brought up to or away from the edge of the razor; first, for large amounts by sliding backwards or forwards the brass tube on the cast-iron lever, then for smaller amounts by turning round the screw, when the pawl is out of gear, by means of a small milled head placed on the top for this purpose. (5) There are no delicate working parts which can get out of order, and the whole instrument is easily taken apart for packing and is very portable. But it is needful also to describe one or more of the best in- struments designed specially for cutting sections by congelation, or freezing of the imbedding mass. Dr. R. A. Hayes designed an ether freezing microtome with the object of affording to those who have occasional need to cut sections of tissues for pathological investigations ckc. the means of doing so quickly, conveniently, and Fig. 354.—Dr. Hayes’s ether freezing microtome. accurately. It is illustrated in fig. 354. It is very compact, solidly constructed, and simple in plan. It freezes rapidly, and permits sections of large surface to be made with precision; sections 1 in. X f in. having been cut by it without difficulty. It consists of a solid cast-iron base, A, 10 in. x 4| in., which rests upon a mahogany block. Extending the whole length of the upper surface of the base is a V-shaped gutter, on the planed sides of which slides a heavy metal block, B, on the flat top of which the razor is secured (any ordinary razor can be used), the tang being grasped 412 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS between two flat pieces of iron, which are pressed together by a winged nut, C. The razor by this arrangement can be secured at

by (xxxvi) : <0 = , 1 --TT73 {x (xxxvi) 8/4 (/a-1)/-5 fi — lj - £ + £ (x - ®)2 4J2 + ® v~ (xxxvii) The aberration 8 P'= — a>P'2?/2 . . . (xxxviii) To find the aberration of two lenses in contact. Let Q and Q' be the object and its conjugate at the second lens,/' be the focus of the second lens, and F the focus of the combination; then P' = — Q. *=i-A. . . . (xx); JL = A+1 /xx\ P' / P V ' Q' /' Q y ] for the first lens, A = *-p + o>^2 .... (xxxvii) 111 for the second lens, — = - + + a'yi . . . (xxxvii) for both lenses, = y ~ p + (« + «') y2 . • (xxxix) Therefore, for n lenses, A_ = 2 + 2 coy2 . . (xxxix) J * The aberration 8 Q'= -2a> Q'2?/2 and 8 F = — 2 to F2/2 .... (xl) Example: Two plano-convex lenses of equal foci have their convex surfaces in contact; find the aberration for parallel rays (fig. 7). Then V'~af>f-f' (xxiii) Jj For the first lens r = oo ; therefore x= -1 (xxxiv) ; P = oo ; therefore a = -1 (xxxii) ; and o> = -9- (xxxvi). 2/3 For the second lens s = 00; therefore a; = l (xxxv); A = — ; there- to f 1 Journal B.M.S. USEFUL TO THE MICROSCOPIST 1045 13 • 20 fore a = - 3 (xxxii); &>' = (xxxvi); = ; 6/ 3 f3 90 f= 2 F (xxiii); therefore 2 w = ; v n 24 F3 SF= _2°JV= (xl) 24 F3 6 F ( ; This is half the aberration of an equi-convex lens (fig. 1) of the same focal length as the combination where (xi) If the front lens of the combination be turned round so that its convex surface faces the incident light the aberration is 8F--F2-$ w or half what it was before (fig. 5). This is nearly a third of the aberration of a plano-convex in the best position (fig. 2), which is = ~\'VJ (viii> The following figures pictorially illustrate spherical aberration in single lenses and in various combinations of two plano-convex lenses, all having the same focus, F, the same aperture, and the same refractive index, §. The dot nearer the lens is the focal point for the marginal, and that farther away the focal point for the central rays; the distance between the dots is the spherical aberration 8 F Fig. 1. Fig. 2. Fig. 8. Fig. 1. An equi-convex, r = F; 8F = —1*6 ~ = -T73 (xi) F Fig. 2. A plano-convex, r = -■ ; A 8F = -1-16 = -T21 (viii) F 7 7 ... Fig. 8. A crossed convex, r = F ; a = — — F (xviii); SF- -1*07 |*= --111 (xvi) Fig. 7. A combination of two pianos with their convex faces in con- tact, the focus / of the first lens being equal to /', that of the second. f The focus of the combination F (xxiii) A 8 F= --838 |2= — ’087 (xl) Fig. 4. The same, only 2/=f; 8 F= -1-611 |2= —-168 (xl) 1046 APPENDICES AND TABLES Fig. 6. The same, only/= 2 f ; 8F= --5 $t* = —-052 (xl) F Fig. 5. The first lens inverted,/=/'; • 7/2 ' SF=--'416|= — -043 (xl) Fig. 8. The same, only 2/=/'; S F = - -6231 = — -065 (xl) F Fig. 9. The same, only/=2/'; dF= - -376 t = — ‘039 (xl) F Fig. 4. Fig. 5. Fig. 6. Fig. 7. Fig. 8. Fig. 9. We see, therefore, that with the same focal length F the aberration of fig. 1 is the greatest, and that of fig. 9 the least. We also see in the com- binations that by decreasing up to a certain point the focus of the first lens the aberration is increased, and vice versa. The best form of a combination of plate glass, p = 1-516, for parallel rays similar in arrange- ment to fig. 9 is when /= . O The Aplanatic Meniscus.—A spherical refracting surface has two aplanatic foci, such that if converging rays, which have their focus at P', meet a convex spherical refracting surface, whose centre of curvature is r, and if the distance between the points P' and r = pr, then those rays will be refracted aplanatically to some other point, say P, which will lie on the same side of the surface as P'. This fact is of great service, because it enables an aplanatic meniscus to be constructed ; thus, if we make r the radius of the curve A, we can make s, the radius of the curve B, a radius from the point P. If, then, P is a radiant, the light travelling from left to right will pass through the curve B without refraction, because P is the centre of the curve B. The light will then pass on unchanged to the curve A, and will by it be refracted aplanatically, as if it had come from P\ P will be negative and P' positive. The formulae for finding r and P' when P is given are : . . , „ r= ; P'= -MP (xli) p + 1 and those for finding r and P when P' is given are : -or p/ . . r = ; P = — — (xlii) u + J u USEFUL TO THE MICROSCOPIST 1047 An excellent combination, suitable for a bull’s-eye, can be made of an aplanatic meniscus and a plano-convex of flint, or a crossed plate lens. The following are the radii of some examples. A doublet of plate glass = 1-516. 1st lens, a meniscus, diam. P7"; r— + '964:" ; s= +T375''. 2nd lens, bi-convex crossed, diam. 2T"; r'= + P816"; s' = — 12D7". The flatter side of the crossed lens to face the meniscus, the distance between the lenses •05", P= T6", P' = 2-425", §F = — T68", angle 70°. A better combination can be made by substituting a flint plano- convex for the second lens, diam. 2T"; = l-62; r' = + 1-83"; S F = — T32". The aberration is therefore ’036" less than before. The aberration may be further reduced by adding another meniscus and by making all the lenses of flint \i — T02. 1st lens, a meniscus, diam. P65" ; r= + -958"; s= + P35". 2nd lens, a meniscus having its concave side facing the convex side of the first lens ; diam. 2-0"; r' = + l-67" ; s' = + 2-55". The third lens is a plano-convex, with its plane side facing the convex side of the second meniscus ; diam. 2T"; r" = + 2-914"; P - 1-55 ; SF= — -0226 ; angle 70°. The aberration is therefore T45" less than that of the first example. The distance between the lenses is ‘05" as before. To find the radii r and s of a lens which will refract light from a point p to point p' with minimum aberration. K- . . (xliii); /= (xx) (H-.1)*'. . 2>+P' K ’ r = + 2l.l (xliv) H (2 p. + 1) K — 4 (p + 1) K ' s = _pr p — rK Let ft be the coefficient of in formulae v, viii, xi, and xvi, then for Vs parallel rays in each particular case the lateral aberration = ft J . (xlvi) 1 'IT* • • Diameter of least circle of aberration = --ft?- (xlvii) Distance of least circle of aberration from focus = - ft J . (xlviii) AYhen the rays are not parallel 1 3 (xlvi) = cop'y3 (xlvii) = <»p'yz (xlviii) = cop'2y2 It is interesting to note that y- = 2 (/x - ....... (xlix) 3 <2/^ Therefore, when p = r. = t. To find m, the magnifying power of simple lenses or magnifying : glasses. Let d be the least distance of distinct vision apart from the lens, and f be the principal or solar focus of the lens. Then m = 1 + (1) It may be of interest to note that formula (xx) on this page may be used do determine the focus of spectacles required to bring the abnormal focus APPENDICES AND TABLES of either a presbyopic or myopic person to a normal focus. Make p the- abnormal, and p' the normal focus ; then /will be the focus of the spectacles required. In both cases p is a negative quantity, because it is on the same side of the lens as p'; it is usual to make p' 10 or 12 inches. Achromatism Let p be the refractive index of a mean ray (E line nearly) fo a certain material, pv that for a blue ray, and pr that for a red ray ; the is- persive power of the material is ——; this is usually written , or w. p-1 ' #* —1 The formula for achromatism is 8 p 1 p' 1 = Q . p-l'f p'-lf' ’ that is, W + p = 0 (li)* The foci of the two lenses are therefore directly as their dispersive powers, and the focus of one will be negative. An aehromatic effect, which is not achromatism in the strict mean- ing of the term, can be obtained with two lenses of the same kind of glass by making d the distance between the lenses : ........ (Hi)' 2 p-f If p is large, / in the denominator may be neglected ; this will make d half the sum of the foci, which is the formula for both the Huyghenian and Bamsden eyepieces ; but when p = f,d is the sum of the foci. Formula: relating to Spherical Mirrors Let p = one focus, p' = its conjugate, / = principal focus, and r = radius of curvature ; then in concave mirrors v'-jr , ,f=zsL.f,r 2 p - r P~J P +P 2 r-*/i P+P' p' f To find p interchange p and p'. If o is the size of an object, and i the size of its image, and v the dis- tance of the image from the principal focal point, then • op' .-of . ... * = -- ; »=*-i (xxn)> p v In convex mirrors prefix a negative sign, thus: r = — 2/, and so on with the other formulae. The formulae for mirrors may be derived from those of lenses by sub- stituting-1 for p ; thus r = — 2/(vii). Let y = the semi-aperture; then the spherical aberration 5/= “J ‘ Vy (v) or (viii)1 A mirror to be aplanatic for parallel rays must have a parabolic curve. A mirror to reflect ray 3 diverging from a point p, so that they may converge aplanatically to another point p', must be elliptical, having «- and p' for its foci. USEFUL TO THE MICROSCOPIST Formula relating to Prisms Let t = the refracting angle of the prism, cf> the angle of incidence on the first surface, + yfr' — t; //■', ' = \f/ = L, and 2 . t + D sin 0 M = —f- (Uv> sin 2 by which formula the refractive indices of media can be found, because both i and D are capable of accurate measurement. Formula relating to Conic Sectio?is Ellipse.—Let A = major axis; a = minor axis. Then Focus = ~ —— (lv) 2 Parabola.—Let A = height; a= base. Then Focus = (lviy 4 A Hyperbola.—Let A = major axis ; a = minor axis. Then Focus = -+ a (lvii) 2 Works consulted:—Coddington,Camb. 1830; Parkinson, Camb.; ‘Ency- clopaedia Britannica’ ; ‘ Journal R.M.S. ’; Heath, Camb. 1887, &c. It will be- seen that several of the formulae have been entirely reset, while some appear now for the first time. APPENDICES AND TABLES APPENDIX F EXAMPLES USEFUL TO THE MICBOSCOPIST Square inch . . . = 10-08045 square millimetres. „ i „ . . . = 6-45148 „ » IT » • • • = 4*48021 „ » T5o. » . . . = *06451 „ „ = 64514-8 „ fi „ T5oo » • . . = 645-148 „ „ Square centimetre = 15*5003 square i inch. „ millimetre = 15*5003 „ ~ „ » 100 n =15*5003 „ i „ >> 10 fx ....... = *1550 ,, ,, ,, » P = '00155 „ „ „ Multiples of the above may be found by multiplying the values given ■by the square of the multiplier. Thus, square i inch = i x 4 ; the square of 4 = 4 x 4 = 16, and 6-45148 x 16 = 103*22368 square millimetres, the answer required. Cubic £ inch . . . . = 32-00508 cubic millimetres. To » . . . . = 16-38662 „ „ „ TT „ • • • • = 9*48300 „ „ Ti5 „ . . . . = *01638 „ „ >, To5o » .... =16386*62 „ ix Cubic centimetre = 61*02537 cubic ~5 inch. „ millimetre = 61*02537 „ „ „ =61*02537 „ jig „ „ 10 ix = *061025 „ „ „ „ fx = *000061025 „ „ „ Multiples of the above may be found by multiplying the values given by the cube of the multiplier. Thus, 2 cubic millimetres: 2 cubed = 2 x 2 x 2 = 8, and 61*02537x8 = 488*20296 cubic Tg5 inch, the answer required. Areas of Circles inch diameter = T22718 sq. ~ inch = 7*9171 sq. millimetres, /o » .. = ‘78589816 „ „ „ = 5*0670 „ T5 „ „ = ‘54541 „ „ „ = 3-5187 „ Too ” >> = '<8540 ,, ygg ,, = -0506/ ,, „ = 50670-0 „ fji To5o » » = '78540 „ jggg „ = 506*7 „ „ 1 millimetre diam. = *78539816 sq. mm. = 12-1737 sq. gi inch. ... . = 7854*0 „ ft = 12-1737 „ TJgg „ 10 fi .... = 78*54 „ „ = *1217 „ „ „ P = '7854 „ „ = *001217 „ „ „ USEFUL TO THE MICROSCOPIST 1051 Multiples of any of the above may be obtained in the same manner as in the preceding example. Thus, if the diameter of the circle = T§5 inch, then the square of 3 being 9 and -7854 x 9 = 7-0686 sq. inch and "05067 x 9 = -45603 sq. millimetre, are the areas required. Volumes of Spheres in. diameter . = 1'02266 cubic inch= 16-758 cubic millimetres. h .. » • = -52360 „ „ = 8-580 „ jj >, »> • = *30301 „ „ = 4-965 „ „ jlo » • = -52360 „ „ = -00858 „ looo >> »> • = *52360 ,, j(joo » = 8580*0 ,, p 1 mm. diam. . . . = ‘52360 cubic mm. = 31*953 cubic inch. 100 p „ ... =523600-0 „ p =31-953 „ „ 10 p „ . . . = 523-60 „ „ = -03195 „ „ „ p „ . . . = -52360 „ „ = -00003195 „ Multiples of any of the above follow the preceding example of cubic measures. Thus, if the diameter of the sphere = 30 p, then the cube of 3 being 27 and 523-6 x 27 = 14137*2 cubic p and -03195 x 27 = *86265 cubic inch, are the volumes required. APPENDICES AND TABLES 1052 APPENDIX G USEFUL NUMBERS AND FORMULAE Paris line = -088813783 inch. Statute mile = 5280 ft. = 1609-330 metres. Geographical mile = 6082-66 ft. = 1853-978 metres. Nautical mile = 6080 ft. = 1853-167 metres. Cubic foot of water weighs 62-2786 lb. avoirdupois at 62° Fahr. Cubic inch of water weighs 252-286 grs. at 62° Fahr. Gallon of water weighs 10 lb. avoirdupois at 62° Fahr. 1 gallon = 277-46288 cubic inches. Cubic foot of sea water weighs 63-963 lb. Weight of sea water = 1-027 weight of fresh water. 1 inch of rainfall = 100 tons per acre. Pressure of water in lb. per sq. inch = -433 head of water. Expansion of water between 32° Fahr. and 212° Fahr. = *04775. Dip of horizon in nautical miles = 1-23 \/height. Marks on hand lead line for sea soundings 1, 2, and 3 fathoms, 1, 2, and 3 tags of leather respectively; 5 and 15 fathoms white rag; 7 and 17 fathoms red rag; 10 fathoms leather with hole in it; 13 fathoms blue rag ; 20 fathoms 2 knots ; 30 fathoms 3 knots &c. A small knot is placed at intermediate 5 fathoms after 20 fathoms—viz. at 25, 35, 45, &c. Pressure of wind in lb. per sq. foot = 0-002288 (velocity in feet per second)2. Areas and Volumes. Area of triangle = base x £ perpendicular. Volume of wedge = area of base x £ perpendicular height. Volume of cone or pyramid = area of base x ± perpendicular height. Surface of side of cone = circumference of base x £ length of side. Area of parabola = base x § height. Velocity of light = 187,272 statute miles per second.1 'Wave-length of yellow light = —^— inch. Number of vibrations per second 593,270,600,000,000. Falling Bodies. S, space fallen in feet; V, velocity in feet per second ; g = 322; t, time in seconds. S = Y = V27 • VS= 8-025^/s. Arithmetical Progression A, first term ; B, last term; S, sum ; d, difference between terms; n,. number of terms. 1 Latest determinations by Young and Forbes with Fizeau’s method. USEFUL TO THE MICROSCOPIST 1053 A = B-d(w-l). B = A + . S = Em~A. m ("_1 m—i Properties of Circles and Spheres dc. tr . . =3-14159205358979+ log tt . =0-4971499 7r3 = 9-86902 \/tv . =1-77245 1 = -31831 7r h, . . = -10132 JL= -56419 " . >/«■ | . . =-52360 = 1-41421 = 8-02496 Circumference, C. Area, A. Radius, r. Diameter, d. Volume, V. Surface, S. C = 2 7r r — 7T d. A = 7T r2. S = tt<72. Y=~.d=9. 6 TT Area of sector of circle -clesrees 2 ar0 x area °Lc5?le. 360 Length of arc = number of degrees x -017453 r. Unit of circular measure = 57°-29578. Side of square of equal area to a circle = r Vn. Side of inscribed square = r 2 Area of ellipse = £ major axis x | minor axis x tt. Volume of ellipsoid = major axis x (minor axis)2 x 6 Number of Threads in Whitworth's Standard Screws Sizes TV No. of threads 48 »>&••••• »> 40 „ i „ 20 >> s' » 10 »£••••• » 12 Convenient Approximations for rapid Calculations 6 knots = 7 miles, more correctly 13 knots = 15 miles. 5 kilometres = 3 „ „ „ 50 kilometres = 31 ,, 1 metre = 3 ft. 3| in. „ „ 04 metres = 70 yards. 5 centimetres = 2 inches „ „ 33 centimetres = 13 inches. 3 millimetres = § inch „ „ 5 millimetres = | inch. 1 pole = 5 metres; 1 furlong = 2 hectometres. 5 fi = sijjjy inch ; inch = { mm.; XTniW inch = |/x. 2 are = 239 sq. yds.; 1 rood = 10 are ; 2 acres = 81 are ; 100 hectare = 247 acres; 3 cubic yards = 23 decisteres; 1 decastere = 13 cubic yards ; 2 millilitres = 34 iq_ (minims); 2 decilitres = 7 / 3 (ounces); 4 litres 1054 APPENDICES AND TABLES USEFUL TO THE MICKOSCOPIST = 7 pints (imperial) ; 2 grammes = 31 grains ; 4 grammes = 15 (drachm)- (apothecaries’) ; 7 grammes = 4 dr. (drachms) (avoirdupois). 5 kilogrammes = 11 lb. (avoirdupois). 50 kilogrammes = 1 centner = 1 cwt. Nobert's 19 Band Test Plate Band Lines per inch Band Lines per inch 1 11259-5 r 15 90076-1 5 33778-5 19 112595-1 10 61927-3 Difference between each band = 5629-75. Nobert's last 20 Band Test Plate Band Lines per inch Band Lines pey inch? 1 11259-5 . 15 168892-7 5 56297-6 20 225190-3 10 112595-1 Difference between each band = 11259-5. Convenient Formula for Lantern Projection or Enlargement and t Reduction. Let D be the distance of the screen, and d the distance of the object from the optical centre of the lens, F the equivalent focus of the lens, M the magnifying power or ‘ number of times ’ for enlargement or reduction, then— D = F (M + 1); *-*♦*-£; F Example : It is required to project by a lens of 6 inches equivalent focus a slide having a 3-inch mask so that it may give a 10-ft. disc, what must be the distance of the screen ? Here M the magnification will be 40 times. D = F (M + 1) = 6 (40 + 1) = 246 inches = feet. Note, in a double combination the optical centre may be assumed to be half way between the lenses. To reduce, interchange the object and the screen. INDEX ABB A Abbe (Prof.), his compensation eye-piece, 42, 323 ; binocular eye-piece, 103; stereoscopic eye-piece, 103; achromatic condenser, 212, 256-259, 329; chro- matic condenser, 212,256, 267 ; camera lucida, 237; apertometer, 255, 337 ; condenser, iris-diaphragm fitted to, 259; diffraction theory and homo- geneous immersion, 312, 313; method of testing object-glasses, 326-333 ; test plate, 330, 331; experiments in dif- fraction phenomena, 876 — on amplifying power of lens, 25; on homogeneous immersion, 28; on im- provement of optical glass, 31; on classification of eye-pieces, 34; on principle of microscopic vision, 43, 44, 45; on definition of aperture, 45; on aperture, 48 note; on radiation, 57; on angle of aperture, 60, 61, 62; on diffraction, 63-75; on ‘intercostal points,’ 73; on ‘ penetration,’ 82 ; on over-amplification, 90; on stereoscopic vision, 90, 93; on ‘ aplanatic system,’ 94; on orthoscopic effect, 95 ; on Bams- den’s circles, 107; on solid cones of light, 362 Aberration, 19; positive, 21, 309 note; negative, 21, 27, 309 note) chromatic, 31; spherical, 81, 251, 254, 331; errors of spherical and chromatic, corrected by Boss, 306 Abies balsamea, 383 Abiogenesis, 686 Abraham’s prism, 344 Absorption or dioptrical image, 64 — and diffraction images due to diffrac- tion, 65 note — of light rays, Angstrom’s law, 278 — bands, 273, 274, 275, 277 Abstriction of spores, 562 Acalephce, sexual zobids of polypes, 786 ; relationship to hydroids, 796 ; develop- ment of, 798 ; medusan phase of, 801 Acanthometra xiphicantha, 774; echi- noides, 777 Acanthometrina, 772,776; central capsule of, 776 ACT Acarina, eggs of, 928-929; anatomy of,. 983-936 ; larvae of, 933; nymph of, 933; integument of, 934 ; legs of, 934; eyes- of, 935; classification of, 936 Accommodation, of the eye, 88 ; depth, 8S> Acetabularia, 493 ; pileus of, 493 Acetic acid, as a test for nuclei, 440 Acheta, 911 — campestris, eggs of, 929 Achlya, zoospores of, 494; oospores of,, 495 ; zodsporanges of, 569 — prolifera, 493 and note, 494 Achnanthece, characters of, 545 Achnanthes, frustules of, 517, 544 ; stipe of, 518,544; ‘stauros’ of, 545; struc- ture of frustule, 545 Achnanthes longipes, 545 Achromatic, comparison of, with chro- matic and apochromatic lenses, 315 — condenser, Abbe’s, 212, 256-259, 267 ; Powell and Lealand’s, 251, 258; for ob- servation of pycnogonids, 883 — doublet, Fraunhofer’s, 146; meniscus, 322 — lenses, Charles’s, 146; Marzoli’s, 302; Selligue’s, 803 — objectives, 19,32 ; Powell & Lealand’s. dry, 190; Tully’s, 303 ; Wenham’s, 310; cover slips for use with, 380 — oil condenser, Powell & Lealand’s, 267 Achromatism, 17, 19, 148; in photo- micrography, 33; rise of, 145 ; in- augurated, 313 ; imperfect, causing yellowness, 360 Acineta, 697 ‘ Acinetiform young ’ of Ciliata, 712 note Acinetina, 696; food of, 697 * Acorn ’ monad, 684 ‘Acorn-shells,’ 891 Actinia, reproduction from fragments, 787, 801 — Candida, thread-cells of, 803 — crassicornis, thread-cells of, 803 Actinocyclus, 518, 539, 550 Actinommainerme, 774, 776 Actinophrys, 770 — form of Microgromia, 662 — sol, 662-665 Actinoptychus, 518, 540, 541 1056 INDEX ACT Actinospharium Eichornii, 665 Actinotrocha, 874 Actinozoa,787, 801-806 Actius on myopy, 120 Actuarius on myopy, 120 Adams’ variable microscope, 141; non- achromatic microscope, 146 ‘ Adder’s tongue ’ fern, 604 ; sporanges of, 601 Adipose substance, 969 Adjusting objectives, Ross’s, 306, 309 Adjustment, coarse, 156,157,185; Wale’s, 185 — fine, 157-164 • long lever system, 151; Ross’s, 151, 161; to Pritchard’s microscope, 151; applied to stage, 153; Powell’s, 161; short side lever, 162 ; Swift’s vertical side lever, 162, 181; Camp- bell’s differential screw, 164, 193; to the sub-stage, Nelson’s, 169; for Powell & Lealand’s sub-stage, 174; in Beck’s No. 1,180 ; Wale’s, 185 ; in Beck’s third-class microscope, 190 •— screw collar, 309 AUcidiospore generation of Puccinia, 566 JEcidium berberidis, relation to Puc- cinia, 566 -— tussilaginis, 567 JEthaliurn septicum, plasmode of, 563 Agaricus, 575 -— campestris, 576, 577 Agate, 1017 Agave, leaf of, 611; raphides of, 621 Agrion, 911 — pulchellum, wing of, as test for defi- nition, 368 •—puella, pupa of, 918; wing of, 918 Air-angle, 50, 78 Air-bubbles, microscopic appearance of, 370, 371 Air-chamber of leaves, 641 Airy’s modification of Huyghenian eye- piece, 321 Alee of Surirella, 535 ‘Alar prolongations’ in Fusulina, 750; in Nummulites, 752, 756; of Calca- rina, 755 Albite, 1001, 1003 Albuminous substances, tests for, 440 Alburnum, 629, 633 Alcohol, as solvent for resins, &c., 441; as hardening agent, 428 Alcyonaria, 801, 803; spines of, imi- tated, 1022 Alcyonian, resembled by polyzoan, 832 Alcyonidium, 832 ; polyzoary of, 833 — gelatinosum, calcareous spicules in, 832 note Alcyonium, 781 — digitatum, 803; spicules of, 804 Alder on branchial sac of Corella, 836 note Alexander on myopy and presbyopy, 120 Alga;, preparation of, 427; included under general term of ‘ thallophytes,’ 470; symbiotic in radiolarians, 773 AND Algal constituents of lichens, 579 Alkaloids, micro-chemical examination of, 1024 Allman’s experiments on luminosity of Noctiluca, 694 Allman on Polyzoa, 833; on the ‘ Haus ’ of Appendicularia, 842 ; on Myrio- thela, 787 note Aloe, raphides of, 621 Alternation of generations in Batracho- spermum, 504; in Fungi, 563; in ferns, 605; in Medusa, 801 Althcea rosea, pollen-grains, 646 Alveolina, 729; resembled by Loftusia, 743; resembled by Fusulina, 750 Amaranthacea, pollen-grains, 646 Amaranthus hypochondriacus, seeds of, 648 Amarouciumproliferum, as example of compound ascidian, 836 Amici suggests oil for immersion lenses, 29 Amici’s invention of immersion system, 27; horizontal microscope, 146; camera lucida, 235; objectives, 304; triple- back objectives, 310; water-immersion objectives, 310; oil-immersion objec- tives, 312 Ammodiscus, 739 Ammothea pycnogonoides, 882 Amoeba, 658, 667-669, 942 Amoeba-phase of Monas, 681 Amoeba proteus, 667 — radiosa, experiments on, 668 Amoeba, cells of sponges resembling, 779 Amoeboid phase of Tetramitus, 686 Amphibians, plates in skin of, 950 Amphibolites, 1000 Ampliioxus, affinities with ascidians, 841 note Amphipleura pellucida, with oblique illumination, 59, 75 ; resolution of, 85 ; markings measured, 230; markings on, 521 Amphistegina, 752; internal cast of, 766 Amphitetras, 543 Amphiuma, red blood-corpuscle of, 960 Amphonyx, haustellium of, 916 Amplification, 83 — linear, 25, 26, 89 ; of images, 45 Ampullaceous sacs of sponges, 780, 781 Anaboena, 491 Anacharis, 458 — alsinastrum, cyclosis in, 613, 615; habitat, 614 Anagallis, raphides of, 621; seeds of, 649 — arvensis, petals of, 644 Anal plate of Antedon, 827 Analgesina, 937, 938 Analyser, 269 Analysing nose-piece, 244 Anarapliidea, 527 Anchor-like plates of Synapta, 819 Andalusite, 1000 Androspore of (Edogonium, 503 INDEX ANE — Anemones, 787. See Actinozoa Anemophilous flowers, 647 Anethum graveolens, seeds of, 649 Angle of incidence, 8; of refraction, 3; of aperture, 61 Angles of aperture, air, balsam, oil, water, 83-87 Angstrom’s law for the absorption of light rays, 273 Anguillula aceti, 869 — fluviatilis, 869 — glutinis, 869 Anguillulce, 869 Angular aperture, 338 of dry objective, 334 ; of oil immer- sion, 334 -of aperture, resolution dependent on, 44 -of water immersion, 334 Angular distribution of rays, 56; grip, 61; semi-aperture, 77 Anguliferce, characters of, 542 Anilin dyes for blue and green stains, 436 Animal kingdom, two divisions of, 652 Animalcule cage, 294 Animalcules, 678. See Rotifera, Infu- soria, Rhizopoda, &c. Animals and plants, differences between, 461 Anisochelse of sponges, 783, 784 Anisonema, 690 Annelida, larvae of, collecting, 459; ma- rine, 872 ; appendages of, 873 ; jaws of, 873; development of, 873 ; eggs of, 874 ; fresh-water, 879 ; luminosity of, 879 ; bibliography, 880; - ‘liver ’ of, 971 Annual layers in trees, 628 Annular cell, Weber’s, 299 — ducts of Phanerogams, 623 — illumination and false images, 362 — illumination for examining perforated membrane of diatom, 362 Annulus of sporange of fern, 601 Anodon, pearls in, 847 ; glochidia of, 857 ; for observation of ciliary motion, 864 Anomia, prismatic layer in, 848 Anopla (Nemertines), 875 Anoplophrya circulans, 702 Anorthite, i003 Antedon, food of, 696 ; pentacrinoid larva of, 825,826; pseudembryo of, 827 Antennae of insects, 911; preparation of, 912, 913 note Antherid of Yaucheria, 492; of Char a, 507, 508; of Fucacece, 556, 557; of Flo- ridece, 561; of Peronosporece, 567; of Marchantia, 590, 592 ; of mosses, 595; of Sphagnacece, 598 ; of ferns, 602; tapetal cells in, 603 Antherozoids, 467, 470 ; of Volvox, 483; of Vaucheria, 492 ; of Sphceroplea, 501; of CEdogonium, 503 ; of Bdtra- chospermum, 504 ; of Chara, 507, 508; of Phceosporece, 556; of Fucacece, 558; of ferns, 603; of Rhizocarpece, 606 AEA ! Anthers, 644 ! Anthony (Dr.) on pseudo-tracheae of fly’s | proboscis, 915 note i A nthophysa, 690 Anthracite coals, 1006 Antirrhinum majus, seed of, 648 Apertometer, 174,833 ; Abbe’s, 255, 337 Tolies’, 333 ; use of, 337 Aperture, in microscopic objectives, 33,- 43-47, 60-67 ; how obtained, 45 ; Abbe on definition of, 45, 48 note — relation of, to power, 82, 83, 311 ; as- certained by vertical illumination. 286 — angular, 49 note, 53, 338 — numerical, 49 note, 53, 76, 333; for dry objective 50; for oil immersion, i 50 ; for water immersion, 50 — numerical, of Zeiss’s apochromatic series of objectives, 318 i — of objective, 332, 333 | — numerical, table of, 84-87 Apertures, relative, 49 Aphctihizomenon, 491 Aphanocapsa, 477 Aphides, wings of, 922, 923 ; agamic re- production in, 930 : Aphodius, antennae of, 912 Apidce, 911 . Apis mellifica, mouth-parts of, 915 Aplanatic system, 20, 23 — objective, use of, 21 — cone, 255 - . — aperture, 257, 262 — foci, Lister’s discovery, 304 Apochromatic objectives, 19, 30, 32, 84, 80, 211; advantages of, 33, 34; objective, Zeiss’s, 314-320 ; dry, 315 ; • comparison of, with chromatic and achromatic lenses, 315; homogeneous objectives, value of, in study of monads, 687; objective, use with • various test scales, 900 — condenser, Powell and Lealand’s, 254 • Apochromatism, 314 Apocynacece, laticiferous tissue of, 620 Apogamy in ferns, 605 Apospory in ferns, 605 Apotheces of lichens, 578, 579 Apparent creation of structure, 68 Appendicularia, 835, 841; pharyngeal sac of, 841; tail of, 842 ; notochord, 842; ‘ Haus ’ of, 842 Apple, raphides in bark of, 621 Apposition, growth by, 463 — mode of growth of starch, 620 Apus, 883, 886; parthenogenesis of, 888 note — cancriformis, carapace of, 886 Aquarium microscopes, 219-225; Collins’s, 221, 222; Schultze’s, 222, 224 Aquatic microscope, 145 Arachnida, 932 — eggs of, 929; related to Pycnogonida, - 883 note; reproductive organs of, 935 Arachnoidiscus, 518, 541 Arachnosphcera obligacantha, 774, 776 1058 ARA Aralia papyrifera, parenchyme of, 611 Araneida, 932 Arcella, 670 Archegones of Vaucheria, 492; of Chara, 507, 508; of Marchantia, 590, 593 ; of mosses, 595, 596; of Sphagnacece, 598; of ferns, 602, 603; of Lyco- podiece, 606; of Rliizocarpece, 606 Archer, on amoebiform phase of Stepha- nosphcera, 485 note; on desmids, 509 note; classification of, 515; on Clatlirulina, 656 note; on rhizopods, 677 Archerina Boltoni, 655 Arctium, stem of, 634 Arcyria flava, sporanges of, 564 Arenacea, 735-739 Arenaceous character of Textularinice, 748 — Foraminifera, varying size of particles in test of, 743 — test of Foraminifera, 735 Arenicola, 872 Areolae of frustule of Coscinodiscus, 520 Areolar connective tissue, 964, 969 Argas, bite of, 936 Argasidce, 936 Argonauta, 853 Argosince, 932 Argulus foliaceus, 890 Aristolochia, stem of, 634 ‘ Aristotle’s lantern ’ of echinids, 814 Aristotle on myopy and presbyopy, 120 Arragonite, 1017 — in shell of Pholas, 848 Arsenic, micro-chemical analysis of, 1023 Artemia, 886 — salina, movement of, 884 ; habitat of, 887 Arteries, 980 Arthrodesmus incus, 498 Abthbopoda, 881-940 ; smallest of, 932 ; eye of, 907 — limbs of Pedalion compared with those of, 718 Arthrosporous Bacteria, 582 Artificial light, 359 ‘ Artificial lightning,’ 607 Ascaris lumbricoides, 868 Asci of Ascomycetes, 571; of lichens, 578 Ascidians, diatoms in stomach of, 544, 552; solitary, 835 ; branchial sac of, 836, 837, 839; circulation in, 836, 839 ; compound, 836; cloaca of, 837; stolons of, 838 ; bibliography of, 838 ; social, 838; general structure of, 840; de- velopment of, 840; tadpole of, 841; affinities with Ampliioxus, 841 note Asclepiadece, pollinium of, 647 Ascogone of Ascomycetes, 572 ; of lichens, 579 Ascomycetes, 571-574; as fungus-con- stituents of lichens, 579 Ascopores of Ascomycetes, 571; of lichens, 578 Asellus aquaticus, ciliated parasite in blood of, 702 INDEX BAC Asilus, eye of, 911 Aspergillus, fermentation by, 575 Asphalte for cells, 386 — varnish, 383 Aspidisca, a phase in development of' Trichoda, 709 Aspidium, indusium of, 600; sori of, 600 Asplanchna, in confinement, 458 Astasia, 475; mouth in, 690 Asteroidea, skeleton of, 815; spines of, 815; larva of, 821 Asterolampra, 524, 539 Aster omphalus, 539 Astromma, 774 Astrophyton, spines of, 815 Astrorhiza, 736, 740 Astrorhizida, 737 Athecata, 792 Athyrium Filix-foemina, apospory in, 605 • Atrium of Noctiluca, 691 Auditory vesicles of Mollusca, 865 Audouin on ‘ muscardine,’ 574 Augite, 995; zonal structure in, 996 Aulacodiscus, 541 — Kittonii, markings on, 521 — Sturtii, markings on, 521 Autofission of diatoms, 523 Auxospore, 528-530 Avanturine, 1016 Avicularia of Polyzoa, 834, 835 ‘ Awns ’ of Chcetocerece, 543 Axile body of tactile papilla, 977 Axinella paradoxa, 782 Axis cylinders of nerve-tube, 975, 976 B Bacillariacece of Kiitzing, 517 Bacillaria paradoxa, movements of, 528, 531, 535 Bacilli, form of, 581 Bacillus, 1 granular spheres ’ of, 588 note — anthracis, 582; spores live in absolute alcohol, 587 — megaterium, 582 — subtilis, 582, 583, 585 ; spores of, 587 — of anthrax, 961 note — of tuberculosis, modes of staining, 438, 439 ‘ Bacon-beetle,’ 904 Bacon (Roger), inventor of simple micro- scope, 128 Bacteria, use of large and small cones in examining, 363; photo-micrographs, 365; as test for definition, 368; preserved by osmic acid, 428; violet of methyl- anilin as a stain for, 437; methyl- blue as a stain for, 438; staining, 437, 438; (see Scliizomycetes), 579; affinities to Algce, 580; to Flagellata, 580; to Nostocacece, 589 ; forms of, 581; move- ments of, 581; mode of multiplication, 581; classification of, 582; nutrition of, 585; flagella of, 587; germinating power of, 587 ; spores of, 587 Bacteriastrum fur cat urn, 543 INDEX 1059 BAC Bacteriology, 589 Bacterium lineola, compared with Cerco- rn onas, 580 — lineola, 586 — termo, flagellum of, 72; movement of, 581; zoogloea of, 585, 586 ; germination of, 587 Bailey, on internal casts of Foraminifera, 753 note Bailey’s method of isolating diatoms, 553 Baker on Cuff’s microscope, 140 Baker’s students’ microscope, 193; optic axis of, 194 — lamp, 350 Balanulce, 891 Balanus balanoides, 891; disc of, 892 Balbiani on supposed sexual reproduction of Ciliata, 709 Balsam angle, 50, 78 — refractive index of, 77 Banksia, stomates of, 641 Barbadoes earth, 771, 774 Bark, 625, 627, 633 Barker’s Gregorian telescope, 144 Bar movement, 215 ‘ Barlow lens ’ applied to a microscope, 147 Barnacles, 891. See Cirripedia Basals of Antedon, 825 Basidiomycetes, 575 ; as fungus-con- stituent of lichens, 579 Basidiospores of Basidiomycetes, 576; of Hymenomycetes, 576 Basids of Puccinia, 566; of Basidiomy- cetes, 576 Bast, 635 Bat, parasite of, 936 ; hair of, 954 ; carti- lage in ear of, 970 ‘ Bathybius,’ 672 Batrachia, red blood-corpuscles, 959; lungs of, 987 Batrachospermece, 503 Batrachospermum moniliforme, 504 — protoneme of, 505 * Battledore scale ’ of Lyccenidce, 899 Bausch and Lomb’s microscope, 185-188 ; chemical microscopes, 217-220; ‘ labo- ratory ’ microscope, 218 ; ‘ University ’ chemical microscope, 219,220 ; neutral tint camera lucida, 235 Bdella, maxillary palps of, 934 Bdellidce, 937 Bdelloida, 717, 718 Bead-moulds, 573 Beale’s microscope for class demonstra- tion, 225 ; camera, 234, 235, 239 ; carmine, 435 ; bioplasm, 435 ; glycerin method of preserving, 444 Beale on organic structure, 942 Beck’s No. 1 microscope, 180, 182 ; sub- stage in, 181, 182 ; small first-class mi- croscope, 189; third-class microscope, 190-192; ‘ Star ’ microscope, 194 ; ‘ eco- nomic ’ microscope, 194,196; histological dissecting microscope, 197, 198; port- able microscope, 199, 202; binocular dissecting microscope, 207; rotatory BIP nose-piece, 242 ; variable condenser,; 260; mode of using parabolic speculum,, 281; light modifier, 284 ; vertical illu- minator, 285 ; disc-holder, 288 ; rings for locking coarse adjustment, 301 ; lamp, 348, 349; achromatic binocular magnifier, 396 note ; disc-holder for examination of Foraminifera, 770 Beck (R.) on markings of Podura scale,. 902 Beck-Jackson model, 162 Bee, hairs of, 904; head of, 906; wing- of, 918, 922 ; sting of, 927 Beeldsnyder’s achromatic objective, 145 Beetles. See Coleoptera Beggiatoa, form of, 581 — alba, 583, 584 Begonia, seeds of, 649 Behrens’ method of analysing minerals,. 1004 Bell (Jeffrey) on the spines of Cidaris,. 813 Bell’s cements, 383, 448 Beneden (Ed. Van), on Gregarina gigantea, 674 note; on movement of gregarines, 675 Benzol, uses of, 441 Bergh on Flagellata, 689 ‘ Bergmehl,’ 551 Berkeleya, 528 Bermuda earth, 538, 540 Beroe, collecting, 459 —- Forskalii, 805 — ovatus, Eimer on, 806 note Bicellaria ciliata, 834 Bichromate of potash, 430 Biconvex lens, formuloe relating to, 21 Biddulphia, 542 — cyclosis in, 517; chains of, 517, 525 ; structure of frustule, 519 note Biddulpliiece, character of, 541 Biflagellate monad, 684 Bignonia, seed of, 648 Bignoniacece, winged seeds, 648 Biloculina, 727 Binary subdivision of cell, 465, 466 Binocular eye-piece, Tolies’, 102; Abbe’sr 103 Binocular magnifier, Beck’s achromatic, 396 note Binocular microscope, 61, 97 Riddell’s, 96 ; Nachet’s, 98 ; stereo- scopic, Wenham’s, 98; Stephenson’s, 100; Stephenson’s erecting, 102; stereoscopic, for study of opaque ob- jects, 105, 107 ; use of, 105 ; non- stereoscopic, 106 ; Powell & Lealand’s. liigh-power, 107 ; portable, Rousselet’s, 200 ; body in Beck’s portable, 200 ; Stephenson’s for dissection, 201, 203, 344, 395; dissecting, Beck’s, 207 7 spectrum microscope, 276 Biology, 460 Bioplasm, 435 ‘ Bipinnaria,’ resemblance of Adi no- trocha to, 874 Bipinnaria astcrigera, 821 INDEX BIR Birch, pollen-grains of, 647 Bird, parasite of, 936, 938; lacunae in bone of, 946 ; epidermic appendages of, 958; red blood-corpuscles of, 958, 959 ; lungs of, 988 Bird’s egg, concretions on shell imitated, 1028 : 4 Bird’s head processes,’ see Avicularia, . 834- - Bismarck brown, 437 Bivalves, structure of ligament in, 964 '‘ Black dot,’ Nelson’s, 233 4 Bladderwrack,’ 556 Blanchard (R.) on osmic acid, 428; on Sporozoa, 674 note Blatta, antennae of, 912 — orientalis, eggs of, 929 Blenny, scales of, 951 Blood, colourless corpuscles, 942; method of mounting, 962 ; circulation of, 978; • flow of, 979; micro-chemical examina- tion, 1024 <— of insects, circulation of, 917, 918; of Vertebrata, 958 Blood-corpuscle, relation of size to that of bone lacunae, 946 Blood-corpuscles of Vertebrata, 958 Blowfly’s maxillary palpus, hairs on, examination of, 365 Blowfly, proboscis of, examination with apochromatic, 318; hairs on, as test for definition, 368 — development of 981; ‘ imaginal discs ’ of, 981 Blue-black, for nerve-cells, 437 Blue glass for softening light, 360 ‘ Blue mould,’571 Bodo, 475 Body of the microscope, 155 ‘ Bog-mosses,’ 598 Boletus, 575 Bombyx, 911 — mori, eggs of, 929 Bonanni’s microscope, 134; his hori- zontal microscope, 135 ; his compound condensers, 248, 249 Bone, 943; structure of, 944; prepara- tion of, 947; matrix of, 968 ; decalcifi- ■ cation of, 426 Bones, fossilised, 1012, 1018 ‘ Bony pike,’ scale of, 946 Borax carmine, 435 Bordered pits in the trache'fdes of conifers, 622, 628 Boscovich on chromatic dispersion, 42 Botryllians, 838 Botryllus violaceus, 839 Botryocystis, 475 Botrytis bassiana, 573 Botterill’s growing slides, 289; his , zoophyte trough, 298 Bouguet on uniform radiation, 51 Bowerbank on sponge spicules, 783 note; on structure of molluscan shells, 845 Bowerbankia, gizzard of, 829; stem of, 832; polyzoaries oi, 833 1060 BUL ‘ Box-mite,’ 936 Brachinus, antennae of, 912 Brachionus rubens, 714, 715, 718; male of, 717 Brachiopoda, shells of, 843, 849, 851 relation of shell to mantle, 850; affinities to Polyzoa, 851 Bracliyourous decapods, young of, 894 Brady (H. B.) on Foraminifera, 785 ; on arenaceous Foraminifera, 736; on; affinity of Carpenteria, 748 Brady and Carpenter on fossil LituoJce 742 Brain, Hill’s method of preparation of,- 484 ‘ Brake-fern,’ 600. See Aspidium Bran, 649 Branchiae of annelids, 872, 873 Branchiopoda, 883; divisions of, 885 Branchipus, movement of, 884 — stagnalis, 886, 887 Branchiura, 889 note, 890 Brandt (K.) on artificial division of Actinosphcerium, 666 note ; on zoiixanthellae, 773 Braun on Pediastrum, 497 Brewster, his hand magnifier, 37 ; on modification of stereoscope, 91 ; on" ‘ lens ’ from Sargon’s palace, 121; his ‘Treatise on the Microscope,’ 122; on. achromatic condensers, 249 Bright-line spectro-micrometer, 274 ; Brightwell on Triceratium, 543 note; on CheetocerecE, 544 note Brilliancy of image, 826 , ‘ Brimstone moth,’ eggs of, 929 Brine shrimp, 884, 887 ‘ Brittle stars,’ 815. See Ophiuroidea Brooke’s nose-piece, 241 Brownian movement, experiments, 373-. 874 Briicke lens, 38 Brunswick black as a black ‘ ground,’ 884 ; for cells, 386 Bryacece, 598 Bryobia, 937 1 Bryony, cells of pollen chambers, 645 Bryozoa, 828. See Polyzoa Bryum inter medium, peristome of, 597 Bubbles in cavities of crystals, 997 Buccinum, 861 — undatum, palate of, 854, 856, 857; nidamentum of, 858; Buchner’s experiment on spores of Bacteria, 587 Buckthorn, stem of, 628 Bug, mounting medium for, 897 Bugula, polyzoary of, 883 — avicularia, 884, 835 Built-up ‘ cells,’ 889 Bulbils of Nitella, 507 Bulloch’s modification of Zentmayer’s microscope, 184, 186, 187 Bull’s-eye, 250; use of, 278, 279, 351, 857 ; with high power, 280; for use in study of saprophytic organisms, 280 ; Powell and Lealand’s, 280 INDEX BUB Bull’s-eye stand, 201 ’ - • ■ Bundle-sheath, 635 Burdock, stem of, 634 Butschli on Bhizopoda and Sporozoa, 677; on mouth of Astasia, 690; on — Vorticellce, 701 note — and Engelmann on conjugating vorti- cellids, 711 Butterflies, wing of, 918 Butterfly. See Lepidoptera C •Cabbage-butterfly, eye of, 907; number of facets in, 907 ; eggs of, 929 Caberea Boryi, vibracula of, 831 note Cabinet for slides, 454; arrangement of, 454 Cactus, cells of pollen-chambers, 645 Cactus senilis, raphides of, 621; brittle- ness of, 621 Cacumaria crocea, development of, 824 note ■Calamites, 1005 Calathus, antennae of, 912 Calcarina, 750, 755; compared with Fozob.n, 763 Calcispongice, spicules of, 783 Calcite in shells, 848 Calco-globuline, 1022 Callithamnion, 559 Calosanthes indica, winged seed of, 649 Calotte diaphragms, 247 Calycanthus, bark of, 634 Calycine monad, 685 Calycles of hydroids, 792 Calypter of mosses, 596 Calyx of Flagellata, 689 Cambium, 635 — in Exogens, 622 — layer, 633 Cambridge rocking microtome, 408 Camera lucida, 233; Beale’s, 234, 235, 239; Soemmering’s, 234 ; Wollaston’s, 234 ; Amici’s, 235 ; Bausch and Lomb’s, 235; Schroder’s, 236; Abbe’s, 237 Campani’s microscope, 130; eye-piece, 321 Campanula, pollen-grain of, 646 Campanularia, 794 — gelatinosa, 789 Campanulariida, 794; zoophytic stage of, 801 Campbell’s differential screw, 158, 193; j adapted to the Continental model, 164; I fine adjustment, 164, 193; used in photo-micrography, 194 Campylodiscus, 518, 524, 536; move- j ments of, 531; structure of frustule, 536 — clypeus, 536 — spiralis, cyclosis in, 517 Canada .balsam, 383 as a preservative medium, 441; mode of preparation, 441; as mounting j medium, 444, 449; refractive index, CAH x 445 ; capped jars for, 447; for mounts ing insects, 897 Canal system of Calcarina, 750; of Polystomella, 752; of Nummulites, 752 Canaliculi of bone, 943, 945 Cancellated structure of bone, 944 Cancer pagurus, skeleton of, 892 Canna, starch-grains of, 620 Cannel coals, 1006 Cannocchiale, 127 Capacity of object-glass, 326 , Capillaries, 980, 986 Capillitium of Myxomycetes, 565 Capsule, central, of Badiolana, 772 — of mosses, 595 ; of Purpura, 858 — silicious, of Clathrulina, 666 Carapace of Copepoda, 884; of Qlador cera, 885 Carbolic acid: for mounting prepara- tions, 442 ; for dehydration, 450 • Carbon bisulphide as a solvent for oils,. &c., 441 Carboniferous epoch, vegetation of, 606 — limestone, 1011 Carchesium, collecting, 457 Carcinus mcenas, metamorphosis of, 894 Carnallite, 998 Carnation, parenchyme of, 613 Carnivora, arrangement of enamel in, 949 Carp, scales of, 951 Carpenter (H. P.) on crinoids, 827 note Carpenter (W. B.) on stereoscopic vision, 93 ; on classification of Foraminiferq y 724 ; on Eozoon, 763; on alternation of generations in Medusce, 801; on the so- called excretory pores of Ctenophora, 806-note; on development of Antedon, 827 note; on structure of molluscan shells, 845 , Carpenteria, 747; mode of growth com- pared with EozoOn, 763 — rhaphidodendron, 748 , Carpogone of Floridece, 561; of Ascomy- cetes, 572 Carpospores of Floridece, 561 , Carrot, seeds of, 649 Carter (H. J.) on affinity of Carpenteriar, 748 Cartilage, 970; mounting, 971 , Carum carui, seeds of, 649 ' _ . C ary ophy Ilia, lamelke of, 802 — Smithii, thread-cell of, 803 , Cascarilla, raphides of, 621 Cassowary, egg-shell of, 1021 Castracane, on beaded structure of di- atoms, 522; on Pfitzer’s auxospOres, 523, 524; on sporangial frustules pf diatoms, 524; on reproduction of dia- toms, 526 ; on diatoms, 528 Cat, Pacinian corpuscles of, 977 Catadioptric illuminator, Stephenson’.s,. 170, 263-265 Caterpillars, ‘ pro-legs ’ of, 926 ; feet of, 926 Catlicart’s freezing microtome, 412, 413 1062 INDEX CAT •Catoptric form of microscope, 144 Cattle-plague, 588 Caulerpa, 493 Cauterisation by focussing the sun’s rays (Pliny), 119 Cedar, stem of, 630 Cell, contents of, 463-465; binary sub- division of, 465 Cell ’ of Polyzoa, 828 Cell-division and nucleus, 943 and note Cell-sap, 464 Cell-structure, Strasburger on, 467 Cell-wall, 463; mode of growth of, 463; apposition, 463 ; intussusception, 463 — of Phanerogams, 617 Cells of plants, 462; multi-nucleated, 464 ; primordial, 465 ; of vertebrates, 942 Celloidin as an imbedding mass, 417 ; as congelation mass, 419; for freezing, Hill’s method, 419; clearing agents ‘ Cells ’ for mounting Infusoria, &c., 299; for dry mounting, 385; sunk, 388 ; of cement, 382; paraffin, 386 ; paper, 386 ; of plate glass, for zoophytes, &c., 388; built up, 389; mounting in, 451, 452; of bone, 452 ; of tin, 452 for, 419; Scbering’s, 419 Cellular cartilage, 970 — parenchyme, 613 Cellulose, 463 — tests for, 440; envelope of desmids, 509 ; in Dinojlagellata, 695; in zoo- cytium of Ophrydium, 706 Cement-cells, 386 Cements, 382 ; liquid, 382 ; Bell’s, 383, 448; japanner’s gold size, 388 ; Bruns- wick black, 384 ; glue and honey, 384 ; shellac, 384; Hollis’s liquid glue, 384, 449 ; Venice turpentine, 884 ; marine glue, 385 ; Heller’s porcelain, 445 Cementum of teeth, 949, 950 Centipedes. See Myriopoda Central capsule of Badiolaria, 659 Centring, 826 Centring nose-piece, 242; as sub-stage, 193 Centro-dorsal plate of Antedon, 825 Cephalolithis sylvina, 771 Cephalophorous mollusca, palates of, 854-857 Cephalopoda, 853 — organs of hearing in, 865 ; chromato- phores of, 866 Ceramiacece, 559 <7eramium, 559 Gnathostomata (Crustacean), 889 note Goadby’s solution for mounting cartilage, 971 i Goes (Dr.) on affinity of Carpenteria, 748 Goette on development of Antedon, 827 I Gold size, 388 : Gomphonema, stipe of, 518, 544; move- ments of, 581; attacked by Vampyrella, 655 | — geminatum, 545, 546; stipe of, 545 : — gracile, 551 | Gomvhonemecp, characters of, 545 Goniaster equestris, spines of, 815 i Gonidial cells, 470 — fructification, 470 — layer of lichens, 577 Gonidiophores of Peronosporece, 568 Gonids, or non-sexual spores of Crypto- gams, 470 note; of Vaucheria, 492; of Podosphenia, 526 ; of Floride.ee, 561; of Fungi, 562; of Peronosporece, 568 i Goniocidaris florigera, spine of, 812 Gonium, 475 Gonotliecae of Campanulariida, 794 Gonozoid of hydroids, 792; of Syncoryne, 793; of Tubularia, 793 Gonozoids of Sertulariida, 794 Gordius, 868, 869 Gorgonia, spicules in, 853 — guttata, spicules of, 804 Gorgonia!, 801; spicules of, in mud of Levant, 1007 Goring (Dr.) on magnification of objects, 44 ‘ Gory dew,’ due to Pahnella cruenta, 486 Govi on invention of microscope bv Galileo, 122 Graduated rotary stage, 338 Gramma top kora, chains of, 517, 534 — angulosa, 550 — marina, 587 GRA Grammatophora parallela, 550 — serpentina, 536 — subtilissima, 537 Granite, 1016 — fluid inclusions in, 997 Grantia, 781, 785; spicule of, 1008 Grasses, nodes of, 626; silex in epiderm of, 629 ; paleas of, 640 ; seed of, 649 Grasshopper, gizzard of, 917; wings of, 928 Greensands, microscopic constituents of, 1012 Gregarina, characters of, 674; movement of, 675 — gigantea, in lobster, 674 note — Scenuridis, 676 Gre gar inula, 674 Gregory (J. W.) on Eozoon, 768 note Gregory (W.) on species of diatoms, 530 note Greville on Spatangidium, 539 ; on Triceratium, 548 note Grey matter, 976 Griffith’s turn-table, 391 Griffithsia, 559 Grinding sections of hard substances, 420 Grindl’s microscope, 134 Gromia, 659, 660, 721 — and Arsella, pseudopodia of, con- trasted, 671 Ground-mass of rocks, 995 Groundsel, pollen-grains of, 647 Growing slides, Botterill’s, 289; Mad- dox’s, 289, 290; Lewis’s, 289 Guard-cells, 640 ‘ Gulf-weed,’ 559 Gum and glycerin, 443; and syrup, as a preservative medium, 443 — imbedding for vegetable substances, 427 — arabic, formula, 385; for freezing, 418 — resins, latex of, 620 — styrax, as a mounting medium, 444 ; index of refraction, 445 Gyges, 475 Gymnochroa, 792 Gymnolcemata, 833 Gymnosperms, fossilised, 1005 — generative apparatus in, compared with Cryptogams, 609 Gypsina, 749 H Haddon on budding in Polyzoa, 831 note Haeckel (E.) on Monera, 677 note — on the Gastrcea theory, 677 note — on Badiolaria, 772; on nature of sponges, 789; on Hydrozobn affinity of Ctenophora, 801 note — and Hertwig on classification of radiolarians, 773 note Hcematococcus, red phase of Proto- coccus, 473 — sanguineus, 486 Hematoxylin, alcoholic solution, 433; INDEX HEM aqueous solution, 432; Weigert’s, 433 ; Hill’s method, 433 Hcemionitis, sori of, 600 Haime (Jules) on development of Tri choda, 707 ‘ Hair-moss,’ 596 * Hair-worm,’ 868 Hairs of leaves, 639; of insects, 904; of Acarina, 934 ; of mammals, 953 Halicaridce, 937 Haliomma Humboldtii, 776 — hystrix, 772 Haliotis (diatom), 542 — (mollusc), shell structure of, 852; palate of, 855 Haliphysema, 739 ; sponge-spicules in, 747 Haller on auditory organs of Acarina, 934 Halteres of Diptera, 924 Hand-magnifier, Brewster’s, 37 Hansgirg on movement of Oscillariacece, 490 Hantzsch’s glycerin method for desmids, 444 Haplophragmium, 739 — globigeriniforme, 738 Hardening agents, 427, 428 absolute alcohol, 428; chromic acid, 428 ; osmic acid, 428 ; picric acid, 428 Hardy’s flat bottle for collecting, 457 Harpalus, antennas of, 912 Harting on Janssen’s microscope, 122 ; his experiments on formation of con- cretions, 1022 Hartnack on immersion system, 27 Hartnack’s model, 210 ; his stage, 211 Hartsoeker’s simple microscope, 135 ; his condenser, 248 ‘ Hart’s-tongue,’ 600. See Scolopen- drium ‘ Harvest-bug,’ 937 ‘ Haus ’ of Appendicularia. 842 Haustellate mouth, 916 Haustellium, 916 Haversian canals in bone, 946, 947 Haycraft (J. B.) on structure of striated muscle fibre, 973 Hayes’s ether freezing microtome, 411; minimum thickness of sections there- with, 412 Hazel, peculiar stem of, 628; pollen- grains of, 647 Hearing, organs of, in Gastropoda, 865 ; in Cephalopoda, 865 Heart of ascidians, 836; of Acarina, 935 Heartsease, pollen-tubes of, 648 ‘ Heart-wood,’ 629 Heating-bath, Mayer’s, 393 Heliopelta, 518, 540 Heliozoa, characters of, 659; examples of, 662-667 ; pseudopodia of, 770 Helix pomatia, teeth of, 854 — hortensis, palate of, 854 Heller’s porcelain cement, 445 Helmholtz on aperture, 47 Hematite in carnallite, 998 1073 1074 INDEX HYD Hooklets on wings of Hymenoptera, 923 Hoplophora, 936 — maxillse of, 934 Hormogones of Oscillariacece, 490; of Rivulariacece, 490; of Scytonemacece, 490 ; of Nostoc, 491 Hormosina globulifera, 738, 740 — Carpenteri, 740 Hornblende, 1001 — corroded crystals of, 995 ; pleochroism in, 1002 Hornet, wing of, 923 ; sting of, 927 Horns, 953, 957 Horny substances, chemical treatment of, 440 ‘ Horse-tails,’ 605. See Equisetacece Hosts of parasitic plants, 462 House-fly. See Musca Hudson on the functions of contractile vesicle of rotifers, 716 note Hudson and Gosse on classification of rotifers, 717 Human blood-corpuscles, 958 — hair, 954 Husk of corn-grains, 644 Huxley on the ectosarc of Amoeba, 668 note; on coccoliths, 672; on Bathybius, 672 ; on Collozoa, 778 note; on structure of molluscan shells, 846; on pulvillus of cockroach, 924 note', on agamic reproduction of Aphis, 930 Huxley’s simple dissecting microscope, 204, 205 Huyghenian eye-piece and spherical aberration, 42 — Airy’s modification of, 321 Hyacinth, raphides of, 621; cells of pollen-chambers, 645; pollen-grains of, 647 Hyaline shells of Foraminifera, 724 Hyalinia cellaria, palate of, 855 Hyalodiscus subtilis, 537 Hyaloplasm, 468 Hydra, collecting, 457; cells of, 786; intracellular digestion in, 787; struc- ture of, 788; reproduction of, 790; gemmation of, 930 — fusca, 787, 789 — viridis, 787 — vulgaris, 787 1 Hydra tuba ’ of Chrysaora, 798, 800 Hydrachnidce, 932'; eyes of, 985; mandible of, 933; reproductive organs of, 936 ; characters of, 937 Hydrangea, number of stomates in, 641; seeds of, 649 Hydrodictyon, 486, 495 — utriculatum, 495 Hydroida, classification of, 792 Hydroids, compound, 791; habitats of, 795; Medusce of, 792; planulee of, 792, 795; structure of, 791 et seq.; examination of, 795; mounting, 795; polariscope with, 796; preparation of, 796 Hydrophilus, antenna; of, 911, 912 Hydrozoa, 787-801 HEM Hemiaster cavernosus, development of, 824 note Hemiptera, eyes of, 911; wings of, 923 ; suctorial mouth of, 923 Hensen’s stripe, 973 Hepaticce, 590; thalloid, 593; foliose, 593; elaters of, compared with spiral cells, &c., of pollen-chamber, 645 Herbivora, arrangement of enamel in teeth of, 949 ; cement in teeth of, 950 Herring, scales of, 982 Herschellian doublet, 257 Hertel’s compound microscope, 137, 138 Hertwig’s research on Microgromia, 660 note ; on Actinia, 801 note Heterocentrotus, spine of, 809 — mammillatus, spine of, 811 Heterocysts of Nostoc, 491 Heteromita uncinata, life-history of, 685 Heterostegina, 759 Heurck (Van) on markings of diatoms, 522 Hexarthra, 718 Hicks on amoebiform phase of Volvox, 485; on preparation of insect antenna;, 913 note; on structure of halteres and elytra, 924 Hill’s (A.) method of using Weigert’s lue- matoxylin, 433 Himantidium, 533 Hipparchia janira, eggs of, 929 Hippopus, 542 Hippothoa, 838 Holland’s triplet, 37 Hollis’s liquid glue, 384 Hollyhock, pollen-grains of, 646, 647 Holothuria botellus, plates of, 819 — edulis, plates of, 819 — inhabilis, plates of, 819 — vagabunda, plates of, 819 Holothurice, diatoms in stomach of, 544, 552 Holothurioidea, skeleton of, 818; pharyn- geal skeleton of, 819 note; plates in skin of, 819 ; preparation of calcareous plates, 820 ; direct development in, 824 note Holtenia Carpenteri, 785 Homeocladia, 528 Homogeneous immersion, 312,313; Abbe’s combination, 313 — immersion lenses of Powell and Lea- land, 29 ; of Zeiss, 29 — objectives, value of, in study of monads, 687 — system, 28 Homoptera, wings of, 922, 923 Hood of mosses, 596 Hoofs, 953, 957 — sections of, mounting, 450 ; for polari- scope, 450 Hooke’s application of field-lens to eye- lens, 321 — compound microscope, 130 Hooked monad, 685 Hooker (J. D.) on diatoms of Antarctic Circle, 549 INDEX HYD Hydrozoa and marine mites, 937 Hyla, nerves of, 978 Hymene of Ascomycetes, 571; of Rasidio- mycetes, 576; of Hymenomycetes, 576 Hymenomycetes, 576; pileus of, 576; stipe of, 576 Hymenoptera, 897 ; eyes of, 911; mouth- parts of, 915; wings of, 922; sting of, 926, 927 ; ovipositor of, 926, 927 Hyoscyamus, spiral cells of pollen- chambers of, 645; seeds of, 649 Hypericum, seeds of, 649 Hyphee of fungi, 562 Hypnospore of Hydrodictyon, 495 Hypnospores, meaning of, 470 note Hypoblast, 651 note Hvpopial stage of Tyroglyphidce, 937 Hypopus, 937 I ‘Ice-plant,’ epiderm of, 689 Ichneumonidce, ovipositor of, 927 Illuminating power, 367 — power of objectives, 54; compared with penetrating power, 336 Illumination for dissection, 344 — for opaque objects, 147 — oblique, 170, 171, 331; in Zentmayer’s microscope, 184 — of objects, Boss on, 250 ; by reflexion, 278; opaque, 281; from the open sky, 355; by diffused daylight, 355; for dark ground, 356; experiments in, 857 ; monochromatic, means of obtain- ing, 360, 361; annular, 362; double, objects for study with, 366; with small cones, as cause of errors in interpretation, 369 Illuminator, Stephenson’s catadioptric, 170, 268-265; oblique, 170; white cloud, 172; Wenham’s reflex, 265, 266; parabolic, 267-269; Swift’s sub- stage, 271; Powell and Lealand’s, 283 ; Smith’s vertical, 284, 285; Beck’s, 285; Tolies’ vertical, 285 Image, real, 14 note; virtual, 14 note, 321; conjugate, 24 ; inverted conjugate, 24 ; aborption or dioptrical, 64 ; diffrac- tion, 64; negative, 64 ; positive, 64 ; solid, 95; real object, 321; definition of, 326 ; formed by compound eye, 908, 909 Images, by diffraction, dioptric and interference, 72 Imaginal discs in larva of blowfly, 931 Imbedding processes, 414; simple, 414; in wax, 415; in paraffin, 415; metal case for, 415 — masses, 416; paraffin, 417; wax, 417; celloidin, 417 — by coagulation or freezing, 418 Immersion lenses and vertical illumina- tors, 285, 286 homogeneous, outcome of Abbe’s theory of diffraction, 312, 313 water, Zeiss’s, 317 INT Immersion lenses, water, Amici’s, 310 ; Powell and Lealand’s, 810, 813 ; Praz- mowski and Hartnack’s, 310; Tolies’, 810 — objectives, 28; examination of, 331 — system, 27-29 ; invented by Amici, 27 Imperfect achromatism, cause of yellow- ness, 360 ‘ Impressionable organs ’ in Ciliata, 702 Incidence, angle of, 3 Incident ray, 2 Incus of Rotifera, 715 Index eye-piece, 325 — of visibility, 445 Indian corn, epiderm of, 637; stomates of, 640 Indigo carmine, 437 Indirect division of nucleus, 468 Indusium in ferns, 600 Inflection of diverging rays, 62 Infusoria, preserved by osmic acid, 428; as food of Actinophrys, 663; Ehrenberg’s work on, 678; ciliate, 679; character of, 679; unicellular nature of, 680 note Infusorial earth, 536, 538, 540, 542, 546, 550, 552, 771; from Barbadoes, 771, 774 Injected preparations, 984 Inoceramus, portions of shell of, in chalk, 1009 Insects, 896-931 — parts of, wooden slides for mounting, 390 — parasitic fungi in, 573-574 — mounting media for, 897 ; integument of, 898; tegumentary appendages of, 898; scales of, 899-904; hairs of, 904; parts of head, 906; eyes, 906-911; antennas of, 911; mouth-parts of, 913; circulation of blood, 917 ; alimentary canal, 917 ; wings of, 918, 922-924; tracheae of, 918 ; stigmata of, 919 ; sound-producing apparatus, 923; organ of smell, 924 ; organ of taste, 924 ; feet of, 924-926; stings of, 926, 927; ovi- positors of, 926, 927 ; eggs of, 928 ; agamic reproduction of, 930 ; em- bryonic development of, 931; ‘ liver ’ of, 971 Insect work, polarised light for, 366 Integument of insects, 898; of Acarina, 934 Integuments of ovule, 610 Intensity of light, necessaries for, 359 Intercellular substance, 943; in cartilage, 970 Intercostal points, Stephenson on, 73 ; not revelation of real structure, 73 Interference, 62 — image, 72 Intermediate skeleton in Foraminifera, 726; of Globigerinida, 745; of Calca- rina, 750; of Rot alia, 750; of Nummu- lites, 751; of Eozoon, 764 Internal casts of Rotalia, 748; of Textu- laria, 748; of Eozoon, 765; of wood, 1005 ; of shells in greensand, 1012 1075 1076 INDEX INT Interpretation, errors of, 368 1 Interseptal canals ’ of Calcarina, 755 Intestine, cells of villi in, 968 Intine of pollen-grains, 640 Intracellular digestion in zoophytes, 787 Intussusception, 463 — mode of growth of starch, 620 Invagination, 651 Invertebrata, blood-corpuscles of, 962 Inverted conjugate image, 24 — microscope for chemical purposes (Nachet’s), 216 Iodin, as a test for starch, &c., 440 Ipomcea purpurea, pollen-grains of, 646 Iridescent scales of insects, 899 Iris, epiderm of, 637; leaves of, 642; cells of pollen-chambers, 645 Iris-diaphragm, 229, 252, 253, 260; fitted to Abbe’s condenser, 259 Iris germanica, epiderm and stomates of, 640, 641 Irrationality of spectrum, 19, 314 Isochelffi of sponges, 784 Isoetece, 607 Istlimia, chains and frustules of, 517, 542; structure of frustules, 519 note; division of, 525 — nervosa, 542 — areolations in, 521 Italian reed, stem of, 624 1 Itch-mites,’ 937 Ivory, 948 Ixodes, heart of, 935 Ixodidce, 932; integument of, 934; audi- tory organ, 935 ; tracheae of, 935 ; characters of, 936 J Jackson’s modification of Ross model, 78; his limb, 181, 215; his model, 190 ; his eye-piece micrometer, 232 Janczewski on antherozoids of Spha- celaria, 555 Janssen on invention of lens, 122; his compound microscope, 122 Jars, capped, for Canada balsam, 447 Jelly-fish. See Acalephce and Medusa Jones’s compound microscope, 143, 146 Jungermannia, 593 Jung’s (Thoma’s) microtome, 401 K Kaolin, 999 Karop and Nelson on fine structure of diatoms, 521 note Karyokinesis in monads, 688 Kellner’s eye-piece, 42, 322; as a conden- ser, 177 Kent (Saville) on contractile vacuoles of Volvox, 481 note; on Flagellata, 689 Keplerian telescope, Drebbel’s modifica- tion as a microscope, 123 Keramospliara Murrayi, 735 note LAE Keratose network of sponges, preparation of, 779, 781 Kidneys of Vertebrata, 971 King-crab, 881 Kirchner on the oospores of Volvox, 484 Klebs on mucilaginous sheath of des- mids, 510; on movement of desmids, 510 — and Biitschli on the ‘ cilia ’ of Dino- flagellata, 695 Klein on Volvox, 484 note Knife, two-bladed, Valentin’s, 898 ; special, for microtome, 399 Koch’s method of sectionising corals, 802 Kowalevsky on development of ascidians, 841 note Krukenberg on digestion in sea-anemones, 787 Kiitzing on Palmodictyon, 487; on struc- ture of frustules of diatoms, 519; his classification of diatoms, 532 L Labarraque’s fluid for bleaching veget- able substance, 427 Labels, permanent, 454 Labryinthic structure of Cyclammina, 741; of Parkeria, 748 Labyrintliodon, tooth of, 1013 Lacunae and canaliculi of bone, misinter- pretation of, 870 — of bone, 943-945; dimensions of, in various animals, 946 — relation of size to that of blood-cor- puscle, 946 Lagena, 721, 744 Lagenida, 744 Laguncula, 835, 874 — stolon of, 828 ; polypides of, compared with Clavellinida, 839 — repens, anatomy of, 828, 829 ‘ Lamellae ’ of corals, 802 — of Hymenomycetes, 576 Lamellibranchiata, shell of, 843 Lamellicornes, antennae of, 912 Laminaria, 555, 556 Laminariacea, 556 Lamna, tooth of, 948 Lamp, Nelson’s, 347; Beck’s, 348, 849; Baker’s, 350 Lampyris, antennae of, 912 — splendidula, photograph through eye of, 908 Land-crab, young of, 898 Langley on use of osmic vapour for mucous glands, 429 Lankester (E. Ray) on Bacteria, 581; on movement of gregarines, 675 ; on Pro- tozoa, 677 note\ on intracellular di- gestion in Limnocodium, 787 Lantern-flies, wings of, 923 Lapis lazuli, 1016 Larva of Ecliinodermata, 820; of As- ter oidea, 822 ; of Echinoidea, 822; of INDEX LAT Ophiuroidea, 822; of Crinoidea, 824; of ascidians, 840; of fly, 931; of Acarina, 933 Latex of Phanerogams, 620 Lathrcea squamaria, embryo of, 648 Laticiferous tubes, free-cell formation in, 464 — tissue of Phanerogams, 620 Laurentian rocks, 762, 767 ‘ Laver,’ or green seaweed, 487 Lawrence’s glycerin jelly, 443 Leaves, epiderm of, 637 ; internal struc- ture of, 641; mode of preparation for examination of, 642 Leech, 880 Leeuwenhoek’s simple microscope, 134 Legg’s method of selecting Foraminifera, 769 Legs of insects, 924,926; of Acarina, 932, 934 Leguminosce, seeds of, 610 Leiosoma palmacinctum, 932; hairs of, 934 Leitz’s objectives, 320 — semi-apochromatic objective, 320 Lens spherical, 12; biconvex, 12, 13; plano-concave, 13; diverging meniscus, 13; plano-convex, 13, 15, 22, 37; con- verging meniscus, 13; biconcave, 13 ; piano - convex, focal length of, 15 ; crossed biconcave, 16; crossed bicon- vex, 16 ; equiconvex, 16, 22; Stanhope, 37 ; Coddington, 37 ; Briicke, 38 — from Sargon’s palace, 121 — invention of, 121-122 — achromatic, Charles’s, 146; Barlow’s, 147 Lenses, refraction by, 10, 25 — homogeneous immersion, of Powell and Lealand, 29; of Zeiss, 29 — fluorite; for apochromatic objectives, 34, 35 — combination of, 37 — resolving power of, 64 ; amplifying power of, 25, 26 — testing by Coscinodiscus, 333 Lepadulce, 891 Lepidium, seeds of, 649 Lepidocyrtus curvicollis, scales of, 903 Lepidodendra, 607, 1005 Lepidoptera, scales of, 899, 900; wings of, 905, 923; scales of, mounting, 906; eyes of, 911; antennas of, 912 ; mouth- parts, 916; eggs of, 929 Lepidosteus, bony scale of, 946, 952 Lepidostrobi, 607 Lepisma saccharina, scales of, 900, 901 Lepismidce, 903 Lepralia, 833; extension of perivisceral cavity of, 851; mode of growth in, 828 Leptodiscus (ally of Noctiluca), 694 note Leptogonium scotinum, 578 Leptothrix, form of, 581 Leptus autumnalis, 937 Lerncea, 889 note, 890 Lessonia, 556 Lettuce, laticiferous tissue, 620 LIV Leucite, mineral inclusions in, 998; anomalies in, 1002 Lever of contact, Ross’s, for testing covers, 881 Libellula, 911; respiratory apparatus of larva, 921 Liber, or inner bark, 633 Lichens, 576-579 ; fungus-constituents of, 579 Licmojphora, stipe of, 518-533, 534; flabella of, 534 — flabellata, 517, 533 Licmophorece, 545 — characters of, 533 ; vittse of, 534 Lieberkuehnia, movement of, 657 — paludosa, 658 — Wagneri, 656-658 Lieberkiihn’s microscope, 138; bis specu- lum, 282-284 1 Ligamentum nuchas,’ structure of, 964 Light; refraction of, 2; recomposition of, by prisms, 18; convergence of, 18 ; path of, through compound microscope, 40; quantity of, 50, 51, 54; emission of, 51, 54; quantity of, and aperture, 54 note \ cone of, 170; monochromatic, 271, 372; intensity of, necessaries for, 359 — modifiers, 284 Lignified tissue, test for, 440 Lignites, 1005 Lignum vitce, wood of, 629 Lilac, pith of, 611 Lilium, experiments with pollen-grains of, 646 ‘ Lily-stars,’ 824. See Crinoidea Limax maximus, palate of, 854 — shell of, imitated, 1023 — rufus, shell structure of, 852, 856 Lime, raphides of, 621 Limestone rocks, 1007 Limnceus stagnalis, nidamentum of, 858; velum of, 860 Limnocaridce, characters of, 937 Limnocharis, seeds of, 649 Limnocodium, intracellular digestion in, 787 Limpet. See Patella Limulus, 881 Linaria, seeds of, 649 Lister’s struts for support of body, 147; his influence on improvement of Eng- lish achromatic object-glasses, 148; his zoophyte trough, 297 ; his discovery of two aplanatic foci, 304 ; his note on Chevalier’s objectives, 304; his influ- ence on microscopical optics, 305; his triple-front combination, 309 Listrophorus, 932 Lithasteriscus radiatus, 550 Lithistid sponges, spicules of, 784 Lithocyclia ocellus, 771 Lituola, 739 Lituolce, large fossil forms of, 741 Lituolida, 739 Live-boxes, 294, 295 Liver, 971 1078 INDEX LIV Liver-cells, 972 ‘ Liverworts,’ 590. See Hepaticce Lobosa, characters of, 659 ; examples of, 667-672 Lobster, 881; metamorphosis of, 893 ‘ Lob-worm,’ 872 Loculi, of anthers, 644 Locust, gizzard of, 917 ; ovipositors of, 928 Locusta, eye of, 911 Loftusia, 743 Loligo, pigment-cells of, 866 Lomas (J.) on calcareous spicules in Alcyonidium, 832 note ‘ London Pride,’ parenchyme of, 613 Longicornes, antennas of, 912 Lophophore of Polyzoa, 829, 874; of fresh-water Polyzoa, 833 Lophopus, collecting, 458 Lophospermum erubescens, winged seed of, 649 Lophyropoda, 883 Lorica of Acineta, 697; of Ciliata, 700; of Rotifera, 715 Loup-holders, 203 — for tank work, Steinheil’s, 224 Loups, Reichert’s, 38; Steinheil’s, 88, 822, 457; Steinheil’s aplanatic, 205; Zeiss’s, 261 Louse, mounting media for, 897 Loven on classificatory value of palates in Gastropoda, 856 Loxosoma, lophophore of, 833 Lubbock on Thysanura, 901; on Podura scale, 903 Lucanus, 911; antennae of, 912 Luminosity of Noctiluca, 690 ; of Cteno- phora, 806; of annelids, 879 Lungs, circulation in, 980, 984 Lychnis, seeds of, 649 Lychnocanium falciferum, 771 — lucerna, 771 Lycoperdon, 575; hymene of, 576 Lycopodiacece, 606; in coal, 1006 Lycopodiece, 606 Lyminas, collecting, 457 Lymph corpuscles, 961 Lysigenous spaces in Phanerogams, 613 M Maceration of vegetable tissues, 624; Schultz’s method, 625 Machilis polypoda, scale of, 902 Machines for cutting hard sections, 424, 425 Macrocystis, 556 Macrospores of Polytoma, 685; of sponges, 781 Macrourous Decapoda, young of, 893,894 Madder, cells of pollen-chambers, 645 ‘ Madre,’ Acanthometra occurring in, 777 Madrepores, 802 Magenta as a selective stain, 436 Magma, 996 Magnetite, 995 MEA Magnification, range of, 147 Magnifying power, 367; determination of, 238 Mahogany, size of ducts of, 624; stem of, 630 Malacostraca, 892 ‘ Male ’ plants of Polytrichum, 596 Mallei of Rotifera, 715 Mallow, pollen-grains of, 646, 647 Malpighian vessel of Gamasidce, 935 — layer of skin in mammals, 966 — bodies in vertebrate kidney, 971 Maltwood’s finder, 246 Malva sylvestris, pollen-grains of, 646 Malvacece, pollen-grains of, 646 Mammalia : lacunas in bones of, 946; plates in skin of, 950; epidermic ap- pendages of, 953; red blood-corpuscles of, 958, 959; epidermis of, 966; muscle fibre of, 973; lungs of, 989 Mammary glands, 971 Man, arrangement of enamel in teeth of, 949 ; cement in teeth of, 950; hair of, 955; muscle fibre of, 973; lung of, 989 Mandibulate mouth, 913 ‘ Mantle ’ and growth of shell in Mollusca, 849 Marble derived from limestones, 1011 Marchantia, 590-593 ; archegones of, 596; prothallium of, 602 ; stomates of, 640 ; elaters of, 645 — androgyna, 590 — polymorpha, 590-593 Margaritacece, 843; nacreous layer of, 846; prismatic layer of, 847 ‘ Marginal cord ’ of Calcarina, 755 — of Nummulites, 759 Marine forms, collecting, 458 — glue for forming ‘ cells,’ 385 — mites, 937 — work, tow-net for, 458; dredge for, 458; stick-net for, 459 Marshall’s compound microscope, 185, 136 Marsipella elongata, 738 Martin’s ‘ pocket reflecting microscope, 138; his large microscope, 139; his achromatic microscope, 145; his re- flecting microscope, 145; his achro- matic objective, 145 Marzoli’s achromatic lenses, 302 Masonella, 736 Mastax of Rotifera, 715 Mastigophora Hyndmanni, 830 Mastogloia, stipe of, 518, 548 ; gelatinous sheath of, 518, 548; development of, 526; range of variation in, 547 — lanceolata, 548 — Smithii, 548 Matthews’s method of sectionising hard substances, 420 Mayall on history of microscope, 119; on Assyrian ‘ lens ’ from Sargon’s palace, 121; on Divini’s microscope, 132 Mayall’s removable mechanical stage, 194 Mayer’s heating hath, 393 ‘ Meadow-brown,’ eggs of 929 INDEX 1079 MEA ‘ Measley pork,’ due to Cysticercus, 868 ‘ Mechanical finger ’ for selecting diatoms, 554 — movements of the stage in Tully’s microscope, 147 — stage, 215 Turrell’s, 165; Tolies’, 166; Zeiss’s, 167 ; Mayall’s removable, 194 — tube-length of microscope, 155 Continental, 156 Medullary rays, 629 in dicotyledons, 627 ‘ Medullary sheath ’ of Exogens, 623 ; of dicotyledons, 627 Medusa of fresh water, 787 Medusce, mounting, 888 ; of Hydroids, 792 ; naked-eyed, 792 ; development of, 798 ; alternation of generations in, 801; nerves of, 976 Medusoids, collecting, 459 Megalopa, 894 Megaloscleres, 783 Megasphere of certain Foraminifera, Til Megaspores of Bhizocarpece, 606; of carboniferous trees, 607; of Isoetece, 607 ; of Selaginellece, 607 Megatherium, teeth of, 950 Megatricha of Ehrenberg, a phase in development of Suctoria, 698; Badcock on, 698 Megazobspores of Ulothrix, 486; of Ulva, 489 ; of Scenedesmus, 496 Megerlia lima, shell of, 851 Melanosporece, 554 Meleagrina, 843, 846 — margaritifera, 847 Melicerta, collecting, 457; in confine- ment, 458 Melicertadce, 717 Melolontha, eye of, 911; antennae of, 912; spiracle of larva, 920 — vulgaris, eye of, 907 Melosira, frustules of, 517, 530 ; auxo- spores of, 525, 526, 530; sporules of, 526; zygospore of, 530 — ochracea, 537 — subflexilis, 523, 524 — varians, 523, 524 ; endochrome of, 527 Melosirece, characters of, 537 ; resem- blance to Confervacece, 537 Membrana putaminis, 962 Membranipora, 832, 833 Membraniporidce, 832 Mercury nitrate as a test for albuminous substances, 440 Mereschkowski on movements of diatoms, 581 Meridiece, 545 — characters of, 533 Meridion circulare, 517, 532, 533 Merismopedia, 477 ‘ Mermaid’s fingers,’ 803. See Alcyonium Mesembryanthemum, seeds of, 649 — crystallinum, epiderm of, 639 Mesocarpus, conjugation of, 478; zygo- spore of, 478 MIC Mesoderm of sponges, 780 Mesogloea of Hydra, &c., 788 note Mesophlceum, 638 Metal case for imbedding, 415 Metamorphism of rock-masses, 999,1000; of limestones, 1011 Metamorphosis of Lerncea, 890 ; of Cirripedia, 892 ; of Malacostraca, 892, 893 Metazoa, 652, 779 Meteorites in oceanic sediments, 1015 Metschnikoff on acinetan character of Erythropsis, 702; on intracellular di- gestion, 787; on phagocytes,x961 note Mica, 1000, 1001 Michael’s (A.) opalescent mirror, 172 Micrasterias denticulata, binary divi- sion of, 512 ; form of cell of, 575 Micro-chemistry in Petrology, 1004; of poisons, 1023 Micrococci, form of, 581 Microcysts of Myxomycetes, 565 Microgromia socialis, 660, 661 Microlites, 996 ; in glass-cavities, 997 Micrometer, Cuff’s, 140 — use of, 231 — eye-piece, 328 Nelson’s new, 227, 228, 229; Jack- son’s, 232 Micrometers, 226-233 Micron, a, 82 note, 400 Micro-petrology, 991 ‘ Microplasts ’ of Bacterium rubescens, 588 note Micropyle in ovule, 610 ; of Euphrasia, 648 ; in orchids, Ac., 648 Microscleres, 788, 784 Microscope, Mayall on the, 119 ; history and evolution of the, 119-225 ; inven- tion of, 122 ; inventor of the name, 126 ; essentials in, 154-172; adjustments in, 156-165 ; stage of, 165-168; desiderata in, 215 ; preservation of, 278 — Galileo’s, 129 ; Campani’s, 130 ; Pritchard’s, with Continental fine ad- justment, 150 ; Ross-Jackson model, 151 ; Powell’s (H.), 153 ; Smith and Beck’s, 153 — achromatic, Euler on 145 ; Martin’s, 145; Chevalier’s, 146, 148; Selligue’s, 146 ; Tully’s, 147 ; Ross’s early form of, 150 — aquarium, 219-225 — binocular, 61; Riddell’s, 97; Nachet’s, 98; Cherubin d’Orleans’, 132; Wen- ham’s stereoscopic, 98; Stephenson’s, 100, 395 ; Ross-Zentmayer’s, 178 ; Powell and Lealand’s, 107,175; Ross’s, 177; Rousselet’s, 200 — chemical, Nachet’s, 216 ; Bausch and Bomb’s, 217-220 — compound, 36, 39, 123, 127; construc- tion of, 39 ; path of light through, 40 ; Rezzi on invention of, 127; Hooke’s, 130; de Monconys’, 130; Divini’s, 131; Janssen’s, 122 ; Marshall’s, 135 ; Hertel’s, 138; Martin’s, 139; Adams’s 1080 INDEX MIC variable, 140, 146 ; Jones’s, 143, 146 Microscope, concentric, 171, 178 — demonstration, 225 — dissecting, Beck’s histological, 197; Stephenson’s binocular, 201 ; Ward’s, 205 ; Baker’s (Huxley’s), 204; Zeiss’s, 205; Beck’s binocular, 207 — horizontal, Bonanni’s, 135 ; Amici’s, 146 — petrological, 992 — photographic, 211 — radial, 171, 178; Ross-Wenham’s, 184 — reflecting, Newton’s, 133; Martin’s, 139, 145; Smith’s, 144 — simple, 36, 128, 201 ; path of light through, 25 ; inventor of, 128 ; Bacon’s, 128 ; Descartes’, 128 ; Bonanni’s, 134 ; Ward’s, 205 ; Muschenbroek’s, 134 ; Leeuwenhoek’s, 135 ; Hartsoeker’s, 135 — spectrum binocular, 276 Microscopes, modern, Powell and Lea- land’s, 172, 190; Beck’s, 180,189,190,194, 196; Swift’s, 181, 190, 194, 197; Zent- mayer’s, 184 ; Baker’s, 193; Continen- tal models, 208; Zeiss’s, 208-213 — portable, 198-200 Microscopic and macroscopic vision, 62 — determination of geological forma- tions, 1012 — dissection, single lenses for, 38 •— investigation of rocks, &c., 990 —- vision, principles of, 43 Microscopical optics, principles of, 1 Microscopist’s work-table, 341, 345 Microscopy, definition of, 340 Microsomes, 461, 468 Micro - spectroscope, Sorby - Browning, 272, 273 '— use of, 277 ; in petrology, 1003 Microsphere of certain Foraminifera, Til Microspores of Sphagnacece, 599; of Rliizocarpece, 606; in carboniferous trees, 607; of Isoetece, 607 ; of Selagi- nellece, 607 ; of Polytoma, 685; of sponges, 781 Microtome, ether-spray, 48 — Ryder’s, 344; simple, 398 ; Rivet’s model, 401; Thoma’s (Jung’s), 401-408; freezing apparatus for, 405, 406 — Cambridge rocking, 408; advantages of, 411 — freezing, Hayes’, 411; minimum thickness of sections with, 412 ; Cath- cart’s, 412 Microzoospores of Ulothrix, 486; of Ulva, 489; of Hydrodictyon, 495 ‘ Mildew,’ 566. See Uredinece Miliola, shell of, 724; encrusted with sand, 735 Miholce, 111 Miliolida, 726 ; in limestone, 1011 Miliolina, 111 Milioline Foraminifera, fossils of, 726 Miliolite limestone, 1011 MOR Millepore, resemblance of Polytrema to, 749 Millon’s test for albuminous substances, 440 Mineral nature of Eozotin, 767 Minerals, analysis of, 1003, 1004; micro- scopic testing, 1004 Minnow, circulation in tail of, 981 Mirror, 171, 172 — opalescent, as a substitute for polaris- ing prism, 172 — replaced by rectangular prism, 172 Mites, 932. See Acarina Miibius on mineral nature of Eozoon, 767 Mohl (Yon) on protoplasm, 460 note Moist-stage, Dallinger and Drysdale’s, 289 Molecular coalescence, 1021 Molgula, development of, 841 Moller’s diatom type-slide, 286 Mollusca, larvae of, collecting, 459 — shells of, 843 ; shell-structure of, 843- 849; colour of shell, 845; mantle and shell-growth, 849; palate of, 854; de- velopment of, 857; ciliation of gills, 864; organs of sense in, 864; biblio- graphy, 866 ; resemblance of barnacles to, 891; ‘ liver ’ of, 971; muscle fibre of, 974; internal casts of, 1012; concre- tionary spheroids in shells of, 1021 Molluscan shells in mud of Levant, 1007 Molybdate of ammonia as a general stain, 437 Monad-form of Microgromia, 662 Monadince, life-histories of, 680-686; saprophytic affinities of, 681; effect of temperature on, 686; nucleus in 687 Monads, 680. See Monadince Monas, 475 — Dallingeri, life-history of, 681 — lens, 680 Monaxonida, spicules of, 783 Monconys (De), inventor of field-lens, 130 Monera, Haeckel on, 677 Monerozoa, 552-658 Monocaulus, 795 Monochromatic light, 271, 372 — illumination, means of obtaining, 360, 361 Monocotyledons, 625; stem of, 625; nodes of, 626; epiderm of, 637 Monocotyledonous stem, fossilised, 1005 Monocular, Powell and Lealand’s, 173, 174 Monocystis agilis, cyst of, Monophytes, digestion in, 787 note Monosiga, fission of, 689 Monothalamous Foraminifera, 721 Monotropa, seeds of, 649 Moracece, laticiferous tissue of, 620 Mordella beetle, eye of, facets in, 907 Mormo, scales of, 904 Morpho Menelaus, scales of, 900 Morula of sponges, 781 — compared to higher Protozoa, 651 INDEX 1081 MOK Morula of Gastropoda, 859 Moseley (H. N.) on skeleton of pharynx of holothurian, 819 note; on Chiton's eyes, 865 Mosses, 594-599 — capsules of, wooden slides for mounting, 390 * Mother-of-pearl,’ 846 Moths. See Lepidoptera Motion, spiral, 375 Motor nerves, 976 Motorial end-plates, 977 ‘ Moulds,’ 569, 571 Moults of Entomostraca, 888, 889 ‘ Mountain-flour,’ 551 Mounted objects, keeping, 453; labelling, 453 ; arrangement of, 454 Mounting plate, 391, 392 — instrument, James Smith’s, 394 — thin sections, 447 — in natural balsam, 449; in aqueous liquids, 450; in deep cells, 451 — diatoms, 450; Ophiurida, 450; Poly- cystince, 450 ; sponge - spicules, 450 ; chitinous substances, 450; palates of gastropods, 450 ; sections of horns, &c., 450; Lepidoptera scales, 906; wings of Lepidoptera, 906; hairs of insects, 906; eyes of insects, 910; blood, 962 — media, Canada balsam, 444, 449 Mouse, hair of, 954-955 ; cartilage in ear of, 970 Mouth, suctorial, of FLemiptera, 923 — of Acarina, 933 Mouth-parts of insects, 913 Movement, interpretation of, 374, 375 -— of Lieberkuehnia, 657; of Amceba, 668; of Dallingeria, 683; of plana- rians, 870 ; of Artemia, 884 ; of Bran- chip us, 884; of fly on smooth surface, 925; of white corpuscles, 961; of con- nective tissue corpuscles, 965 ; of Oscillariacece, 490; of desmids, 510; of diatoms, 528; of Naviculce, 531; of Bacteria, 581; of Ciliata, 701 Mucilaginous sheath of desmids, 510 Mucor, fermentation by, 575 — mucedo, 570 Mucorini, 569 ; spores of, 569; epispores of, 570 Mucous glands, Langley’s method of preparing, 429 — membrane, 965, 966; capillaries in, 986 Mud of Levant, microscopic constituents of, 1007 Mulberry, laticiferous tissue of, 620 Mulberry-mass, 651 Muller (J.) on the Badiolaria, 771; on larva of Nemertines, 875 Muller’s (Fr.) ‘ Common Nervous System ’ in Polyzoa, 831 and note Muller’s fluid, 430 Multicellular organisms, 651 Multiplication of Palmoglcea, 472; of Protococcus, 474; of Volvox, 483; of Palmella, 486; of Bacteria, 581; of Microgromia, 661; of Amceba, 669; NAI of Dallingeria, 683; of Heteromita, 685; of Tetramitus, 685; of Noctiluca, 694 ; of Peridinium, 695; of Suctoria, 698 ; of Ciliata, 704 Multiplying power of eye-piece, 240 Munier Chalmas and Schlumberger on dimorphism of Foraminifera, 727 Munier-Charles on certain fossil Fora- minifera, 493 Muricea elongata, spicules of, 804 Musca, eye of, 911; antennae of, 912 — vomitoria, eggs of, 930 ‘ Muscardine,’ 573 Musci, 594-599 Muscinece, 598 Muscle-cells, 975 Muscular fibre, 972 ; structure of, 973; capillary network in, 986 Muscular tissue, preservative for, 443 preparation of, 974 Mushroom, 576 — spawn of, 575 Musk-deer, hair of, 954 Musschenbroek’s simple microscope, 134 Mussels. See TJnionidce and Mytilacece Mya arenaria, hinge tooth of, 848 Mycele of Fungi, 562; of Ustilaginece 565 Mycetozoa, 563 Myliobates, tooth of, 949 Myobia, 932; legs of, 934; maxillae of 934 Myobiidce, 937 Myocoptes, legs of, 934 ‘ Myophan-layer ’ of Vorticella, 701 Myopy, 120 Myriophyllum, a good weed to collect, 458 Mybiopoda, hairs of, 904 Myriothela, intracellular digestion in, 787 Mytilacece, sub-nacreous layer in, 848 Mytilus, for observation of ciliary motion, 864 Myxamcebce, 564 Myxogastres, 563 Myxomycetes, 509 note, 563 ; develop- ment of, 563, 566; spores of, 563, 565; swarm-spores of, 563 ; affinity with Monerozoa, 652 Myxosporidia, 674, 677 N Nachet on ‘immersion system,’ 27; his binocular, 95, 98; his stereo-pseudo- scopic microscope, 208; his changing nose-piece, 243 Nacreous layer in molluscan shells, 843, 846, 848 Naegeli and Schwendener on microscopi- cal optics, 67 Niigeli’s theory of formation of starch, 619 Nails, 953, 957 Nais, 879 1082 INDEX NAP Naphthalin, monobromide of, as a mount- ing medium, 445; refractive index of, 444 Narcissus, spiral cells of pollen-chambers in, 645 Nassula, mouth of, 702 Nauplius, compared with Pedalionidce, 719 Nautiloid shell of Foraminifera, 722 Nautilus, 853 Navicula, 520, 528, 546; markings on, 522 ; cysts of, 526; zygospores of, 526; zobzygospores of, 526 — bifrons, presumed relation to Suri- rella microcora, 532 note — in chalk, 1009 — lyra, as test for definition, 368 — rhomboides, markings on, 521; as test for definition, 368 Naviculacece, frustule of, 518 ; ostioles in, 519 Naviculece, characters of, 546 Nebalia, carapace of, 886 Needles for dissection, their mode of use, 397 Negative aberration, 27, 309 note — crystals, 997 — eye-pieces, 321, 322, 323 Nelson on the sub-stage condenser, 72 ; his model, with Swift’s fine-adjustment screw, 163; his horse-shoe stage, 167, 190; his fine adjustment to the sub- stage, 169; his screw micrometer eye- piece, 227 ; his new micrometer eye- piece, 228, 229; his ‘ black dot,’ 233 ; his plan for estimating edges of minute objects, 233 ; his changing nose-piece, 244 ; his revolving nose-piece, 244; his lamp, 347; his means of obtaining mono- chromatic illumination, 361; his gelatin, 443 — on ghostly diffraction images, 72 note Nelson and Karop on fine structure of diatoms, 521 note Nemalion multifidum, 560 Nematodes, desiccation of, 869 Nematoid worms, 868 Nemertine larva, 875 Nepa, tracheal system, 919 ; wings of, 924 — ranatra, eggs of, 929 Nepenthes, spiral fibre-cells of, 623 Nereidce, 872 Nereocystis, 556 Nerve-cells, 975 — staining with blue-black, 437 Nerve-fibres, 976 Nerve-substance, 975; mode of prepara- tion, 978 Nerve-tubes, 975 Nerves, preservative for, 443 Nervures of wing of Agrion, 918 Nettle, hairs of, 639 Neuroptera, 897; eyes of, 911; circula- tion in wings of pupa, 918 ; wings of, 922 Newt, red blood-corpuscles of, 959; cir- culation in gills of larva, 981 NUC Newton’s reflecting microscope, 183 — suggestion of reflecting microscope, 144 — rings, 1018 Nicol prisms, 269 Nicol’s analysing prism, 244; for re- solving striae, 325 Nicotiana, seeds of, 649 ‘ Nidamentum ’ of Gastropoda, 858 Nitella, 505, 506 Nitrate of silver for tendon-cells, &c., 437 Nitric acid as a test for albuminous sub- stances, 440 Nitrogenous substances, test for, 440 Nitzschia, 528 — scalaris, cyclosis in, 577 — sigmoidea, 535 Nitzschiece, 535 Nobert’s test lines, 286 Noctiluca, collecting, 459 ; tentacle (flagellum) of, 691, 692; cilium of, 691 note; protoplasmic network of, 692 j reproduction of, 694 — miliaris, 690-694 Noctuina, antennae of, 912 Nodes of monocotyledons, 620 Nodosaria, 744 Nodosarince, shell of, 722 Nodosarine shell, sandv isomorphs of, 740 Nonionind, 754 — shell of, 722, 723 Nonionine shell, sandy isomorpli of, 739 Non-stereoscopic binoculars, 106 Non-striated muscle, 972, 974 Nose-piece, centring, used as sub-stage, 193 ; Brooke’s, 241 ; centring, 242; Zeiss’s calotte, 242 ; Beck’s rotating, 242; Powell and Lealand’s, 242; Na- cliet’s changing, 243; analysing, 244 ; Nachet’s changing, 244; Nelson’s re- volving, 244; Yogan’s, 244 Nosema bombycis, cause of pebrine, 588 Nostoc, 490,491; as gonid of lichen, 579; resemblance of Ophrydium to, 705 Nostocacece, 490; affinities with Bacteria, and Myxomycetes, 580 Notochord in Tunicata, 835 ; of Appen- dicularia, 842 Notonecta, 911; wings of, 924 Nucellus, 610 Nuclear stains, 430 — spindle, 468 ; plate, 468 Nuclein, 468 Nucleoli, 464 Nucleoplasm, 467 Nucleus, 464 — of minute organisms, 80 — action of acetic acid on, 440; its im- portance to cell, 465 ; division of, 468 ; fragmentation of, 468; presumed ab- sence of, in some forms, 652; in monads, 687 — and cell division, 943 note Nucule of Char a, 507, 508 INDEX 1083 NUD Nudibranchs, nidamentum of, 858; em- bryos of, 860 Numerical aperture, 29, 53, 60, 333, 367; formula for, 333 ; problems on, 334 of Zeiss’s apochromatic series of objectives, 318; of dry objective, 334; of water-immersion, 334 ; of oil-im- mersion, 334 and resolving power of objective, 336 — apertures, table of, 84-87 Nummuline layer of Eozoon, 764 — plan of growth, Parker and Rupert Jones on, 752 note Nummulinidce, 751 Nummulites, 751, 752, 756 — distans, 757 — garansensis, 757 — Icevigata, 757 — striata, internal cast of, 759 — tubuli in shell of, 725 Nummulitic limestone, 756, 760, 1007, 1011 Nuphar lutea, parenchyme, 612 ; stellate cells of, 612 Nymph of Acarina, 933; of Oribatidce, 933 0 Oak, size of ducts in, 624 — galls, 927 Oberhiiuser’s spiral fine adjustment, 151 Object-glass of compound microscope, 36, 39; of long focus, 40; of short focus, 40; capacity of, 326 Object-glasses, power of, 44 — — testing, 325; Abbe’s method of testing, 326-333; diaphragms for use in testing, 329 ; Tripp’s method of testing, 330 Object-holder for Thoma’s (Jung’s) mi- crotome, 403, 404 — changer, Zeiss’s, 243, 244 Objectives, achromatic, 19,32 ; aplanatic, 19 ; apochromatic, 19, 30, 34, 80, 211; corrected, 20, 21; immersion, 28, 34, 58; aperture of, 43, 65, 333 ; maximum aperture of, 44 ; comparison of, 46; illuminating power of, 54 note; im- mersion v. dry, 54, 79 ; dry, with balsam mounted objects, 55 ; dry, 58; dry, for study of life-histories, 81; penetrating power of, 83, 336; sliding plate with, 241; rotating disc with, 241; of wide aperture, 316; of small aperture, ex- amination of, 332 ; tests for, 332, 337; resolving power of, and numerical aper- ture, 336 — triple-back, 310; Wenham’s duplex front, 311; Leitz’s, 320; Reichert’s, 321; adjusting, 306, 309 — achromatic, Martin’s, 145; Marzoli’s, 302; Tully’s, 303; Selligue’s, 303; Amici’s, 304 ; Ross’s, 305, 309 ; Powell’s, 305, 309 ; Smith’s, 305, 309; Wenham’s, 310 ; covers for use with, 380 OPE Objectives, apochromatic, 314, 315, 320 — oil-immersion, Amici’s, 312 ; Tolies’, 312 ; Zeiss’s, 313, 317 — water-immersion, Powell and Lea- land’s, 310, 313; Prazmowski and Hartnack’s, 310 ; Zeiss’s, 317 Oblique illumination, 170, 171, 331 — illuminator, 170 Obliteration of structure by diaphragms,, 68 Occhiale, Galileo’s, 124, 125 Occhialino, Galileo’s, 123, 126 Oceanic sediments, microscopic examina- tion of, 1014 Ocelli of planarians, 871; of insects, 906, 910 Ocellites of compound eye, 906 Ocular, 40, 321; spectral, 276 CEdogo niacece, 502, 503 (Edogonium ciliatum, 502 CEnothera, pollen-grain, 646; emission of pollen-tubes of, 647 ; embryo of, 648 Oil for immersion lenses, suggested by Amici, 29 — of cassia, used with Stephenson’s illuminator, 265 — of cedar - wood, for immersion ob- jectives, 29 Oil-globules, 370, 371 Oil-immersion, 29 objectives, Amici’s, 312; Tolies’, 312; Zeiss’s, 317 Oils, solvents for, 441 Okeden on isolation of diatoms, 553 note Oleander, epiderm of, 638 ;■ stomates of, 641 Olivine, alteration of, 1001 — corroded crystals of, 995 Onchidium, eyes of, 865 Oncidium, spiral cells of, 618 Onion, raphides of, 621 Oogones of Vaucheria, 492 ; of Sphcero- plea, 501 ; of (Edogonium, 503 ; of Chara, 507; of Fucacece, 556, 557 ; of Peronosporece, 567 Oolitic rocks, structure of, 1011 Oophyte in ferns, 605 Oospheres, use of the term, 467 note; of Volvox, 484; of Vaucheria, 492; of Sphceroplea, 501; of CEdogonium, 502; of Chara, 507 ; of Phceosporece, 556; of Fucacece, 557; of Marchantia, 593; of ferns, 604 Oospores, 470; of Volvox, 484; of Vaucheria, 493; of Achlya, 495; of Sphceroplea, 500; of (Edogonium, 503; of Chara, 509 ; of Fucacece, 558 Ooze, Globigerina, organisms in, 736, 745; compared with chalk, 1007 Opalescent mirror as a substitute for polarising prism, 172 Opalina, 702 Opaque illumination by side reflector, 281 — mounts, 283 ‘ Open ’ bundles, 635 INDEX OPE Operculina, 752, 755; and Nummulites compared, 759 Operculum of mosses, 596 Ophiacantha vivipara, development of, 824 note Ophioglossacece, development of pro- thallium of, 604 Ophioglossum, sporanges of, 601; pro- thallium of, 606 Ophiothrix pentaphyllum, spines of, 815 ; teeth of, 816 Ophiurida, mounting, 450 Ophiuroidea, skeleton of, 815 ; spines of, 815; teeth of, 816; larva of, 822; direct development in, 824 note Ophrydia, quantities of, 706 Ophrydina, colonies in, 705 Ophrydium, cellulose in zoocytium of, 706 — versatile, effect of light on, 702 Ophryodendron, 697 Opium poppy, latex of, 620 Optic axis of Powell and Lealand’s No. 1, 174; of Baker’s third-class, 194 Optical anomalies in petrology, 1002 — centre, 24 — tube-length of microscope, 155 — tube-length, Continental, 156 Orals of Antedon, 825 Orbiculina, 728, 729, 733 — shell of, 722 — compared with Heterostegina, 759 Orbitoides, 757 — Fortisii, 760 Orbitolina, 749 Orbitolince, occurring with flint instru- ments, 749 Orbitolites, 729-735 — shell of, 723; range of variation in, 735; structure of Parkeria resembling, 742 ; deposits of, 1007 — and Cycloclypeus compared, 726, 760 — complanata, animal of, 732-734 — italiaca, 731 note, 733 — tenuissima, 733 Orbulina, 745 Orbuline shell, sandy isomorph of, 740 Orchidece, pollinium of, 647 Orchids, micropyle of, 648 Orchis, pollen-tubes of, 647; seeds of, 649 Organised structure and living action, 460 Organisms, minute, nucleus of, 80 Organs, 463 ‘ Organs of sense ’ in Ciliata, 702 note Oribatidce, nymph of, 938; mouth-parts of, 933 ; legs of, 934; integument of, 934 ; auditory organ, 985 ; reproductive organs, 935 ; supercoxal glands of, 985 ; tracheas of, 935; characters of, 936 Orienting small objects for sectionising, 416; Kingsley’s method, 415 Origanum onites, seeds of, 649 Ornithorhyncus, hair of, 954 Orobanche seeds of 649 PAP Orthoptera, eyes of, 911; antennae of, 912; wings of, 928; nymph of, 983 Orthoscopic effect, 95; with Eamsden’s circles, 107 — eye-piece, 322 Orthosira Dickiei, sporangial frustule of, 524 Oscillaria, movement of, 490 Oscillariacece, 490 — movements of, 375 Oscula of sponges, 780 Osmic acid and fatty structures, 429 Osmunda, sporanges of, 601 — regalis, prothallium of, 604 note Ossein, of bone, 947 Ostiole of conceptacle of corallines, 561 Ostioles of Naviculacece, 529; of Gym- belle or 519 Ostracoda, 884 Ostreacece, shell of, 847 Ostrich, egg-shell of, 1021 Otoliths compared with artificial concre- tions, 1021 -— of Mollusca, 865 Ovarium of Polyzoa, 831 Over-amplification, 88 Over-corrected objective, 20 Over-correction, 307, 308 Overton on Volvox, 484 note Ovipositor of Oribatidce, 936 Ovipositors of insects, 926, 927 Ovule of Phanerogams, 609 — suspensor of, 464 — structure of, 609,610 ; development of, 647 Ovum of Hydra, 790 Oxytricha, a phase in development of Trichoda, 707 Oxyuris vermicularis, 868 Oysters, shell of, 847 P Pacinian corpuscles, 977 Palaeontology, use of microscope in, 1005 ‘ Palate ’ of Gastropoda, 843, 854 ; classi- ficatory value of, 856 ; preparation of, 856 ; viewed with polariscope, 857; bibliography, 857 Palese of grasses, silex in, 640 Palisade-parenchyma of leaves, 641 Palm, stem of, 626 Palmella, as gonid of lichen, 579 — cruenta, 486 Palmellacece, 486 ; frond of, 486 Palmodictyon, 487 ; zoospores of, 487 Palmoqlcea macrococca, life-history of, 471-473 Palpicornes, antennae of, 911 Paludina, infested by Distoma, 870 Pancreas, 971 Pandonna, 475 — morum, generative process of, 485; swarm-spores of, 485 Pantocsek’s finder, 246 Papaveracece, laticiferous tissue of, 620 INDEX 1085 PAP Paper-cells, 386 Parabolic illuminator, 267-269 ; reflector (Sorby’s), 281; speculum, 281 Paraboloid, 267-269 ; Edmunds’, 269 ; Wenham’s flat-topped, 269 Paraffin, solvents for, 417 — for imbedding, melting point of, 417 — cells, 386 Paramecium, Cohn’s experiments on, 668 ; contractile vesicles of, 704 — aurelia, supposed sexual reproduction of, 710 Paraphyses of Puccinia, 567 ; of lichens, 578; of mosses, 596 Parasites, nourishment of, 462 Parasitic Crustacea, 889 — Fungi, 562 Parietal utricle, 463 Parker (T. J.) on osmic acid for Ento- mostraca, 428; on use of osmic acid for vegetable structures, 428; on Hydra, 787 Parkeria, 742 ; a possible Stromato- poroid, 742 note Parnassia, seeds of, 649 Parthenogenesis, 931 note — in Saprolegnice, 569 Passiflora ccerulea, pollen-grains of, 646 Passiflorece, pollen-grains of, 646 Paste-worm, 869 Pasteur’s solution for growing yeast, 574 note; his experiments with Bacteria, 587, 588 Patella, shell structure, 852; palate of, 855 Path of ray of light through a compound microscope, 40 Pathogenic bacteria, 585 Pavement epithelium, 968 Pear, constitution of fruit, 618 ‘ Pearl oyster.’ See Meleagrina Pearls, 847 ‘ Pebrine ’ in silkworms, 588 Peccary, hair of, 954 Pecten, prismatic layer in, 848; pallial eyes of, 864; eye of, 865; fibres of adductor muscle, 974 Pectinibranchiata, 861 Pectinidce, sub-nacreous layer in, 848 Pedalion, 718 Pedalionidce, 718 Pedesis, 373 ; experiments in, 373-4 Pediastrece, 496; affinities of, 496 Pediastrum, zoospores, 496, 497 ; micro- zoospores, 497 — Ehrenbergii, 498 — granulatum, 496, 497 — pertusum, 497 — tetras, 498 Pedicellarise of echinids and asterids, 813 Pedicellina, lophophore of, 833 Pedicularis palustris, 648 — sylvatica, embryo of, 648 Peduncle of Lepas, 891 Pedunculated cirripeds, 891 PHI Pelargonium, petal of, 643; pollen-grain, 646 Pelletan on osmic acid, 428 Pelomyxa palustris, 669 Peneroplis, 726 — variation in shape of shell in, 722 ; shell of, 724 ; varietal forms of, 728 Penetrating power, 367 in objectives, 83 ; of objective, com- pared with illuminating power, 336 Penetration, 38, 82, 83 Penicillium, fermentation by, 575 — glaucum, 571 Pentacrinus asterius, skeleton of, 816 Pentatoma, wings of, 924 Peony, starch in cells of, 619 ‘ Pepperworts,’ 606 Perception of depth, 94 Perch, scales of, 952 Perforated shells of Brachiopoda, 850 Perforation of shell in Foraminifera, 724, 725 Perianth, 643 Perichlamydium prcetextum, 775 Peridinium uberrimum, 695 Perigone of mosses, 595 Periodic structures, 74 Periostracum of molluscan shells, 846; of brachiopod shells, 850 Peripatus, tracheae of, 935 Peritheces of lichens, 578 Peronosporece, 567, 568 Perophora, respiratory sac of, 839; cir- culation of, 839 ‘ Perspicillum,’ Wodderborn’s, 127 Petals, 643 Petrobia lapidum, eggs of, 933 Petrological microscope, Swift’s, 992 Petrology: micro-spectroscope in, 1003; micro-chemistry in, 1004 Pettenkofer’s test, 440 Petunia, seeds of, 649 Peziza, botrytis-iorm of, 572 Pfitzer, on reproduction of diatoms, 523 Phceodaria, 111 Phceosporece, 554, 555 Phagocytes, 961 note Phakellia ventilabrum, 782 Phallus, 575 Phanerogamia, woody structures, pre- paration of, 427 — embryo-sac of, free-cell formation in, 464—466 — relation of, to Cryptogams, 607, 609; structure of stems, &c., 610,625; struc- ture of cells, 612, 613 ; intermediate lamella, 613; intercellular spaces, 613; cell-wall of, 617; sclerogen, 617 ; spiral cells in, 618; laticiferous tissue of, 620; mineral deposits in cells of, 620, 621; woody fibre in, 621 et seq.; fibro-vas- cular bundles, 625 ; root, structure of, 625; flowers of, 643; pollen-grains of, 644; fertilisation of, 647; ovules of, 647; seeds of, 648 Phanerogams. See Phanerogamia Philonthus, antennas of, 912 1086 PHL Phloem, 635 — of Exogens, 622 Pholas, shell of, 848 Phoronis, 875 Phosphorescence of sea, due to Noctiluca, 693 Phosphorus, as a mounting medium, 445 Photographic microscope, Zeiss’s, 211, 212 Photometrical equivalent of different apertures, 50 Photo-micrograph through eye of Lam- pyris, 908 Photo-micrography, 174, 233, 324 — Campbell’s differential screw used in, 194 — illumination for, 356 Phryganea, eye of, 907 Phycocyanin in Chroococcacece, 477 Phyco-erythrin, 560 Phycornyces nitens, 570 Phycophaein, 555 Pliylactolcemata, 833 Phyllites, 1001 Phyllopoda, 886 Pliyllosomata, skeleton of, 892 Physarum album, development of, 564 Physcia parietina, 579 Physma chalaganum, 579 Phytelephas, endosperm of seed of, 618 Phytophthora infestans, 568 Phytopti, mouth-parts of, 934 Phytoptidce, 932 ; characters of, 938 Phytoptus, larva of, 933 Picro-anilin, 437 Picro-carminate of ammonia, 436 Picro-carmine, 436 Piedmontite, 1017 Pieridce, scales of, 899 Pigment-cells of cuttles, 866; of ver- tebrate skin, 966; of fishes, 967; of Crustacea, 967 Pigmentum nigrum, of eye, 967 Pike, scales of, 952 Pileorhiza, 636 Pileus of Acetabularia, 493 Pilidium gyrans, 875 Pilulina Jeffreysii, 737 Pimpernel, petals of, 644 Pines, pollen-grains, showers of, 646 note Pinna, structure of shell of, 815, 848- 846; prisms of shell of, in Globigerina ooze, 1008 ; prisms of, in chalk, 1009 — nigrina, colour of shell of, 845 Pinnularia, 546 — dactylus, 551 — nobilis, 551 Finns canadensis, 383 Pipette, 447 Pistil, 647 Pitcher-plant, spiral fibre-cells of, 623 Pith, arrangement of, 625, 627 Pitted ducts of Phanerogams, 623 Placoid scales, 952 Plagioclase felspar, 1003 Planaria, stomach of, 890 Planarice, 869; movement of, 870; fis- INDEX POL sion of, 871; ocelli of, 871; intracellular digestion in, 787 Planarians. See Planarice — allied to Ctenophora, 806 Plano-concave lens, 13 Plano-convex lenses, 13, 15, 22, 37 Planorbulina, 749 Plantago, cyclosis in, 616 ‘ Plantain,’ cyclosis in, 616 Plants and animals, differences between, 461 Planulce, 792 Planularia hexas, in chalk, 1009 Plasmode in cells of Nitella, 509 note; of Aithalium, 563; of Myxomycetes, 564, 565 Plasmodium of Protomyxa, 654 Plastid, contrasted with cytode, 652 Plastidules, flagellated, of Protomyxa, 654 Plates, calcareous, of Holothurioidea, 819 Pleocliroism, 1002 Pleurosigma, 518, 546 — diffraction image of, 71 — angulatum, 69-71; as test for defini- tion, 368 ; markings on, 521, 522 — formosum, as test for definition, 368 — Spencerii, sporules of, 526 Pliny on cauterisation by focussing sun’s rays, 119; on sight, 120 Ploima, 717, 718 Plumatella, collecting, 458 ‘ Plumed-moths,’ wings of, 923 Plumule of Pieridce, 899 Plutarch on myopy, 120 Pluteus larva of echinoids, 821, 822, 823 Podocyrtis cothurnata, 771 — mitra, 771, 776 — Schomburgkii, 773, 776 Podophrya quadripartita, 697; imma- ture form, 698 — elongata, 697 Podosphenia, sporules of, 526 Podura scale as test for high powers 332 ‘ Podura scales,’ 900, 903 Poduridce, 903 Pointer in eye-piece, 325 Poisons, micro-chemistry of, 1023 Polariscope, condensers for use with, 262; for examination of gastropod palates, 857; crystals for use with, 1017; list of objects for, 1020 Polarised light for insect work, 366; use of, in micro-petrology, 992 Polariser, 262, 269 Polarising prism, substitution of opales- cent mirror for, 172 ‘ Polierschiefer,’ 546 Polishing ground sections, 424 — sections of hard substances, 420 slate, 546 — -stones, 421, 546 Polistes (wasp), with attached mould. 571 Pollen-chambers of anthers, 644 Picro-carmine, 436 1087 POL Pollen-grain and tube, 609 — grains, 644; form of, 645; experi- ments with, 646 — mass, of orchids, 647 — tube, 645 — tubes, traced through the style, 647 Pollinium of orchids and asclepiads, 647 Pollinoids of Floridece, 561; of lichens, 578 Polyaxial spicules, 783 Polycelis levigatus, 871 Polyclinidce, 837 Polycystina, 771, 772, 776 — skeleton of, 659 Polycystina, as test for low powers, 332; mounting, 450 Polydesmidce, 905 * Polygastrica, Ehrenberg’s erroneous views on, 678 Polygonum, pollen-grains of, 646 Polymorphina, 745 Polyommatus Argus, scales of, 900 Polyparies of zoophytes, 786 Polypary of hydroids, 791 Polypes, 787. See Hydrozoa Polypide, of Polyzoa, 829; formation of buds from, 830 Polypidom of zoophyte, 828 Polypite, of hydroids, 791 Polypodium, sori of, 600 Polyporus, 575 Polystichum angulare, apospory in, 605 Polystomella, shell of, 723 — craticulata, 752, 753 — crispa, 752, 754, 765 Polythalamous Foraminifera, 721 Polytoma uvella, life-history of, 684 Polytrema, 749; mode of growth com- pared with Fozoon, 763 — miniaceum, colour of, 724 Polytrichum commune, 595, 596 Polyxenus lagurus, hair of, 905 hair of, as test for objectives, 332; as test for definition, 368 Polyzoa, collecting, 457, 458; keeping alive, 458 ; ‘ cell ’ of, 828 ; structure of, 828; gemmae of, 830; muscular system, 831; sexual reproduction of, 831; ‘ colonial nervous system,’ 831 and note; fresh-water, lophophore of, 832; epistome of, 833; classification of the group, 833; bibliography of, 834; relation to Brachiopoda, 851; ‘ liver ’ of, 971 Polyzoaries in coralline crag, 1011 Polyzoary, 828 Pond-stick, 456 Poplar, pollen-grains of, 647 Poppy, laticiferous tissue, 620; seed of, 648 Porcellanea, 726-735 Porcellanous shells of Foraminifera, 724; of Gastropoda, 852 — and vitreous Foraminifera, difference in, 725, 726 Porcupine, hair of, 954 Pores of sponges, 780 INDEX Pill Porphyra, trichogyne of, 561 Porphyritic crystals, glass inclusions in, 997 ‘ Portable ’ microscope, Powell and Lealand’s, 198, 199 ; Beck’s, 199, 202 ; Rousselet’s binocular, 200; Swift’s, 198, 200 Por tun a, skeleton of, 892 Positive aberration, 309 note — eye-piece, 43 — eye-pieces, 321, 322, 323 Potash, caustic, action on horny sub- stances, 440 Potato-disease, 568 — starch-grains of, 620 — tubers, starch in, 619 Powell, T., formula for objective, 84 Powell and Lealand’s homogeneous im- mersion objective, 29 ; fluorite lenses, 34, 35; high-power binocular, 107; sub-stage, 170, 174 ; their microscopes, 173, 189, 190; binocular, 176; achro- matic dry, 190; portable microscope, 198; rotating nose-pieces, 242; a.hro- matic condenser, 251, 267; new low- power condenser, 252 ; apochromatic condenser, 254; dry achromatic con- denser, 258; chromatic oil condenser, 258; condenser for polariscope, 262; achromatic oil condenser, 263, 267; latest condenser, 267 ; bull’s - eye, 280; vertical illuminator, 285; com- pressor, 296 ; protecting ring for coarse adjustment, 301; water- immersion objectives, 310, 313; inch objective, for observation "of cyclosis, 614; objectives, for study of monads, 687 Powell’s (H.) microscope, 152, 153; fine adjustment applied to the stage, 153 — fine adjustment, 161 Prawn, skeleton of, pigment of, 893 Prazmowski and Hartnack’s water-im- mersion objectives, 310 Preparation of vegetable tissues, 427 Presbyopy,120 Preservative media, 441-443 Primary tissues of Vertebrata, 941 Primordial cells, 465, 466 — utricle, 463; of desmids, 510; of Pha- nerogam cells, 613 — chamber in Foraminifera, 728; of Orbitolites, 731 Primrose, cells of pollen-chambers, 645 ‘ Prince’s feather,’ seed of, 648 Principle of microscopic vision, 43 Principles of microscopical optics, 1 Pringsheim on generative process of Pandorina, 485 ; on Vancheria, 492 Prism, refraction by, 8, 9; Wenham’s, 99 ; Stephenson’s erecting, 102 — polarising, substitution of opalescent mirror for, 172 — rectangular, in place of mirror, 172 — Nicol’s, 244, 269; Nicol’s analysing, 325; Abraham’s, 344 — refracting angle of, 9, 18 1088 INDEX PRI Prismatic epithelium, 968 — layer in molluscan shells, 844, 845, 847, 848 — layer of shells compared with enamel, 949 — shell-substances imitated, 1022 Prisms, recomposition of light by, 18 Pristis, tooth of, 948 Pritchard’s doublets, 249 — microscope with Continental fine ad- justment, 150, 151 Privet hawk-moth, eggs of, 929 Problems on refractive index, 5 Procarp, of Floridece, 561 Projection eye-piece. 323 Promycele of Puccinia, 566 Prosenchymatous tissue, 621 Proteus, red blood-corpuscle of, 960 Prothallium of Sphagnacece, 599; of ferns, 602; of Equisetacece, 606; of Rhizocarpece, 606; of Lycopodiacece, 607 Protococcus, as gonid of lichens, 579 — pluvialis, 473-480; life-history of, 473 ; multiplication of, 474 ; zoospores of, 474, 475 ; mobile and still forms of, 475-477; encysted, 480 Protomyxa aurantiaca, 652-654 Protoneme of Batrachospermum, 505 Protophytes, 460, 580, 651 — mounting, 442; mode of nourishment of, 462; movement by cilia and con- tracting vacuoles of, 465 Protoplasm, 461; vital attributes of, 461; continuity of, 469; of Rhizopoda, 658 ; of Noctiluca, 692 Protoplasmic substance in Vertebrata, 941 Protoplasts, 435 Protozoa, 651-712 — mode of nourishment of, 462 — Lankester’s papers on, 677 ‘ Pseudembryo ’ of Antedon, 827 Pseudo-navicellas, 675 Pseudo-parenchyme of Fungi, 562 Pseudopodia of Protomyxa, 653; of Vampyrella, 655; of Lieberkuehnia, 656; of Rhizopoda, 658; of Reticu- laria, 658; of Heliozoa, 659; of Gromia, 660; of Microgromia, 661; of Acti- nophrys, 663; of Amoeba, 668; of Arcella, &c., 671; in Amoeba-phase of monad, 682; of Eozoon, 766; of Glo- bigerina, 746; of Radiolaria, 772; of endoderm cells in zoophytes, 787 Pseudoraphidece, 527 Pseudoscope, Wheatstone’s, 92 Pseudoscopic effects, 95 — effect with Ramsden’s circles, 107 — vision, 92 Pseudo-scorpions, 932 Pseudo-stigmata of Oribatidce, 935, 936 Pseudo-trachese, on fly’s proboscis, 915 note ‘ Psorosperms,’ 677 Pteris, sori of, 600; indusium of, 600 — serrulata, apogamy in, 605 RAP Pterocanium, 772, 776 Pterodactylus, bones of, 1014 Pterophorus, wings of, 923 Pteroptus, 936 Ptilota, 560 Puccinia graminis, 566 Puff-ball, 576 Pulvilli of insects, 924 ; cockroach, 924 note Pumice, 1014 Pupa of Neuroptera, circulation in, 918 — stage of fly, 931 ‘ Purple laver,’ 561 Purpura, method of examination of egg- capsules of, 863; supplemental yolk of, 931 — lapillus, nidamentum of, 858 ; develop ■ ment of yolk-segments of, 861, 862 ‘ Puss-moth,’ eggs of, 929 Pycnogonida, 881; related to Arachnida, 883 note Pyrola, seeds of, 649 Pyroxene, alteration of, 1001 — andesite, 999 Q Quadrula symmetrica, 671, 672 Quartz-porphyries, 995 Quartzite, 1001 Quekett (E.) on Martin’s microscope,. 139; cn production of raphides, 621; on preparation of tracheae of insects, 921; on minute structure of bone, 1013 Quekett’s loup-holder, 204 ‘ Quills ’ of porcupine, 954 Quinqueloculina, 727 R Radials of A n ‘edon, 825 Radiating crystallisation, 1017 Radiation of light in different media, 53- 58 ; in air and balsam, 55-57 Itadiolaria, collecting, 459 ; skeleton of, 659, 773-777 ; fossilised forms of, 771, 778 note; central capsule of, 772; zoo- xanthellae in, 773; bibliography of, 778 — colonies of, 773; distribution of, 778; mounting, 778 Radiolarian shells in ‘ ooze,’ 1008 Rainey, on presumed cause of cattle plague, 677 ; on molecular coalescence, 1021 Ralfs on British desmids, 509 note; classification, 515; on Eitzschia and Bacillaria, 535 Ralph on dehydration by carbolic acid, 450 Ramsden circles, 107 Ramsden’s eye-piece, 43; ‘ screw micro- meter eye-piece,’ 227,228; positive eye- piece, 323; micrometer eye-piece, 325 Rapliidece, 527 Raphides of Phanerogams, 620, 621 RAT of plants and sponge-spicules compared, 784 Rat’s intestine, villi of, 986 Rays, scales of, 952 Reagents, mode of labelling bottles, 345 Real image, 14 note; formation of, 28, 24 — object image, 321 Recomposition of light by prisms, 18 Red ant, integument of, 898 — blood-corpuscles of Vertebrata, 958 ; size of, in various Vertebrata, 959; re- lative sizes of, in various Vertebrata, 960 — coral, 801 — corpuscles, flow of, 980 ‘ Red snow,’ due to Palmella cruenta, 486 ‘ Red spider,’ 937 Red spots in Infusoria, 7C2 Reflector, Sorby’s parabolic, 281 Refracted ray, 2 Refracting angle of a prism, 9, 18 Refraction, 57 — angle of, 3 — of light, laws of, 2, 3 — by plane surface, 3, 4 ; by curved sur- face, 5; by prisms, 8, 9; by lenses, 10- 25 Refractive index, absolute, 2 ; of water, 3; relative, 4, 5; of crown glass, 5; of flint glass, 5; of balsam, 77 ; of gum styrax, 445 ; of Canada balsam, 445; of monobromide of naphthalin, 445; of phosphorus, 445 Refractive index of silicious coat of dia- toms, 445 — indices of air, of cedar oil, of water, 60 Regulator, Reichert’s, 393 Reichert’s loups, 38; his objectives, 321; his thermo-regulator, 393 Reindeer, hair of, 954 Reproduction in Actinophrys, 664 ; of Actinosphcerium, 666; of Glathrulina, 667 ; of Euglyplia, 671; of sponges, 781; of Campanulariida, 794 ; sexual, of Polyzoa, 831; agamic, of Entorno- straca, 887 ; agamic, 930 ; of Acarina, 932 Reproductive organs of Acarina, 935 ; of Arachnida, 935 Reptiles, lacunae in bone of, 946; cement in teeth of, 950, plates in skin of, 950 ; epidermic appendages of, 958 ; red blood-corpuscles of, 958, 959 ; muscle- fibre of, 973; lungs of, 987 Beseda, seeds of, 649 Residuary secondary spectrum, 313 Resins, solvents for, 441 Resolving power of objectives, 867 of object-glasses, 44 ; of lenses, 64 ; of objective and numerical aperture, 75, 336 Respiration of insects, apparatus of, 918 Respiratory organ of spiders, 938 Bete mucosum, 966 Betepora, calcareous polyzoaries of, 833 Beticularia, 720 INDEX ROS Beticularia, characters of, 658; examples of, 659-662 Reticulated ducts of Phanerogams, 623 Retinulae, 907 Revolving nose-piece, Nelson’s, 244 Rezzi on invention of compound micro- scope, 127 Bhabdammina, 738 — abyssorum, 740 Bhabdolithus pip a, 771 — sceptrum, 771 Rhabdom, 907 Bhabdopleura, 833 Bhamnus, stem of, 628 Bheophax sabulosa, 740 — scorpiurus, 740 Rhinoceros, horn of, 957 Bhizocarpece, 606 Rhizoids of mosses, 594 Rhizome of ferns, 600 Rhizopoda, 658-674, 770 — protoplasm of, 461; ectosarc of, 464 ; Archer’s papers on, 677 ; Biitschli on.. 677 ; skeletons of, 720; sarcode of, 942 ; pseudopodial network of, 977 Bhizosolenia, 543 — cyclosis in, 517 Bhizostoma, 798, 800 Bhizota, 717 Bhododendron, pollen-grains of, 647 Bhodospermece, 503 Rhodospermin, 560 Bhodosporece, 554 Bhopalocanium ornatum, 778, 776 Rhubarb, stellate raphides of, 621; spiral ducts of, 623 Bhynchoflagellata, 694 note Bhynchonellidce, shell structure of, 851 Ribbons of sections, 408 Bibes, pollen-tubes of, 648 Rice, silicified epiderm of, 640 ‘ Rice-paper,’ 611 Rice-starch, 620 Riddell’s binocular microscope, 96, 97 Ring-cells, 386, 887 Rivalto (Giordano da) on invention of spectacles, 120 Bivulariacece, hormogones of, 490 Roach, scales of, 952 Bochea falcata, epiderm of, 639 Rock, ground-mass of, 995; fluxion- structure of, 996 Rocks, method of making sections of, 991; metamorphism of, 999 Rodents, hair of, 954 Roe-stone, structure of, 1011 Root of Phanerogams, structure of, 625, 636 et seq. Root-cap, 635 Bosalina varians, 723 Rose, glandular hairs of, 639 Ross (Andrew) on correction of object- glass, 19-21; his early form of achro- matic microscope, 150; mechanical movements of his stage, 151; his fine adjustment, 151,161; on illumination of objects, 250; his arrangement for lock- 1090 ROS ing coarse adjustment, 301; his achro- matic objectives, 305-306 ; his lever of contact for testing covers, 381 Ross, model, 177 — and Co.’s tank microscope, 221 Ross-Jackson model, 178 Ross’s ‘ Jackson ’ microscope, 151 Ross-Wenham’s radial microscope, 178, 180 Ross-Zentmayer model, 178 Botalia, 749; intermediate skeleton of, 750 — aspera, in chalk, 1009 — Beccarii, shell of, 722 — Schroeteriana, 750 Rotalian series, 748 Rotaliince, colour of shell, 724 Rotaline shells of Foraminifera, 722 — shell, sandy isomorph of, 739 Rotating disc of objectives, 241 Rotatoria, 678. See Rotifera Rotifer vulgaris, 713 Rotifera, preserved by osmic acid, 428 ; collecting, 457 ; keeping alive, 458; as food of Actinophrgs, 663 ; divisions of, 678, 712-719, 867, 869; habitats of, 713; structure of, 714-717; mastax of, 715; lorica of, 715; contractile vesicle of, 716; males of, 717; eggs of, 717; classification of, 717; desiccation of, 718, 887; bibliography of, 719; wheel apparatus of, compared with velum of gastropods, 860, 863; winter eggs of, 888 ; non-sexual reproduction of, 930 Rotten-stone, 546 ‘ Round worm,’ 868 Rousselet’s binocular portable micro- scope, 200; his tank microscope, 224, 225; his compressorium, 295; his live- box, 295 Rowland’s reversible compressor, 295 Rugosa, 801 Rumia cratcegata, eggs of, 929 Rush, stellate tissue in, 612 Rutherford on freezing process, 419 Rutile in clastic rocks, 998 Ryder’s microtome, 344 S Sabellaria, tubes of, 872 Sable, hair of, 954 Saccammina in limestone, 1012 — Carteri, 737 — spherica, 737 Saccharomyces cerevisice, 574 Saccliaromycetes, 574; zymotic action of, 574; endospores of, 575 Saccolabium guttatum, spiral cells of 618 Sachs on Ghara, 509 note Sago, starch-grains of, 620 Salicylic acid, for mounting, 442 Salivary glands, 971 Salmon, scales of, 952 — disease, 569 INDEX SCH Salpce, diatoms in stomach of, 544, 555 Salpida, 835 Salpingceca, calyx of, 689 Salt solution as a preservative medium, 442 Salter (J.) on the ‘ teeth ’ of Echinus, 814 Salvia verbenaca, spiral fibres in seeds of, 618 Sand-grains surrounded by silica, 999 ‘ Sand-stars.’ See Ophiuroidea ‘ Sand-wasp,’ 898 Sandy isomorphs (Foraminifera), 739 — tests of Lituolida, 739 Santonine, crystallisation of, 1017 Sap-wood, 629 Saprolegnia, 493, 494 note — ferox, 569 Saprolegnice, 569 Saprophytes, 575 Saprophytic organisms, study of, 280 — Bacteria, 585 — fungi, 562 Sarcocystids, 677 Sarcode, 460 note, 461; of Iihizopida, 658 Sarcolemma, 973 Sarcoptes scabiei, 937 Sarcoptidce, mandibles of, 933 ; maxillas of, 934; hairs of, 934; legs of, 934; characters of, 937 Sarcoptince, 937 Sarcosporidia, 674 Sargassum bacciferum, 559 Sarsia (Medusa of Syncoryne), 793 ‘ Saw-flies,’ ovipositor of, 927 Saxifraga, seeds of, 649 — umbrosa, parenchyme of, 613 Saxifrage, cells of pollen-chambers, 645 Scalariform ducts of ferns, 599 ; as modified spiral ducts, 623 ‘ Scales,’ covering epiderm of leaves, 639; of Elceagnus, 639 — of Lepidoptera, 899, 900; of Coleo- ptera, 899; of Curculio imperialis, 899; of Lyccenidce, 899, 901; of Pieridce, 899 ; as tests for objectives, 900; of insects, markings of, 900; of Thysanura, 901; on wing of Lepido- ptera, 923; of fishes, 950; of reptiles, 950, 953; Scallops. See Pecten Scarabeei, antennas of, 912 ‘ Scarfskin,’ 965 Scatophaga stercoraria, eggs of, 930 Scenedesmus, megazobspores of, 496 Schists, 1000, 1001 Sehizogenous spaces in Phanerogams, 613 Schizomycetes, 579-589 Schizonema, 528 — Grevillii, 547 — mucous sheath of, 518, 547 Scliizonemece, character of, 547 Schnetzler, on movement of Oscillaria, 490 Schott (Dr.) on improvement of object- glasses, 31 INDEX SHR Selective staining, 431 — stains, 436 Selenite, 270 — with mica film, 271 — stage, 270 Selenites, 262 ; blue and red, 271 Selligue’s achromatic microscope, 146, 148; objectives, 303 Semi-apochromatic objective of Leitz, 320 Sempervivum, seeds of, 649 Seneca, on magnifying by water, 120 Sense, organs of, in Mollusca, 864 Sensory nerves, 976 — organs of sponges, 780 Sepals, 643 Sepia, pigment-cells, 866 Sepiola, eggs of, 866 1 Sepiostaire ’ of cuttle-fish, structure of, 853 ; imitations of, 1023 Septa in shell of Foraminifera, 721, 728, 729 Serialaria, presumed nervous system in, 831 Serous membrane, 965, 966 Serpula, tubes of, 872 Serricornes, antenme of, 911 Sertularia cupressina, 795 Sertulariida, gonozobids cf, 794; zoo- phytic stage of, 801 Sessile cirripeds, 891 Seta of Tomopteris, 877 ‘ Sewage fungus,’ 583 Sexual fructification, 470 — generation of Volvox, 483 Shadbolt on structure of Arachnoidiscus, 541 Shadbolt’s turn-table, 386, 391 Shadow effects, 61 Shark, dentine of, 947 Sharks, scales of, 952 Sheep-pox, 588 Sheep-rot, 869 Shell, bivalve, of Ostracoda, 889 — calcareous, of Retir.ularia, 658; of Microgromia, 661 — silicious, of Dictyocysta, Costonella, 700 — of Foraminif era, 721-726 ; of Latnel- libranchiata, 843 ; of Brachiopoda, 843 Shellac cement, protection against cedar oil, 384 ‘ Shell-fish,’ 843. See Mollusca Shells of Mollusca, nacreous layer of, 843, 846, 848; prismatic layer of, 844, 845, 847, 848; colour of, 845; an ex- cretory product, 846 ; sub-nacreous layer of, 847, 848 — of Brachiopoda, 849; periostracum of, 850 ; perforations of, 850 — of Gastropoda, structure of, 852 — of Girnpedia, 892 ‘ Shield ’ of Ciliata, 700 Shrimp, concretionary spheroids in skin of, 1021 Shrimps, skeleton of, 893 1091 SCH Schroder on binocular vision, 107; his fine adjustment, 160; his camera lucida, 236 Schultz’s method of macerating vege- table tissues, 625 Schultze (Prof. E.), his aquarium micro- scope, 222 Schultze (Prof. Max) on identity of ‘ sarcode ’ and ‘ protoplasm,’ 460 note; on cyclosis in Diatomacece, 517 ; on affinity of Carpenteria, 747 Schulze (Prof. F. E.) on soft parts of Euplectella, 785 note Schwendener on lichens, 577 Scirtopoda, 717, 718 Scissors, spring, 396; for section cutting, 397 Sclerenchyme of ferns, 600 Sclerogen, 621 Sclerotesin Fungi, 562; of Myxomycetes, 565 Scolopendrium, indusium of, 600; sori of, 600; sporanges of, 601 Scorpions, 881, 932 Screw-collar adjustment, 309 Scrophularia, seeds of, 649 * Scyphistoma ’ of Cyanea, 799 Scytonema, as gonid of lichen, 579 Scytonemacecz, 490; hormogones of, 490 Scytosiphon, conjugation of, 556 Sea-anemone. See Actinia Sea-anemones, intracellular digestion in, 787 Sea-fans, 801. See Gorgonice ‘ Sea-jellies,’ 777 Sealing-wax varnish, 384 ‘ Sea-mats,’ 832. See Flustra and Mem- hranipora Searcher eye-pieces, 323 * Sea-slugs.’ See Boris, Folis ‘ Sea-urchin,’ 808. See Echinus Sea-weeds, 554 — continuity of protoplasm in, 469 — red, 503 Secondary minerals, 1001 — spectrum, 19, 31; overcome by Abbe’s objectives, 314 Section cutting, scissors for, 397 — lifters, 431, 432 ; cover glass as, 432 — mounting, 447 Sections, ribbons of, 408; of hard sub- stances, 420; of bones, 420, 423; of coral, 420, 423; of enamel, 420; of fossils, 420; of shells, 420; of teeth, 420, 423; of hard and soft substances together, 423; of Phanerogam tissues, 624 Sedum, pollen-grains of, 646; seeds of, 649 Seeds, 609, 648 Segmentation of Gastropoda egg, 859; of annelid body, 872 Seiler’s solution for cleaning slides, 380 Selaginella, archegone of, homology of, | 610 Selaginellece, 607 1092 SID Side reflector, 281 — lever, short, fine adjustment, 162 Swift’s vertical fine adjustment, 162, 181 Siebold on agamic reproduction in bees, 930 Sieve-plates, 635 Sieve-tubes, 635; in Exogens, 622 Sigillarice, 607, 1005 Silene, seeds of, 649 Silex in Equisetacece, 605; in epiderm of grasses, 639 Silk glands of spiders, 939 ‘ Silk-weeds,’ 499 ‘ Silkworm,’ eggs of, 929 Silkworm disease, 573 Silpha, antennas of, 912 Simple magnifier, 37 — microscope, 201 Sines, law of, 3 Siphonacece, 491-493 ; Munier-Charles on fossil forms of, 493 Siphonostomata, 889 note Siricidce, ovipositor of, 927 Sirodot on alternation of generations in Batrachospermum, 504 Skate, muscle fibre, 973 Skeleton, dermal, of Vertebrata, 950; fossilised, 1012 — fibrous, of sponges, 781 — silicious, of Heliozoa, 659; of Radio- laria, 771 — of sponges, 779; of zoophytes, 786; of Echinoidea, 808 ; of Asteroidea, 815; of Ophiuroidea,815; of Crinoidea, 816; of Holothurioidea, 818; of Ante- don, 825; of Vertebrata, structure of, 944 Skin, 965; pigment-cells in, 966; capil- laries in, 986 Skip-jack, antennae of, 911 Slack on the costae of Pinnularia, 546 Slack’s optical illusion, 370 Slide-forceps, 393 Slide-glass, 379 Slides for cultures, 288, 289 — Seiler’s solution for cleansing, 880 Sliding-plate of objectives, 241 Sloths, fossil, teeth of, 948 Slug. See Limax Slug’s eye, 865 Slugs, Rotifera in, 713 Smell, organ of, in insects, 924 Smith and Beck’s microscope, 153, 154 Smith’s Cassegrainian microscope, 144; his reflecting microscope, 144 Smith (H. L.) on Tolies’ binocular eye- piece, 103 ; his vertical illuminator, 284, 285; on classification of diatoms, 527 Smith (James) on use of bull’s-eye with high powers, 280 ; his separating lenses, 309; his mounting instrument, 394 Smith (T. F.) on markings of diatoms, 522 Smith (W.) on cyclosis in Diatomacccc, 517; on species of diatoms, 530 note; on habits of diatoms, 548 INDEX SPH Smith (W. H.) on structure of frustules, 519 note; on movements of diatoms, 531 Snail, 854 ; eye of, 865. See Helix — muscle of odontopliore, 974 Snake, lung of, 987 Snapdragon, seed of, 648 Snell’s ‘ Law of Sines,’ 49 Snow, crystals of, 1016 Snowberry, parenchyme of fruit of, 613 Snowdrop, pollen-grains of, 647 Soda, caustic, action on horny substances, 440 Soemmering’s simple camera, 234 Sole, scales of, 950, 951, 952 Solen, prismatic layer in, 848 Solid cones of light for minute observa- tion, 362 — eye-pieces, 42, 322 — image, 95 — objects, delineation of, 83; correct appreciation of, 88 — vision, the consequence of oblique illumination, 61 Sollas on sponges, 783 note; on the ex- tensions of the perivisceral cavity in Polyzoa, 851 Sorby (H. C.) on microscopic structure of crystals, 990 Sorby’s parabolic reflector, 281 Sorby-Browning’s micro-spectroscope, 272, 273 Soredes of lichens, 577 Sori of ferns, 600 Sound-producing apparatus of crickets, 923 Spatangidium, 539 Spatangus, spines of, 813 ‘ Spawn ’ of mushroom, 575 Spectacles, invention of, 120 Spectra, diffraction, 67 — artificial, 274 Spectral ocular, Zeiss’s, 276 Spectro-micrometer, bright-line, 274 Spectroscope in micro-chemical opera- tions, 1024 Spectroscopic test, 273 Spectrum, 19; irrationality of, 19 — binocular, microscope, 276 — map, 275 — natural. 274 — of dark lines, 273 ; of bright lines, 273 Speculum, parabolic, 281; Lieberkiihn’s, 282-284; in Smith’s illuminator, 284, 285 Spermathecse of Gamasidce, 936 ; of Tyroglyphidce, 986 Spermatia of Puccinia, 567; of lichens, 578 Spermatic fluid, preservative for, 443 Spenn-cells, 467 ; of Volvox, 483; of ferns, 603; of sponges, 781; of Hydra, 790 ; of Polyzoa, 831 Spermogones of Puccinia, 567 ; of lichens, 578 Sphacelaria, 555 Spliacele, 555 INDEX 1093 SPH Sphceria in caterpillars, 574 Sphceroplea annulina, 500, 501 Sphcerozosma, rows of cells in, 512 JSphcerozoum ovodimare, 111 Sphagnacece, 598, 599 Sphagnum, leaf of, 598 Sphenogyne speciosa, winged seed of, 649 Spherical aberration, 14, 15, 31, 249, 251, 254, 331 diminished by Huyghens’ objective, 42 Spheroidal concretions of carbonate of lime, 1021 Sphingidce, antennae of, 912 Sphinx, eye of, 911; antennae of, 912 — ligustri, eggs of, 929 Spicules of alcyonarians, 804 — of sponges, 772; their names, 783-784 — silicious, of sponges, 781 — calcareous, of sponges, 781 Spiders, 881, 932,938; microscopic objects furnished by, 938 ; spinning apparatus, 939 Spinal cord, Hill’s method of preparation of, 434 Spindle fibres, 468 Spinnerets of spiders, 939 Spiny lobster, metamorphosis, 893 Spiracles of insects, 919, 920 Spiral cells in Phanerogams, 618; mode of preparation of, 619 — crystallisation, 1018 — focussing for projection-lens, 324 — vessels of Phanerogams, 622, 623 ; observation of, in situ, 644; of plants compared with tracheae of insects, 919 Spiriferidce, perforation in shells of, 851 Spiriferina rostrata, shell of, 851 Spirillina, 744 — sandy isomorph of, 739 Spirillum, movement of, 375; granular spheres of, 588 note — undula, 586 — volutans, movement of, 581, 583, 586 Spirit, dilute, as a preservative medium, 442 Spirochcete, 581 Sprogyra, 478 ; attacked by Vampy- rella, 654 Spirolina, a varietal form of Peneroplis, 728 Spiroloculina, 121 Spirula, 853 — shells of, bearing Protomyxa, 652 Spirulina, movement of, 490 Splachnum, sporange of, 594 Splenic fever, 588 — due to Bacillus anthracis, 582 Sponge-spicules, 781-784 — mounting, 450 — in Carpenteria, 747 ; in mud of Levant, 1007 Sponges, 779-786; skeleton of, structure of, 779, 780 ; reproduction of, 781; habitat of, 785 ; preparation of, 785, 786; bibliography of, 786; pseudopodia STA of cells in, 786; intracellular digestion in, 787; fresh-water form of, 787 Spongilla, 785 Spongolithis acicularis, 550 Spongy parenchyma of leaves, 641 Spontaneous generation, 686 Sporange of Fungi, 562; of Marchantia, 590, 593; of mosses, 596; of Sphag- nacece, 599; of ferns, 600 ; of Fquise- tacece, 605 ; of Myxomycetes, 565 Sporangia of Lycopodiacece in coal, 1006 Sporangiophores of Mucorini, 569 Spore, use of the term, 467 note Spores of Nostoc, 491 ; of Myxomycetes, 563, 565; of Peronosporece, 568; of Bacteria, 587; of Marchantia, 593; of mosses, 597; of ferns, 601; of ferns, method for studying development of, 604 note ; of Equisetacece, 605 ; of Lycopodiece, 606 ; of gregarines, 675 ; of Monas Dallingeri, 682; of Lycopo- diacecc in coal, 1006 — different kinds of, 470 note — resting, of Chcetophoracece, 583 Sporids of Ustilaginece, 565; of Puccinia, 566 Sporocarp of Ascomycetes, 572 Sporogone of mosses, 597 Sporophores of Myxomycetes, 565; of Peronosporece, 568 ; of Ascomycetes, 571 Sporophyte in ferns, 605 Sporozoa, 674-677 Sporules of Melosira, 526 ; of Pleuro- sigma, 526; of Podosphenia, 526 Spot-lens, 267 Spring-clip, 394 — press, 394 — scissors, 396 ‘ Spring-tails,’ 903. See Poduridce Squid, 866 Squirrel, hair of, 954, 955 Stag-beetle, antennae of, 912 Stage, horse-shoe, Nelson’s, 163, 190; of the microscope, 165-168; concentric, rotatory motion of, 167 ; qualities need- ful in a, 167 ; in Hartnack’s model, 211 ; mechanical, 215 ; graduated rotary, 338 -—forceps, 287 micrometer, 226, 230, 239, 240 — moist, 290 plate, glass, 288 — thermostatic, 292, 293 — Turrell’s, 165, 189 ; Tolies’, 166, 184 ; Zeiss’s, 167 vice, 287 ‘ Staggers ’ of sheep, due to Ccenurus, 868 Stahl on movement of desmids, 510 Staining, process of, on glass slides, 430, 431; multiple, 438 ; double, 438; me- thods, 439; differential, 439 — Bacteria, 437, 438 — fluids, 432-437 — processes, 430 Stains, violet of methanilin for Bacteria, 437 ; methyl-blue for Bacteria, 638 1094 INDEX SUN Sterigmata of Puccinia, 5(50 Sterile cells of Volvox, 483 Stibbite, 998 Stichopus Kefersteinii, 819 Stick-net for marine work, 459 Stickleback, parasite of, 890 ; circulation in tail of, 981 Stigmata of insects, 919, 920 ‘ Stinging hairs ’ of nettle, 639 Stings of insects, 926, 927 Stipe of diatoms, 517, 518 ; of Licmo- pliora, 534 ; of Gomphonema, 545 Stolon of Foraminifera, 721; of Eozoon, 764 ; of Laguncula, 828 ; of ascidians, 838 Stomach, follicles of, 971 Stomates, 640 — of Marchantia, 591 Stomopneustes variolaris, spines of, 812 Stone-cavities in crystals, 997 Stone-mit % eggs of, 933 Stones of fruit, preparing sections of, 624 — of stone fruit, constitution of, 618 Stone-wort, 505 Stony corals, resembled by polyzoaries, 828 Stop, introduction of, 37; in the eye- piece, 42; use of, 261, 263 Strasburger’s borax carmine, for study of embryo-sac, 435 Strawberry, parenchyme of fruit, 613 Streptocaulus pulcherrimus, 795 Striated muscle, 972; size of fibres in different groups, 973 Striatella unipunctata, 527 Striatellece, characters of, 536 ‘ Strobila ’ of Cyanea, 799 Stromatopora, doubtful character of, 767 Stromatoporoids, 742 note Strophomenidce, perforations in shells of, 851 Stylodyctya gracilis, 775 Suberous layer of bark, 633 Sub-nacreous layer in molluscan shells, 847, 848 Sub-stage, 169-171, 215; Nelson’s fine adjustment to, 169; Powell and Lea- land’s, 170, 174; swinging, 170, 171, 184; in Ross-Zentmayer’s model, 178; in Beck’s microscope, 181;1 centring nose-piece used as, 193 ‘ Sub-stage condenser,’ Nelson’s, 72; Stephenson’s, 101; compound, 135 — illumination, 248 — simplest form of, 261 Succulent plants, stomates in, 640 Sucker on legs of Sarcoptidte, 934 Suckers on foot of Dytiscus, 925; of Cur- culionidce, 926 Suctoria (Protozoa), 696-699 — (Crustacea), 890, 891 ‘ Sugar-louse,’ 901. See Lepisma Sulphuric acid, as a test, 440 ‘ Sun-animalcule,’ 662 ‘ Sundew,’ glands of, 639 Sunk-cells, 388 STA Stains, Nicholson’s blue, 439 Stanhope lens, 37 Stanhoscope, 38 Staphylinus, antennae of, 912 Star-anise, tissue of testa of, 617; testa of seeds of, 649 Starch, tests for, 440; formation of, 619 — grains, 464, 465 ; mode of growth, 620; hilum of, 619; in Ganna, 620; in potato, 620 ; in wheat, 620; in rice, 620 1 Star-fish,’ 815. See Asteroidea Statospore of Protomyxa, 653 Staurastrum, binary division of, 512; form of cell, 515 — dejection, 498 Stauroneis, 546 ‘ Stauros ’ of Achnanthes, 545 Steenstrup on alternation of generations, 801 Stein on affinities of Volvox, 479; on contractile vacuoles of Volvox, 481 note; on Flagellata, 689; on Nocti- luca, 694 note ; on Acinetina, 699 note Steinheil’s loups, 38; his combination of lenses, 38; his aplanatic loup, 205 ; his loup for tank work, 234 ; his formula for combination of lenses, 316; his triple loups, 322 Stellaria, seeds of, 649 — media, petals of, 644 Stem of mosses, 594; of Bryacece, 598 ; of Sphagnacece, 598; structure of, in — Phanerogams, 625 ; of Phanerogams, development of, 634, 635; treatment of, for examination of their structure, 636, 637 Stemmata of insects, 910; of spiders, 938 Stentor, collecting, 457; impressionable organs of, 702; contractile vesicle of, 704 ; conjugation of, 711 Stephanoceros, collecting, 457 ; in con- finement, 458 Steplianolitliis spinescens, 771 — nodosa, 771 Stephanosphcera pluvialis, amoebiform phase of, 485 note Stephenson on Pleurosigma. angulatum, 70 ; on ‘ intercostal points,’ 73 — his suggestion on homogeneous im- mersion, 28 — on Coscinodiscus, 538 Stephenson’s binocular, 100, 344 ; sub- stage condenser, 101; erecting binocu- lar, 102; erecting prism, 102; cata- dioptric illuminator, 170,263; binocular dissecting microscope, 201, 203, 395; tank microscope, 220 Stereocaulon ramulosus, 579 Stereo-pseudoscopic microscope, Na- chet’s, 208 Stereoscope, 90 ; Brewster’s modification of, 91 Stereoscopic binocular, Wenliam’s, 98; for study of opaque objects, 105, 107 — eye-piece, Abbe’s, 103 — vision, 89-98 INDEX 1095 SUP Super-amplification, 33 Super-stage, 169 Supplemental yolk in Purpura, 862, 863, 931 Surirella, 518, 535; conjugation of, 528; zygospores of, 529; movements of, 531; frustule of, 535 — biseriata, cyclosis in, 517 — caledonica, 551 — constricta, 536 — craticula, 551 — plicata, 551 Surirellece, 535 Suspensor of ovule of Phanerogams, 464 Sutural line of desmids, 590 Swarm-spores, 466; not a new genera- tion, 467; meaning of term, 470 note', of Pandorina, 485; of Hydrodictyon, 495; of Gutleria, 556; of Clathrulina, 667 ; presumed, of Pelomyxa, 670 Sweat-glands, 966 ‘ Sweetbread,’ 971 Swift’s side-lever, 158; vertical side- lever fine adjustment, 162,181; micro- scopes, 181, 190, 194, 197; portable microscope, 198; low-power condenser, 252 ; condenser for polariscope, 262; sub-stage illuminator, 271 ; micro- spectroscope, 275; live-box, 295; petro- logical microscope, 992 Symbiosis in lichens, 578 Symbiotes tripilis, hairs of, 934 Symbiotic algse in radiolarians, 773 Sympathetic nerves, 978 Symphytum asperrimum, seeds of, 649 Synalissa symphorea, 579 Synapta digitata, ‘ anchors ’ of, 819 — inhcerens, ‘ anchors ’ of, 819 Synaptce, rotifers in, 713 Syncoryne Sarsii, gonozobids of, 792 Syncrypta, 475 Syne dr a, 535 Syringammina, 736 Syringe for catching minute aquatic objects, 300 Syrup, as a preservative medium, 442 — and gum, as a preservative medium, 443 T Tabanus, 911; ovipositor of, 927 Tabellaria vulgaris, 551 Table of numerical apertures, 84-87 — for microscopists, 341-345; for dis- secting and mounting, 342 Tactile papillae of skin, 966; nerve to, 977 Tadpole, pigment-cells of, 967; circula- tion in tail of, 980 ; general circulation in, 981; blood-vessels of, 983, 984 — of ascidians, 841 Tadpole’s tail, epithelium of, 968 Tcenia, 867 Tank microscopes, 219-225 Tannin, test for, 440 Tapetal cells in fern antherid, 603 THA ‘ Tape-worm,’ 867 Tardigrada, desiccation of, 869 Tarsonemidce, 937 Taste, organs of, in insects, 917, 924 Teeth, decalcification of, 426 — fossilised, 1012 — in palate of Helix, 854; of Limax, 854; of Buccinum, 854 ; of Mollusca, 854 — preparation of, 947 — of Echinus, 814 ; of Ophiothrix, 816 ; of Vertebrata, 947 — of elephant, Rolleston on enamel in, 852; of Bodentia, Tonies on enamel in, 852 Tegeocranus cepheiformis, 932 — dentatus, 932 Tegumentary appendages of insects, 898 Telescope, Barker’s Gregorian, 144 Teleutospore generation of Puccinia, 566 Temperature, effect of, on various monads, 686 Tendon, 943 Tentacle of Noctiluca, 691, 692 ‘ Tentacles ’ of Drosera, 639 ; of Suctoria, 697 ; of Hydra, 788; of annelids, 873 Tenthredinidce, ovipositor of, 927 Terebella, tubes of, 872; gills of, 873 — conchilega, 872 Terebratula bullata, shell of, 851 Terebratulce, shells of, 849, 850 Terpsinoe musica, 537 Terpsinoece, character of, 537 Tertiary tints in crystalline bodies, 1018 Tesselated epithelium, 968 Test of Gromia, 660; of Arcella, 670; of Difflugia, 671 Testa of seeds, 649 Testaceous amoebans, 670, 671 Testing object-glasses, 325; diaphragm for use in, 329; Fripp’s method, 330; Abbe’s method, 326-333 Test-plate, Abbe’s, 330, 331 Tests, sandy, of Lituolida, 739 Tethya, spicules of, 1008 Tetramitus rostratus, life-history of, 685 ; nucleus of, 688 Tetranychi, 937 Tetranychus, mandibles of, 933 Tetraspores of Floridece, 561; of Vam- pyrella, 655 Textularia, 248 — aculeata, in chalk, 1009 — globulosa, in chalk, 1009 Textularian form of shell, 723 — series, 748 Textulariidce, 736 Textularinice, arenaceous character of, 748 Thalassicolla, 772, 777 Thallophytes, 467, 470 Thallophytic type, passage to cormo- phytic, 594 Thallus of TJlva, 488; of Phceosporece, 555 ; of lichens, 577 Thaumantias Eschscholtzii, 797 THA Thaumantias pilosella, 797 ‘ Theca ’ of mosses, 596 Thecaphora, 792 Thecata, 792, 794 — zoophytic stage of, 801 Thermo-regulator, Reichert’s, 393 Thermostatic stage, Dallinger’s, 292-293 Thoma’s (Jung) microtome, 401 Thompson (J. Vaughan) on pentacrinoid larva of Antedon, 825 ; on Cirripedia, 891 Thomson (Wyville) on development of Antedon, 827 Thread-cells of zoophytes, 701; of Hydra, 788 ; of Zoantharia, 801, 802 ; of pla- narians, 871 ‘ Thread-worm,’ 868 Threads of spiders’ webs, 939 Thurammina papillata, 738, 740 Thwaites on conjugation of Epithemia, 529 ; of Melosira, 530 Thysanura, scales of, 901 Ticks, 932. See Acarina Tineidce, wings of, 923 Tinoporus baculatus, 749 Tipula, eye of, 911; antennae of, 912; spiracle of, 920 Tolies’ binocular eye-piece, 102; his me- chanical stage, 166, 184; his vertical illuminator, 285; his water-immersion objectives, 310 ; his apertometer, 333 Tomes (Charles) on teeth, 949 Tomopteris onisciformis, 876, 877; de- velopment of, 878 — quadricornis, 878 ‘ Tongue ’ of Gastropoda. See Palate ‘ Tortoise-shell butterfly,’ eggs of, 929 Torula cerevisice, 574 Total reflexion, 6, 7 Tourmaline, pleochroism in, 1002 Tow-net, 458 Tow-nets of Challenger Expedition, 459 7iote Tracheae of insects, 918 ; of Acarina, 935 Trachei'des of ferns, 600; of conifers, 622, 628 Traclielomonas, 475 Tradescantia virginica, cyclosis in hairs of, 615, 616 Tragulus javanicus, red blood-corpuscle of, 960 Trematodes, 869 Triceratium, 518, 520 — experiments with, 357, 358 —favus, 542 — markings on, 522 —fimbriatum, as test for higher powers, 332 Trichocysts of Ciliata, 701 Trichoda lynceus, crawling of, 702 ; re- production of, 707-709 Trichodina grandinella, a phase in de- velopment of Vorticella, 707 Tricliogyne of Florideie, 561; in lichens, 578 Trichonympha, 702 INDEX UND Trichophore of Floridece, 561 Trichophrya, a phase in development of Suctoria, 698 Trigonia, prismatic layer in, 848 Triloculina, 727 Triple-backed objectives, 310 Triplet, Holland’s, 37 Triplex front to objectives, 317 Tripoli stone, 546 Trochus zizyphinus, palate of, 855 Trombidiidce, 932 ; legs of, 934 ; hairs of, 934 ; eyes of, 935 ; tracheae of, 935 ; characters of, 936 Trombidium, maxilla; of, 934; larvae of, 937 —- holosericum, 937 Trophi of Roiifera, 715 Truncatulina rosea, colour of, 724 ‘Tube-cells,’ 382 Tube-length, English and Continental, 155 Tuberculosis, bacillus of, 437; Pittion and Roux’s method of staining, 439 Tubifex rivulorum, gregarine of, 676 Tubipora, 801 Tubularia, gonozobids of, 793 — indivisa, 793 Tubuli in Nummulites, 751; of dentine, 948 Tubulipora, 833 Tulip, raphides of, 621 Tully’s achromatic microscope, 147; his live-box, 294 ; his triplet, 303; his achromatic objective, 303 ‘ Tunic ’ of Tunicata, 835 Tunicata, 828, 835-842 ; zoological posi- tion of, 835; bibliography of, 842 ; ‘ liver ’ of, 971 Turbellaria, 867, 870 — larvae of, collecting, 459 Turbinoid shell of Foraminifera, 722 Turbo, shell structure of, 852 Turkey-stone, use of, 421; constituents of, 546 Turn-table, Shadbolt’s, 386, 391 ; Griffith’s, 391 Turpentine, uses of, 441 Turrell’s mechanical stage, 165, 189 Twin lamellae in leucite, 1002 Tylenchus tritici, 869 Tympanum of cricket, 923 Typhoid of the pig, due to Bacillus, 588 Tyroglyphi, nymph of, 933 ; legs of, 934 Tyroglyphidce, reproductive organs of, 936; characters of, 937 U TJlothrix, conjugation of, 486 Viva, 488, 489 UlvacecB, 487 Umbelliferous plants, seeds of, 649 Umbonula verrucosa, 830 Under-corrected objective, 20, 21 Under-correction, 255, 307, 308 UNG Unger on the zoospores of Vaucheria, 492 Unicellular plants, 469 XJnio, pearls in, 847 ; glochidia of, 857 — occidens, formation of shell in, 849 Unionidce, nacreous layer of, 847 Unit (standard) for microscopy, 400 Uredinece, 565 ; alternation of genera- tions in, 565 Uredo-form of Puccinia, 567 Uredospores of Puccinia, 567 Urinary calculi and molecular coalescence, 1023 Urine, micro-chemical examination, 1024 JJrochordata, 835 TJropoda, tracheae of, 935 ‘ Urticating organs.’ See Thread-cells Ustilaginece, 565 TJvella, 475 V Vacuoles in cell, 464 — contractile, in protophytes, 465 ; of Volvox, 481 — of Actinophrys, 662 Vagine of mosses, 596 Valentin’s two-bladed knife, 398 Vallisneria, habitat, 613, 614; mode of demonstration of cyclosis, 613, 614 Valvulina, shell of, 723 Vampyrella, 654, 655 — gomphonematis, 655, 656 — spirogyrce, 654, 655 Vanessa, 911; haustellium of, 916 — urticce, eggs of, 929 Variation, range of, in Astromma, 774 Varley’s live-box, 294 Varnish, test for, 383; asphalte, 383 Varnishes, 382; sealing-wax in alcohol, 884; red, 385; white, 385; various colours, 385 ‘Vascular Cryptogams,’ links with Pha- nerogams, 607 Vascular papillae of skin, 966 Vaucher, on Siphonacece, 492 Vaucheria, 491-493 — Rotifera in, 713 ‘Vegetable ivory,’ endosperm of, 618 Vegetable substance, preparation of, 427 ; gum-imbedding for, 427; bleach- ing of, 427 ; Cole’s staining method, 436 — structures, hardened in osmic acid, 428 Veins of vertebrates, 980 Velum,’ in gastropod larva, 860 Venice turpentine cement, for glycerin mounts, 384 Ventriculites, 785, 1010 Venus’ flower basket, 783, 784 ; spicules of, 784 Verbena, seeds of, 649 Vertebrata, 835; bone of, 944 ; teeth of, 947 ; dermal skeleton of, 950; blood of, 958; red blood-corpuscles, 958; white blood-corpuscles, 960; distribu- tion of ciliated epithelium, 968; kidney of, 971 INDEX WAR Vertebrated animals, 941 Vertical illuminator for ascertaining ‘ aperture,’ 206 Vespidce, 911 Vibracula of Polyzoa, 834, 835 Vibrio, movement of, 375 — rugula, 586 ‘ Vibriones,’ as applied to certain nema- todes, 869 Vibriones, form of, 581, 586 Vigelius on tentacular cavity of Polyzoa, 829 note Vignal on osmic acid for Noctiluca, 428 Vine, size of ducts of, 623 Viola tricolor, pollen-tubes of, 648 Violet, cells of pollen-chamber, 645 — of methanilin, for staining Bacteria, 437 Virginian spiderwort, cyclosis in, 615, 616 Virtual image, 14 note, 24, 25, 321 Vision, depth of, 88, 89, 90; stereoscopic, 89 Visual angle, 27 Vitrea (Foraminifera), 744 Vitreous cells (arthropod eye), 907 — optical compounds, 31 — shells of Foraminifera, 724 ‘ Vittse ’ of Licmophorea, 534 ; of seeds of umbellifers, 649 Vocal cords, structure of, 964 Vogan’s changing nose-piece, 244 Volcanic ashes, microscopical examina- tion of, 101 —- dust, examination of, 999 Volvocinece, 479-485 Volvox associated with Astasia, 690 — vegetable nature of, 484 note; amce- biforrn phase of, 485 ; Rotifera in, 713 — aureus, cellulose in, 481; starch in, 481 — globator, 479-485; flagellate affinities of, 479; contractile vacuoles in, 481; endochrome of, 482 ; multiplication of, 483; reproductive cells of, 483, 484 Vorticella, foot-stalk of, 701; contrac- tion of foot-stalk, 702 ; fission of, 704 ; gemmiparous reproduction of, 711; conjugation of, 711 — microstoma, encystment of, 706, 707 Vorticellina, encystment of, 706 W Waldheimia australis, shell of, 850 Wale’s coarse adjustment, 185 ; his fine adjustment, 185 ; his limb, 185, 189 Wallflower, pollen-grams of, 647 Wall-lichens, 577 Wallich, on structure of diatom frustule, 519 note; on Triceratium, 543 note; on Chcetocerece, 544 note; on cocco- spheres, 672 ; on Polycystina, 776 note Wallich’s plan for sectionising a number of hard objects, 421 note ‘ Wanghie cane,’ stem of, 626 Ward’s simple microscope, 205 1097 1098 WAR ‘ Warm-stage’ for observing blood-cor- puscles, 958 Warmth, mode of applying, for cyclosis, 616 Wasps, wings of, 922, 923; sting of, 927 Water, refractive index of, 3, 7 — distilled, for mounting Protophytes, 442 — milfoil, 458 Water-angle, 50 Water-bath, 392 Water-boatman, wings of, 924 ‘ Water-fleas,’ 883, 88(5 Water-globules in oil, 370, 371 Water-immersion objectives, 310; Zeiss’s, 317 Water-lily, leaf-structure of, 642; cells of pollen-chambers, 645 ‘ Water-mites,’ 937 ‘ Water-net,’ or Hydrodictyon, 495 Water-of-Ayr stone, 421 Water-scorpion, 919. See Nepa ‘ Water-snail.’ See Limnccus Water-vascular system of Tania, 867 Wavellite in My a, 848 Wax and olive oil for imbedding, 417 Web of spiders, 939 Weber’s annular cells, 299 Webster condenser, 256 Weismann on development of Diptera, 931 Wenham on binocular vision, 107; on cyclosis of Vallisneria, 615 Wenham’s suggestion of homogene- ous immersion, 29; his stereoscopic binocular, 95-98; his prism, 99; his reflex illuminator, 265, 266 ; his disc and button illuminators, 266; his paraboloid, 267-269; his achromatic objective with single front, 310; his duplex front objective 311 West on Cluetocerece, 544 note ‘Whalebone,’ 957 Wheat, starch-grains of, 620 Wheatstone’s stereoscope, 90 ; his pseudoscope, 92 ‘ Wheel - animalcules,’ 678, 712. See Rotifera Wheel-like plates of Chirodota, 820 ‘ Wheels ’ of Rotifera, 713 Whelk. See Buccinum ‘ White ant,’ ciliate parasite of, 702 White blood-corpuscles of Vertebrata, 960 ; flow of, 980 — fibrous tissue, 963, 964 — of egg, as a preservative medium, 442 Whitney’s directions for examination of frog’s circulation, 984 Wild clary, spiral fibres of, 618 Williamson (W. C.) on Volvox, 484 note', on structure of fish-scales, 951; on structure of coal-plants, 1006 Willow-herb, emission of pollen-tubes, 647 Wing of Agrion, 918 Winged seeds, 648 Wings of insects, 922-924; of Ptero- INDEX zoo phorus, 923; venation of, in Neuro- ptera, 922 Wodderborn on Galileo’s invention of compound microscope, 123 Wodderborn’s ‘ perspicillum,’ 127 Wollaston’s doublets, 36, 151; his camera lucida, 234 Wood, arrangement of, 625, 627; concen- tric rings of, 628; fossilised, 630, 1005 Wooden slides for opaque objects, 890 Woody fibre, 621 — tissue of ferns, 599 Working eye-pieces, 323 Worms, 867-880 X Xylem of Exogens, 622, 623, 635 Xylol-balsam as a preservative medium, 442 Y Yeast, 574; fermentation due to, 574 Yellow cells, in Actinice, 773 ; in radio- larians, 773 — fibrous tissue, 963, 964 Yolk-bag of young fish, circulation on, 981 Yucca, epiderm of, 637; guard-cells of stomates in, 640, 641 Z Zanardinia, swarm-spores of, 556 Zea Mats, epiderm of, 637 ; stomates of, 640 Zeiss’s oil-immersion objectives, 29; his eye-pieces and objectives, 34; his mechanical stage, 167; his dissecting microscope, 205 ; his photographic microscope, 211, 212; his latest mi- croscope, 213; his calotte nose-piece, 242; his sliding objective changer, 243; his iris-diaphragm, 246, 248 ; his apla- natic loup, 261; his apparatus for mono- chromatic light, 272; his spectral ocular, 276; his apochromatic objective, 814-320 ; his water-immersion, 317; his apochromatic, for resolving diatom markings, 521; his apochromatic for study of monads, 687 Zeiss-Steinheil’s loups, 207 Zentmayer’s microscope, 184; swinging sub-stage in, 184 Zeolite, 1017 Zinc, clilor-iodide of, as a test, 440 — cement, Cole’s, 385; Zeigler’s, 385 Zoantharia, 801 Zoea, 894 Zonal structure in crystals, 996 Zobchlorellae of Heliozoa, 659 Zobcytium of Ophrydium, chemical com- position of, 706 Zobgloea of Beggiatoa, 583 ZOO Zodgkete, 581 Zoophytes, 786-807 — mounting, 388, 389 — non-sexual reproduction of, 930 Zoophyte troughs, 297, 298 Zodsporange of Volvox, 483, 485 Zobsporanges of Phccosporece, 556 Zoospores, 466; of Palmoglcea, 472; of Protococcus, 474, 475 ; of Palmodic- tyon, 487; of Ulva, 488; of Vaucheria, 492 ; of Achlya, 494 ; development of, 494 ; of Hydrodictyon, 495; of Con- fervacecc, 500 ; of (Edogonium, 502 ; of Chcetophoracece, 503 ; of Chytri- diacece, 555; of Phccosporece, 556; of Floridece, 561; of Fungi, 562; of radiolarians, 773 Zoothamium, collecting, 457 INDEX 1099 ZYM Zooxanthellee in radiolarians, 778 Zoozygospores of Navicula, 526 Zukal on movement of Spirulina, 490 Zygnemacece, characters of, 477; habitats of, 477 ; conjugation of, 478 Zygosis in Actinophrys, 665; of Amoeba, 669 ; of gregarines, 677 Zygospore, 467; formation of, 470; of Hydrodictyon, 495; in Desmuliacece, 513, 514 Zygospores of Palmoglcea, 472 ; of Meso- carpus, 478 ; of Spirogyra, 478; of Pandorina, 485; of Ulva, 490; of Navicula, 526; of diatoms, 528; of Mucorini, 569 Zygote of Glenodinium, 695 Zymotic or fermentative action of Fungi, 462 CATALOGUE OF MEDICAL, DENTAL, Pharmaceutical and Scientific Publications, WITH A SUBJECT INDEX. 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It includes Small 8vo, Half Morocco, as above, with Thumb Index, $4.25 Plain Dark Leather, without Thumb Index, 3.25 SEVERAL THOUSAND NEW WORDS NOT CONTAINED IN ANY SIMILAR WORK. IT CONTAINS TABLES of the ABBREVIATIONS used in Medicine, of the ARTERIES, of the BACILLI, giving the Name, Habitat, Characteristics, etc.; of GAN- GLIA, LEUCOMAINES, MICROCOCCI, MUSCLES, NERVES, PLEXUSES, PTOMAINES, with the Name, Formula, Physiological Action, etc.; and the COMPARI- SON OF THERMOMETERS, of all the most used WEIGHTS AND MEASURES of the world, of the MINERAL SPRINGS OF THE U. S., VITAL STATISTICS, etc. Much of the material thus classified is not obtainable by English readers in any other work. OPINIONS OF PROMINENT MEDICAL PAPERS. “ One pleasing feature of the book is that the reader can almost invariably find the definition under the word he looks for, without being referred from one place to another, as is too commonly the case in medical dictionaries. The tables of the bacilli, micrococci, leucomai'nes and ptomaines are excellent, and contain a large amount of information in a limited space. The anatomical tables are also concise and clear. . . . We should unhesitatingly recom- mend this dictionary to our readers, feeling sure that it will prove of much value to them.”—American Journal of Medical Science, Sept. 1890. “As a handy, concise and accurate, and complete medical dictionary it decidedly claims a very high place among works of this description. In fact, taking handiness and cheapness into account, we certainly think this is the general practitioner’s model dictionary, and we cordially recommend it to our readers. The definitions are for the most part terse and accurate, and the derivations up to modern lights.”—British MedicalJournal, London, Sept. 1890. May be obtained through all Booksellers. Sample pages free. P. BLAKISTON, SON & CO.’S jVJedical and Scientific publications, No. 1012 Walnut St., Philadelphia. ACTON. 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