NAJXM wmo ''1 / / ARMY MEDICAL LIBRARY WASHINGTON Founded 183 G t "l A.NNE2 Section. iSBBl \ Number .31&Q4.0. Fobm 113c, W. D., S. G. O. 3—10543 (Revised June 13. 1936) V ^ V I ") z \ the Oculist's Uade mecunn mprising all the'important methods of testing the which have been devised by leading ophthalmic su geons of Philadelphia; besides many valuable tables on the properties of lenses and methods of procedure in the examination of the eyes. PUBLISHED BY FERGUSON & WESTON, ---OPTICIANS--- N. E. Cor. Chestnut and Fifteenth Streets, PHILADELPHIA. EDITED BY DR. JAMES WALLACE. CONTRIBUTORS: DR. EDWARD JACKSON. DR. S. D. RISLEY. DR. JAMES THORINGTON. DR. JOHN F. CARPENTER, JR. DR. M. W. ZIMMERMAN. DR. GEO. C. HARLAN. DR. HOWARD HANSELL. DR. S. LEWIS ZIEGLER. DR. WM. CAMPBELL POSEY. DR. E. W. STEVENS. WW \N 191* \ 1898 p-'L, >r ' Copyright by FERGUSON & WESTON, f PREFACE. The aim of this work has been to present a System of test types for the examina- tion of the eye of such accuracy in their construction that the Oculist might confi- dently rely upon them in the determination of the refraction and accommodation of the eye. Reading matter for testing the eyes should be so arranged as to accomplish two purposes: The first of these is the determination of the amplitude of the accom- modation, and is effected by having types of the sizes that represent normal visual acuity, as the distance of the near point varies with age^ Unless the type is small enough to represent normal visual acuity on the near point designated, the real near point may be at some distance beyond. Thus we understand that a vision of j% say in a presby- ope represents a hypermetropia of i or 2 dioptries; similarly a near point of 25 cen- timetres, which represents 4 dioptries of accommodation if obtained by type repre- senting normal vison at 50 centimetres would have a visual acuity of fg, and as that is compatible with a lack of 1 or 2 dioptries of accommodation, instead of 4 we might have 3, or even 2 dioptries of accommodation if the visual acuity is high. With 25 centimetres, type constructed on an angle of 5', the error in measuring the accommo- dation is very much reduced. To still further remove the difficulty, type is introduced that represents at 15 centimetres an angle of 5'. A visual acuity of f is very common, so that a person with ? visual acuity and 4 dioptries of accommodation ought still to read this type at 25 centimetres. It is, however, for people below the age of 30 that such type will be most useful, and may lead to a rearrangement of the amplitude of ac- commodation. The second purpose to be fulfilled in reading matter as test type is the selection of glasses which will be most comfortable at the distance which the patient prefers holding the book. There is a small margin in which the preference of the indi- (3) vidual must be recognized in order to give complete satisfaction. For this purpose one full page of the book is printed in English type of the size that is commonly found in newspapers and magazines. Too small type is apt to mislead the patient into selecting a stronger glass than is necessary. The dispensary services of all the large hospitals have a considerable number of foreigners among their patients, who are mainly Germans, Italians and Hebrews. Many of them are unable to read English, and a difficulty arises on this account, in furnishing them with glasses. To obviate this, types of German, Italian and Hebrew (dialect) have been added to the English. It is hoped that these will be found con- venient, especially for patients who have had their sight restored after cataract opera- tion, who can be tested at first in their own room, if necessary. The types are evenly graded, accurately measured and clearly printed on heavy glazed paper, so that in the smaller types due value is given to the finest details. The types have their distances marked above them at which they represent normal visual acuity. For convenience of all, the distances are denoted both in centimetres and in English inches, and a table is appended of the amplitude of accommodation and the near point of different ages. The great importance of testing the action of the ocular muscles is well recognized, and this work is greatly indebted for methods of testing the muscles as well as descrip- tive papers on the subject to Drs. Risley, Jackson, Hansell and Ziegler. Dr. Harlan has contributed a valuable paper on Toric lenses; Drs. Stevens, Thorington and Car- penter have furnished papers that will be a great help in refraction technique; finally, acknowledgment is made of Dr. Posey's practical paper on testing for simulated mon- ocular blindness, and Dr. Zimmerman's lamp screen. Many valuable tables, some of them original in this work, have been introduced with the view of collecting together every needful adjunct in the examination of the eyes. It is hoped that a large amount of useful information that is usually only to be found scattered through text books has been gathered here where it can be most easily utilized as a work of reference. (5) TABLE OF CONTENTS. I'ACiK. Table of the Amplitude of Accommodation,............. 8 Test Types for Accommodation—English,...........9-11-13-15-17 " —German,..............19-21 " " —Italian, ..............23-25 " " " —Hebrew,.........• • . . 27-29 —Music,............... 31 Test Types for Visual Acuity, angle of 4',.............. 33 Dr. James Wallace's Astigmatic Chart, .............. 35 Ferguson & Weston's Test for Muscle Balance,............ 37 Test for Muscle Balance at Working Distance—Dr. Edward Jackson, . . 39 The Cobalt Glass Test for Heterophoria—Dr. Howard F. Hansell, ... 40 Dr. S. Lewis Ziegler's Greek Cross Test Object for Muscle Balance, . . 41 The Practical Methods for Determining the Ocular Balance—Dr. S. D. Risley..............................42-43 1 >r. James Wallace's Table of Prisms,................44-45 Dr. Edward Jackson's Table of Decentered Lenses,.......... 45 Equivalent Number of Prisms, . :................. 46 Dr. S. Lewis Ziegler's Prism Scales,................. 47 Dr. James Wallace's Tables for Eeducing Two Crossed Cylinders with Oblique Axes to a Sphero-cylinder,.............48-49-50-51 Periscopic Cylindrical and Sphero-cylindrical Lenses—Dr. Geo. C. Har- lan.............................. 52 Notes on Retinoscopy—Dr. James Thorington,............ 53 Subjective and Objective Methods of Measuring Refractive Errors— Dr. John T. Carpenter, Jr.,.............t.....54-55 The Detection of Simulated Monocular Blindness— Dr. William Camp- bell Posey,.........................56-57 An Adjustable Lamp Screen—Dr. M. W. Zimmerman,........ 58 The Ophthalmometer and its Clinical Use—Dr. E. W. Stevens, . . 59-60-61-62 Table of the Value of Lenses expressed in Dioptries and the Equivalents in inches, ........................... 63 (7) Table of the amplitude of the accommodation and the near point of the eye at different ages. Amplitude of Accommodation in NEAR POINT. AGE. Dioptres CENTIMETRES INCHES IO M 7 2-75 *5 12 8-5 3-25 20 IO 10 3-87 25 8-5 12 4-75 30 7 H 5-5 35 5-5 18 7 40 4-5 22 8.62 45 3-5 28 11 5o 2-5 40 15-75 55 i-75 57 22.37 60 1 100 40 65 .50 200 80 70 •25 400 160 75 Infinite. Infinite. Infinite. PUBLISHED BY FERGUSON & WESTON. (8) fit- « it the great bene- i Helmholtx ha title world. The verifi- cation of this mechanism has been supplied by Hen- ion and Volckers by proT- ing indisputably the for- wurd movement of the choroid. In considering the delicacy of movement necessary to sharply focus upon the retina the image formed by a system of lenses whose equivnlent focus is IS millimetres, whose integral parts are .60 M not always well centred, whose errors of refraction not infrequently include a considerable degree of as- tigmatism, and whose ac- tion ia automatic, we are justified in asking whether bond between retina, cili- ary muscle, and crystalline lens, by which the amount of nerve force exerted is always proportioned to the degree of the error and within certain limits modi- fied by its character ? It is no unusual thing to find an astigmatism of from .5 to 1 di- optre obliterated by the accommo- dation .We might also add here that the permanency of the accommo- I M dation for a fixed distance during several consecutive hours, without any sensible ef- fort on our part, argues PUBLISHED BY FERGUSON 6. WESTON. (9) I These Types represent normal visual acuity when read at the distances marked above them. 37 CENTIMETRES OR 15 INCHES. ith the kindred eyes of the child, aw noTed from the familiar chamber in---- ■ her mind all that had lately passed, and she sprang from her bed, hoping nio the church-yard, brushing the dew from the long grass with her feet, that she might not tread npon the graves. She felt a curious kind of pleasure in linger- ing among these house* orthe dead, and read the inscriptions on the tombs of the good people (a great number of good people were buried there), passing on from one to another with increasing interest. It waa a very qaiet place, as such a place should be, sa«e for the cawing of the rooks who had built their nests among the branches of some tall old trees, and were calling to one another, high up in the air. First, one sleek bird, hovering near hie ragged house as it swung and dangled in the wind, uttered his hoarse cry, quite by chance as it would seem, and in a sober tone, as though he were but taking to himself. Another answered, and he called again, but louder than before ; then another spoke, and then another; and each time the first, aggravated by contradiction, insisted on his case more strongly. Other voices, silent till now, struck in from boughB lower down and higher up and midway, ami to the right and left, and from the tree-tops; and others, arriving hastily from the gray church turrets and old belfry windows, joined the clamor which rose and fell, and swelled and dropped again, and still went ' a skimming to and fro, and lighting on fresh branches, and Ireqaent chauge of place, which satirised the old restlessness of ____and turf below, and the strife in wluch they had worn away their lives. Frequently raising her eyes to the trees whence ____3 down, and feeling as though they made the place more quiet than perfect silence would have done, the child loitered from grave to grave, now stopping to replace with careful hands the bramble which had started from some green mound it helped to keep in shape, and now peeping through one of the low latticed windows into the church, with its worm-eaten bo-ks upon the deck*, and baise of whitened-green mouldering from the pew-side* and leaving the naked wood to view. There were the seats where the poor old people sat. worn spare and yellow like themselves ; the rugged font where children had their names. the homely altar where they knelt in after-life, the plain black treasels that bore their weight on their last visit to ths cool old shady church. Everything told 50 CENTIMETRES OR 20 INCHES. She was looking at an bumble stone which told of a young man who had died at twenty-three years old. flfty-five years ago, when Bhe beard a faltering stop ap- proaching, and looking around saw a feeble woman bent with the weight of yeara, win) tottered to the fi>ot of that same grave and asked ber to read the writing on the stone. The old woman thanked her when she had done, saying that she had had the words by heart for many a long, long year, but could not see them now. " Were you his mother V* said the child. " I wa3 his wife, my dear." She the wife of a young man three-and-twenty 1 Ah. true! It was fifty-five years ago. " You wonder to hear me say that." remarked the old woman, shaking her hej.3. *' You're not the first. Older folk than you have wondered at the same thing before now. Yes, I wag bis wife. Death doesn't change us more than life, my dear." " Do you come here often ?" asked thechild. '* I sit here very often in the summer-time," she answered. " I used to come here «>u->. which was new to one listener, though it were but a child, she told her how she had wept and moaned and prayed to die herself when this h ippmed, rm-l li w. when she first came to that place, a young creature strong in love and grief, ?he had hoped that her heart w.ts breaking, as it seemed to be. But that tim-- pa-s^l i>y, and although she continued to be sad when she came there, still Bhe could bear to come, and so went an until it was pain no longer, but a solemn pie i-nr-. tnd :!, duty she had learned to like. And now that five-and-fifty years were gone, she Bpoke of the dead man as if he had been her son or grandson, with a, kind ot pity for his youth, growing out of her own old age, and an exalting of his strength and manly beauty aa compared with her own weakness and decay; and yet she spoke about him as her husband, too, and thinking of herself in connection with him, as she use to be, and not as she was now; talked of their meeting in another world, as if he were dead but yesterday, and she, separated from her former self, were thinking of the happiness of that comely girl who seemed to have died with him. The child left her gathering the flowers that grew upon the grave, and thoughtfully retraced her steps. The old man was by this time up and dressed. Mr. Codlin, still doomed to contemplate the harsh realities of existence, was packing among his linen 62 CENTIMETRES OR 25 INCHES. " Objection," said the little man. " Now be gracious for once, Tommy, and say that you'd rather they went with us. I know you would. Be gracious Tommy. " Trotters ! " said Mr. Codlin. who talked very slowly and eat very greedily, as is not uncommon with philosophers and misanthropes ; " your're too free." " Why, what harm can it do? '* urged the other. " No harm at all in this particular case, perhaps." replied Mr. Codlin ; " but the principle's a dangerous one, and you're too free, I tell you." " Well, are they to go with us or not 7" " Yes, they are." said Mr. Codlin ; ' but you might have made a favor of it. mightn't you ?" The real name of the little man was Harris, but it gradually merged into the less euphonious one of Trotters, which, with the prefatory adjec- tive Short, had been conferred upon him by reason of the small size of his legs. Short Trotters, however, being a compound name, inconvenient for use in friendly dialogue, the gentleman on whom it had been bestowed was known among his intimate friends as "Short" or " Trotters," and was seldom accosted at full length as Short Trotters, except in formal conversations and on occasions of ceremony. Short, then, or Trotters, as the reader pleases, replied unto the remonstrance of his friend Mr. Thomas Codlin a jocose answer calculated to turn aside his discontent; and applying himself with great relish to the cold boiled beef, the tea, and bread and butter, strongly impressed upon his companions that they should do the like. Mr. Codlin, indeed, required no such persuasion, as he had already eaten as much as he could possibly carry and was now moistening his clay with strong ale, whereof he took deep draughts with a silent relish and invited nobody to partake—thus again strongly indicating his misanthropical turn of mind Breakfast being at length over. Mr. Codlin called for the bill, and charging the ale to the company generally (a practice also savoring of misan- thropy), divided the sum-total into two fair and equal parts, assigning one moiety to himself and friend and the other to Nelly and her grandfather. These being duly discharged, and all things ready for their departure, they took farewell of the landlord and land- lady and resumed their journey. And here Mr. Codlin's false position in society and the effect it wrought upon his wounded spirit, 75 CENTIMETRES OR 30 INCHES. Sometimes they played out the toll across a bridge or ferry, and once exhibited, by particular desire, a turnpike. where the collector, being drunk in his solitude, paid down a shilling to have ittohimselt. There was one small place of rich promise in which their hopes were blighted, for a favorite character in the play having gold lace upon bis coat and being a meddling, wooden-headed fellow, was held to be a libel on the beadle, for which rea- son the authorities enforced a quick retreat; but they were generally well received, and seldom left a town without a troop of ragged-children shouting at their heels. They made a long day's journey, despite these inter- ruptions, and were yet upon the road when the moon was shining in the sky. Short beguiled the time with songs and jests, and made the best of everything that happened. Mr. Codlin, on the other hand, cursed his fate and all the hollow things of earth (but Punch especially), and limped along with the theatre on his back, a prey to the bitterest chagrin. They bad stopped to rest beneath a finger-post where four roads met, and Mr. Codlin in his deep misanthropy had let down the drapery and seated himself in the bottom of the show, invisible to mortal eyes and disdainful of the company of tiis fellow-creatures, when two monstrous shadows were seen stalking "towards them from a turning in the road by which they had come. The child was at first quite terrified by the sight of these gaunt giauts—for such they looked as they advanced with lofty strides beneath theshadows oi the trees—but Short, telling her there was nothing to fear, blew a blast upon the trumpet, which was answered by a cheerful shout. " It's Grinder's lot, ain't it ?" cried Mr. Short, in a loud key. " Yes," cried a couple of shrill voices. "Come on then." said Short. " Let's have a look at you. I thought it was you." Thus invited, " Grinder's lot " approached with redoubled speed, and soon came up with the little party. Mr. Grinder's com- pany, familiarly termed a lot, consisted of a young gentleman and a young lady on stilts,"and Mr. Grinder him- self, who used his natural legs for pedestrian purposes and carried at his back a drum. The public costume of PUBLISHED BY FERGUSON & WESTON. (») 1 METRE OR 40 INCHES. the young people was of the Highland kind, but the night being damp and cold, the young gentleman wore over his kilt a man's pea-jacket- reaching to his ankles, and a glazed hat; the young lady too was muffled in an old cloth pelisse and had a handkerchief tied about her head. Their Scotch bonnets, ornamented with plumes of jet-black feathers, Mr. Grinder carried on his instrument. " Bound for the races, I see," said Mr Grinder, coming up out of breath. " So are we. How are you, Short?" With that they shook hands in a very friendly manner. The young people, being too high up for the ordi- nary salutations, saluted Short after their own fashion. The young gentleman twisted up his right stilt and patted him on the shoulder, 1.50 METRES OR 60 INCHES. and the young lady rattled her tambourine. "Practice?" said Short, pointing to the stilts. " No," returned Grinder. " It comes either to walkin' in 'em or carryin' of 'em, and they like walkin' in 'em best. It's weary pleasant for the prospects. Which road are you takin' ? We go the nighest." " Why, the fact is," said Short, " that we are going the longest way, because then we could stop for the night a mile and a half on. But three or four mile gained to-night is so many saved to- morrow, and if you keep on, I think our best way is to do the same." "Where's your partner, inquired Grinder. " Here he 2 METRES OR 80 INCHES. The child considered for a moment, and reflecting that she must shortly beg, and could scarcely hope to do so at a better place than where crowds of rich ladies and gentlemen were assembled together for purposes of enjoyment and festivity, determined to accompany these men so far. She therefore thanked the little man for his offer, and said, glancing tim- idly toward his friend, that if there was no objection to their accompanying them as far as the race town PUBLISHED BY FERGUSON & WESTON. (13) 3 METRES OR 120 INCHES. Cried Mr. Thomas Codlin. presenting his head and face in the proscenmm of the stage and exhibiting an expression of countenance not often seen 5 METRES OR 200 INCHES. there; and he will see his partner boiled alive before 6 METRES OR 240 INCHES. LITTLE NELL AND THE MARCHIONESS, MRS. JARLEY'S WAX WORKS, DICK SWIV- PUBLISHED BY FERGUSON & WESTON. C5) ♦ 90 CENTIMETRES OR 36 INCHES. Another bright day was shining in through the small casement, and claiming fel- lowship with the kindred eyes of the child, awoke her. At sight of the strange room and its unaccustomed objects she started up in alarm, wondering how she had been moved from the familiar chamber in which she seemed to have fallen asleep last night, and whither she had been conveyed. But another glance around called to her mind all that had lately passed, and she sprung from her bed, hoping and trustful. It was yet early, and the old man being still asleep, she walked out into the church- yard, brushing the dew from the long grass with her feet, and often turning aside into places where it grew longer than in others, that she might not tread upon the graves. She felt a curious kind of pleasure in lingering among these houses of the dead, and read the inscriptions on the tombs of the good people (a great number of good people were buried there), passing on from one to another with increasing interest. It was a very quiet place, as such a place should be, save for the cawing of the rooks who had built their nests among the branches of some tall old trees, and were calling to one another, high up in the air. First, one sleek bird, hovering near his ragged house as it swung and dangled in the wind, uttered his hoarse cry, quite by chance as it would seem, and in a sober tone, as though he were but talking to himself. Another answered, and he called again, but louder than before; then another spoke, and then another ; and each time the first, aggravated by contradiction, insisted on his case more strongly. Other voices, silent till now, struck in from boughs lower down and higher up and mid- way, and to the right and left, and from the tree-tops; and others, arriving hastily from the gray church terrets and old belfry windows, joined the clamor which rose and fell, and swelled and dropped again, and still went on ; and all this noisy contention amid a skimming to and fro, and lighting on fresh branches, and frequent change of place, which satirized the old restlessness of those who lay so still beneath the moss and turf below, and the strife in which they had worn away their lives. Frequently raising her eyes to the trees whence these sounds came down, and feel- ing as though they made the place more quiet than perfect silence would have done, the child loitered from grave to grave, now stopping to replace with careful hands the bramble which had started from some green mound it helped to keep in shape, and now peeping through one of the low latticed windows into the church, with its worm-eaten books upon the desks, and baize of whitened-green moldering from the pew-sides and leaving the naked wood to view. There were the seats where the poor old people sat, worn spare and yellow like themselves ; the rugged font where children had their names, the homely altar where they knelt in after-life, the plain black tassels that bore their weight on their last visit to the cool old shady church. Everything told of long use and quite slow decay; the very bell-rope in the porch was frayed into a fringe, and hoary with old age. She was looking at an humble stone which told of a young man who had died at twenty-three years old, fifty-five years ago, when she heard a faltering step approaching, and looking around saw a feeble woman bent with the weight of years, who tottered to the foot of the same grave and asked her to read the writing on the stone. The old woman thanked her when she had done, saying that she had had the words by heart for many a long, long year, but could not see them now. " VVere you his mother?" said the child. " I was his wife, my dear." She the wife of a young man three-and-twenty ! Ah, true ! It was fifty-five years ago. " Do you wonder to hear me say that," remarked the old woman, shaking her head. " You're not the first. Older folk than you have wondered at the same thing before now. Yes, I was his wife. Death doesn't change more than life, my dear." " Do you come here often ?" asked the child. "I sit here very often in the summer-time," she answered. "I used to come here once to cry and moan, but that was a weary while ago, bless God !" PUBLISHED BY FERGUSON & WESTON. (17) 50 CENTIMETRES OR 20 INCHES. gnbtidi tam bit .$cit tjetfcti, ba man fid) ^ut Uebetfabrt fdnifen, bie .Rutfcbcn unb SBagen etrcatten follte, bie unfete gan^e Ztuppc it ad) bemSdjIoffe beS (Vltajeu biniilvt ^u iiibri'ii beftfUi lsaten. 2dion (tun torauf fielen cirojie Stteitia; feiti-n t>ot, toet mit bent ancctn iabten, une man fifccn jollte? Tie Qtbnung^ unb Gintcilung toatb enbtid) nut mtt TOiiSje auegemad)t unb teftgefefct, bedi [eibet cf)ne Bitfung. ,Sur beitimmten istunbe famen iseniget SBagen, at* man eticattet batte, unb man mujite iid' cintid)ten. Set Baton, bet ju 15jctbe nid)t lange bjintetbtein jo';ltc, gab jilt Utiachc an: bafe im idjtoife alle* in gtctKt Seroegur.g fei, lreil nidjt allcin bet giirft eintge Jage ttiibet etntteffen n>etcc, aid man gcglaubt, ionbetn rceil aitd) unetroattetet Sefud) fd)on gegentsiittig angelangt fei;- bet iUat< gelje febt (ufaminen, fte triltben aud) bcjuuegen nicht to gut logictcn, ali man ei tb/nen botbjet beftiinmt babe, lrclcbce ihni aufcetevbenttid) leib tbue. Wan tcilte fid) in bie SSagch, fo gut e$ getjen rooUtc, unb ba leibtid) SBettet unb bad idilcii nut cinige Etunben entjetnt mat, madjten fids bie Ku'itigften'liebet ^u ftufee auf ben Beg, al8 bats fie bie 3tucffebvba «utjd)en batten abroat; ten (ollcn. J>ie Jtatareane ;\og mit' jteubengefdjtei aug, ^um ett'tenmal otjne Sotgen, trie bet ffiitt 511 bejafjtcn fei. SaS EdjloB bed ©tafen ftanb ihnen mie cin Jeengebaube vox bet Seele; fte waten bie gliictlidjjten unb ftoblidnten iDienfdien bon bet SBclt, unb jebet iniipjte untetreegs an biefen lag, nacb jeinet 2ltt ju benfen, etne SRetfie bon (Slucf, Gbjte unb SBotjlftanb. IStn ftatfet SRegen, bet uncniattet einficl, toruite fte nicf)t aui biefen angenefjmen gmpjinbungen teifecn; ba et abet tmmct anbaltenb'et unb ftattct reutbe, fpiitten biele bon itjnen cine fliemlidje Unbequemltdjteit. 35ie Sttadjt fain betbei, unb etn>uufd)ter fonnte iljnen nidjts etjebeinen, aU bet butd) alle Stocfroetfe etleudjtetc $alaft beS (Stafen, bet ifjnen eon cinem §iigel cntgegen glance, jo bafe fie bie Jenftet jablen tonnten. 2U3 fie natjet Retail famen, fanben fie aud) atte ftenftei bet &eitengebaube etbetlet. Sin jebet bacbte bet fid), TOcldjeS luobl fein 3'mmet reetben mod)te? unb bie metften begniigteit fid) befdjeiben mit etnet Stube in bet Sftaniatbe obet ben gliigeln. 9cun jufiten fte butd) ba« J)otf unb am liUvUbaufe botbei. 3Silf)eltn lie(3 bjalten, um bott abjufteigen; attein bet fflitt uetfidjette, bafj et ib>m nidjt ben gctingften :Saum annieifen fbnr.e. Det fiett ©taf t)abe, roeil unetrcattete (Safte angefommen, fog(etd) baS ganjc iKiirte^au's bejptodjen, an alien 3iwniern ftelje fdjon fett geftetn mit Steibe beutltd) angefdjtieben, met batinne 75 CENTIMETRES OR 30 INCHES. toofmen fofle. SBiber fcinen SBillett mu§te alfo unfer grfunb mit ber iibrt'gett ©efeflfc^aft jum ©d)lo§bofe tiiitaufabren. Um bie .Uiidicnfcuer in etnem Seitengebciube faben fte gefiaftige MiJcfye fid) bin unb tytx bewegen unb waren burd) btefen Slnbltcf fd)on erqutcft; etlig famen Sebtcnte mit i'id)tem auf bie Jreppe beg .^auptgebaube^ gefpntngen, unc bag £>er$ ber gnten SBanberer quoll iiber ttefen 3lu^ficbten auf. 3Bie febr uerwunberten fie fid) bagegen, al3 fid) fctefer Smpfang in etn tntfe^lidjei? gludjen auflbfte. Tie 23ebienten fdiimpften auf bie Subrleute, baj fie bier bereingefabren feien ; fie follten umtDenben, rtef man, unb roieber btnaug nad) bent alten ^djloffe ju, bier fei Fein 9faum fiir btefe ©afte! (Sinem fo unfreunblidjen unc unertoarteten 23efd)eibe fiigten fie nod) allerlei ©pbttereten btnju unb lacbten fid) unter einanber am, ba§ fie burd) biefen 3rrtum in ben 9tegen gefprengt rocrben. (£g go§ nod) immer, feine Sterne ftanten am ^immel, unb nunjeurbe bie ©e= fellfcbaft burd) einen bolprid)ten 2Beg jrotfd)en ^roet Wnuern in bag alte btntere ^d)Io§ gejogen, t»eld)eg unberoobnt ba flanb, feit ber i?ater beg ©rafen bag oorbere gebaut batte. leilg im £ofe, tetlg unter etnem langen gerob'lbten 2borroege bitten bie SBagen fttli, unb bie Subrleute, Slnfpanner aug bem Xorfe, fpanuten aug unb rttten tbrer 2Bege. ©a 9?iemanb jum Smpfange ber ©efeu* febaft fid) jetgte, fttegen fie aug, rtefen, fudjten »ergebeng! SUleg blt'eb ftnfter unb ftilie. £er 3Binb blteg burd) bag t>otje ^bor, unb grauerlid; roaren bie atten Jiirme unb ^bfe, roouon fte faum bie ©eftalten in ber gtnfternig unterfd)teben. ©ie froren unb febauerten, bie gratten fitrd)teten fid), bie Ktnber fingen an p roeinen; tt>re Ungebulb oermebrte fid) mit jebem ^ugenbltrfe, unb ein fo fcbnelter ©liicfgroed)fel, auf ben ntemanb corberettet roar, bradjte fte atle gan^ unb gar aug ber gaffung. ©a fte jeben Slugenblitf erroarteten, ba§ jemanb fommen unb ibnen auffd)lie§en roerbe, ba balb 9?egen, balb ©turm fie ta'ufd)te unb fie mebr al3 einmal ben Jrttt beg errounfdjten ©cblo^oogtg gu bbren 1 METRE OR 40 INCHES. gtaubten, bltebcn ffe etne (ange 3^tt nnmuttg unb untbattg; e^ fte( fetnem etn, in ba^ neue ©ct>toj3 ju ^eben unb bort mtttetbtge ©eeten um §tlfe angurufen. ©te fonnten nicf)t begretfen, wo tt)r ^r^wnb, ber ^Baron, a,e* btteben fet, unb maren in etner becl>|t befd)n?er I id) en ^age. (Snblid) famen mirflic^ SRenfc^en an, unb man erfannte an ibren ©ttmmen jene gu0gdnger, t)k auf bem 3©cge binter btn Jabrenben gurucf geblieben tt\iren. ©ie er^ablten, ba§ ber 53aron mit bem ^ferbe gejturst fet, fity am gujje j!arf befd)abigt babe unb baO man aucb {kf ba fte im ©cfyloffe nact)gefragt, mit itngejtiim l)terr)er ^emiefen babe. S)ie gauge @efell* fcf)aft njar in ber grefjten ^erlegenbett; man ratjtyfagte, ma^ man tt)itn follte nnt) fonnte feinen (gntfc^lug faffen. (Snblid) fat) man »on meitem eine ^aterne fommen unb botte frifd)en 5(tem; atlein bk £)offnuug einer balbigen (Srtofung iu'rf(l)it\mb aucf) mieber, inbem bie (Srfi^einung nat)er fam unb beutlict) n)arb. (Sin 3^eitfned)t leud)tete bem befannten ©tall- PUBLISHED BY FERGUSON &. WESTON. (19) 1.50 METRES OR 60 INCHES. metier be3 ®rafen fcor, unb biefer crfun&igtc ftd), aU er ndtyer lam, fet)r eifrtg nadj Sttabemoifelle sptnltnen* orgetreten, aU ev tfjr fetyr brtngenb anftot, jte in ba^ neue <5$lojji ju fitljren, too em $ld£d)en fur fte kt ben ilammerjungfern ber Ordftn kreitet fei. @ie kfann fid) ni$t lange, ba$ SinerMeten banfkr ju ergretfen, fafite il>n Ui bem Slrme unb tooflte, ba fte ben anberen i^ren itojfer empfol)len, mit il)m 3 METRES OR 120 INCHES. f ortetlen; atteiti wan trat Wjitett in ben 28egf fracjte, Bat Be- fdjtoiir ben StaUmetfter, baft tx ettblid), um mix mit feitter @dja- nen fosjutomntett, atte^ toertyrad) 5 METRES OR 200 INCHES. uni> uetfiaierte, in turpi folic oa§ ©d)to| craffnet unb fie auf ooJ befte cin= (litattiert mcrocn. ©Irid) PUBLISHED BY FERGUSON & WESTON. (21) 50 CENTIMETRES OR 20 INCHES. 11 aeguente fatto arrenuto in caaa Cenn, inninzi che egli partisseper Rocca Petrella, fara conoscere di phi 1' cfferato cuore di costul. — Beatrice stava neduta sopra un verone delopalaazo Cenci, che guardava il giardino; in grembo ella tenera un fanciullo, che dagli occhi, da' capelli, da lutte le sue sembianie appariva esaerlo fratello ; ella gli accartinva amorosa lm chioma, e di rratto in tratto gli baci.iva la fronte. 11 fanciullo riposa il buo caposul seno della sorella, ed affiasa in lei Ip pupille immote, ma sennintemione. a guiaa di persona a-»aorta nel penai* ro di qualche coaafuoridi questo mondo. Lainfermitaaveva appasBitoilfi'.redelliigiovineixa: la sua pelle eratenue e candidi di un bi-inco pallido e deli.ato cosi. che i raggi del sole cadenti gli tralueevano in vermiglio traversole orecchie e le dita: talora aospiraYa. piii apeaao schiudeva la bocca con Ubadiglio convulso: pareva un aneelo in pena. Beatrice, sconaolata, gli dLsse: — A che petui, mio diletto Virgilio ? — Pcnso che sarebbe pure Btata la grande carita non fani mui venire al mondo! — Ah! Virgilio. ... — £ poicbe a questo non trovo piA rimedio, il mejlio sara uscirne presto. — Uscirne! E perdu- ? — E perche ? E pcrchc restarci ? II mio cuore qui dentro 6 morto da Umpo; e quando il cuore e morto, oh come pesache gli aopravviva il rnrpol — Tu, si pu6 dire, tiaffacci appena, fratello, alia vita, egia favelli parole disperate ; ci6 non ista bene: vivi e ralletrrati, perche non «ai quali ruse educhi per te la fortuna. — Rose! f«rtuna! Adesso la morte coglie i fiori per la ghirlanda della mia bara. La fortuna mi abba'idono qut 1 giorno che perdVmmo la madre. ... — Ma noi uon ci poasiamo cons derare orfani affatto: forse I'ottima signora Lucr< li i non ci moitra viacere di madre? — Si. ma non e nostra madre. — E poi non hai anchc me, che ti amo tamo ? — Si, si, buona sorella, risposp il fanciullo, gittandole le bracc-a al collo e pian^endo dirotto: ma ne anche tu sei la mamma mia. — Ed oltre a me, ti mancano forse fratelli ? Non hai tu padre ? — Chi p idre ? — Beatrice, atterrita dall' improvriao rimeacolarsi del fanciullo a cotesta parol a, si tacque. Solo, dopo lungo silenxio, con voce esitante sog^iunae : — Francesco Cenci non e per awentura tuo padre.... e mio? — 11 fanciullo abbas.*o il capo, chiuse gli occhi, fece delle braccia al pctio croce, e con suono velato riapose: — Sorella, guardami sulla fronte alia radice de* capelli; vedi la cicatrice che vi porto ? — La vedi? Sai tu chi mi ha ferito? — lo non tel diaai, fin qui, ma ora che mi sen to vicino a morire, io te lo poaao con- fesapa Clemente VIII voleva la morte dei Cenci, e colla morte la conrisca dello sterminato loro ]>atri- monio: ma essi non avevano coiifessato, e quindi non potevasi, a forma delle leggi, confisearne i beni. Ma ad un papa carnefice e ladro non mancano mezzi per ottenere un fine. Kece prometerre dal Farinaccio, celebre magistrato e in questo arfare anch'esso ingannato, ai C6nci die se avessero coiifessato il delito, avreb bero avuta la grazia, e ne impegnava la sua papale parola. I Cenci caddero nell' agguato, si dissero rei, ed allora furono condannati a morte. II Farinaccio, vedendosi ingan- nato, ricorse al Paiia, il quale, battendo il piede sul pulvinare di velluto, con voce tonante rispose: Pisogna che muoiauo. (iiacomo, Pernardo, Lucrezia Petroni e Beatrice furono condannati a morte col tnglio della testa; il primo (Giacomo) doveva essere attanagliato e poi squartato. Le campane col lugubre s(|iiillo invocano la preghiera per gli agonizzanti; i tamburi suonano scordati; il cielo e la terra pareva che con quei suoni si scambiassero 1' annunzio che la strage era per compiersi, e ne rimanessero sbigottiti. Nel cortile stavano attelati parecchi sbirri a piedi, e poi i fratelli della 1 METRE OR 40 INCHES. in cui si trovava e una prigione; l'aspetto della stanza apparisce strano a vedersi: splendido e il letto per cortine ampissime di damasco e cornici dorate, ricuopre il pavimento uno splendido arazzo; le finestre erano chiuse da una ferrata, davanti alia quale, era inchiodato un assito a modo di tramoggia, e sopra questa adattata una graticola fitta di filo di ferro; quivi gemeva la povera Beatrice ; ma anco da questo luogo le era riuscito avvisare l'amato Guido Guerra, supplicandolo di pronto aiuto. — E notte. Uu lieve tocco sospinge la porta; ecco si muove silenziosa sui cardini: prima il capo, poi il petto __finalmente tutta la persona apparisce di un uomo canuto, avvolto in un'ampia zimarra. — E il conte Cenci strascinato dal destine Tende l'orecchio... ascolta 1' alito di Beatrice. Appoggia il corpo intero sul piede di dietro, muove cauto l'altro, e sempre va innanzi; si ferma in fondo al letto. Beatrice ha chiuso gli occhi a sonno travagliato, e PUBLISHED BY FERGUSON & WESTON. (23) 1.50 METRES OR 60 INCHES. agitandosi irrequieta si e scomposta la chioma, che le sta vag- amente sparsa pel seno divino. — Egli la guarda. La vista di forme cosi stupendamente leggiadre rallegra l'annima.— Che ardisce costui? Non basta, ed e anche troppo, vedere quel seno che palpita? — II truce vecchio stende le scarne braccia, e trae a se cautissimo i lini. I tesori di coteste membra appaiono manifesti.—Cheta, cheta la porta della stanza torna di nuopo a polgersi sopra gli arpioni: entra un altr'uomo e si ferma:— guarda.....stupisce.... e non ravvisa il Conte al fioco chiarore del lume, che peglia fra loro. II Conte, lussuriando per ogni fibra, 3 METRES OR 120 INCHES. trema; gli occhi gli si aggrin- ziscono a modo di vipera: una striscia di fiamma d' etico gli imporpora il sonimo delle gote; lascia cadersi giu dalle 5 METRES OR 200 INCHES. spalle la zimarra, e appaiono le pal- lide membra del PUBLISHED BY FERGUSON 4. WESTON. (25) 50 CENTIMETRES OR 20 INCHES. Dj?"a ]jW33'n Din jyjyn ptbi x ts^o x*n ]yj:iS~yj axn jxtasyutp Syanyar y«X3 xn ny3nxii:"x >,c?x "a nS'ii jyaipyjnxs -p?pTii rx hxs ~\yi px ,]y2X-iayj oxn anxi Dxn DyjyaSyr cyn aSnyny JunynsixmyD ts*» axn iy"1""!"1 .Syanyar *iy3"Sp *iyn ps :N's y?:nh\ ]xd *in»x ps ^yna x jyaSxmy Ban piyryn xn ysm X3nN* Dxn :dj?"3 D'nx |3?3'3 ]X "IITX tSp'K' "iy TX pX U13 inyT 01X1 T'X OH'X TX ,«pnyOK DMX ,J?2'J ysoxSp nyaajnyiyj nyi ps trasya x lynyj i'x ys'sx's ypjoSt .jyaixp ;nyx y:nxJ o»» iv ijn:ny3 x ]y^y3 t»x ny troxn ,o:ymx2 u'3 nx.3 onxi oxn iy ,]xa3yiiB> px 1X3 ,iy3»xnp tryn axn y;nj y;nxi a"n ]"t dnii pa nyn oaySy; nu oxn iy .nyva =yj axn iy dxh ps ijn lynyj t'x dxi ,]yiS»3 nrs a:ymys *iy axn nnx' px ixn p'x lyo^y4? px iy axn lyiSu c^a*£3 xn ps nnr>D is dtq 'Sya xh jypyu SiSx trin px osixi bxsis |y3'Sp'bj3ix ix -pn ySyac p'i ps ajxry^ax pyanpyj axn px lyixSiys -px mx1 75 CENTIMETRES OR 30 INCHES. jxj-isd tfnyewa x px pyny: px Dy jyDXii N'm eprnpyjax oxn iy pyi^yii ipn rnjyny: -ix; .dtq ^ya xh ;ypyn ]ypyj^ jys'na pn d*d pyajxjiND'nx ypao^T *pT px px-umruypiyoystrj ps ,vw mixasna mx pyaxipiya iy px lyoDra-iyn px tyax^p ix iyrviyjjx pixst? axn px lyix^ xh ix ;yjjxjyaix -iy px ,nxaDna jyny^ D"1: D'ln lo"1: rx nxaDns nyi .mnvo ix tix n^oc : jyi^xnj d-d lypyo^ pn wn xtix-ix^xj x op^yjD'nx nya'a dxii oxn px lyjy^yj pn pn jyjjnstyyj-nx anyayf) lyonx -iyn ?in,x n^'ii dxii ,xij ,xu ... jyD'm px^xnjxpD x ixd dxt rx dxh ;nyj jx i?nxo nyn pa .ooy-ix jx-j ddx si'ix px ay^p yybvy yj""L" jyayipyj toxn ypjc6r yx-jx'a jyoxj jyDD'r oyi .ybww \"\ jyixbiya dxh -iy px t3jxryj3x d.tx jya ljxm epiK jypy: T'x iy jyn ^>nxio j«k oxn iy .yx'jx'a \"p ~mx\ -iy boxn jyjnpyj iy axn |ya*Myjjx axn x'd d^x ,Daxj^B>x^a yD'nj x ly^xiaix rnayWi w: ny loxh njinn n t3x^^JXDy, dxi ,;ynyj ^ya'^xp'p jyoySx^a rx ,jynya d^ px Dy px Daxas? xh jyair dxt ?pr to-iyn -ix„ : lyntyyao^K yjjv tyj,,^p x oxn ja^jn^a iw ,jy^iD^yj Daxjtr xn 1 METRE OR 40 INCHES. nyrDE^^ j^k lyjKntsyj airs ps ^t losn ny t\s ]yayb« «vq niDir lytDij k |ib x^i ^ius co^dxin t\x dd^j^ ^x^d ^i iSn jjhxnjfa ^)3yn eds^k j^w csrnpyj |yi53n^T o«n x^ |is% ,|j7iKnyj ]y^si}?D rx ]yp3n>T J^d lyror^n ^i^ ra jjnjr?3-]yB PUBLISHED BY FERGUSON «. WESTON. (27) 1.50 METRES OR 60 INCHES. era njfflsj^ njn p< jsrwBtt fcbtt ptr r« Sjrro jnjw?j? 3 METRES OR 120 INCHES. k>t rntr 17 ^i^ii^ji ^sian^ itbn cart -**n 2^n 1371010 ~\yi 5 METRES OR 200 INCHES. PUBLISHED BY FERGUSON t WESTON. Andante espressivo. legato. ^~~h I i J 3 I > m ■+■ i »w m i—»i 11----0—\-m—*—F-yt—j ^S=^=iHE^i=^ Ped. --------1—EH--F-l--*H--FH--FH—fi-M--i-1!--UH--!=-l—#-l—»-!-«#—' ^ •^ ^ ^| ^|TJ» | f|g£| fU U Q . dim. ^H *J "N _______--■ tin 1 a P s is i ■*- **-*,*- 1'':!. ±: -S---r— :t=t- * tim=?=3a=? && ^r=t=i ,Jt—»-l—»- r± -£-~?: tru%rai3%sr ^Fir^ffFTi @ife* ! L > ^ £=$ $:■ :E=P^3- m I—r^-h-F- —I-*-sr utj^jtjr -I I--r- fe H---1---h ^ ^ ^ cresc. m &=3 -9----#■ 3=£ =J PUBLISHED BY FERGUSON & WESTON. (3i) The letters are based on an angle of 4 minutes, as the standard acuity of vision. ENGLISH FEET M£ms 80 p^ p^ pHi 20 40 Z E N H ° 20BARXKTF 6 16 FYTPEZCG 5 13 DCOCARXK 10 ZNHEBSYT PUBLISHED BY FERGUSON & WESTON. (33) DR. JAMES WALLACE'S Astigmatic Chart. Adapted to normal visual acuity at one metre, (or 40 inches), and shorter distances as the visual acuity is lowered. PUBLISHED BY FERGUSON & WESTON. (35) /09 8 7 6 5 + 3 2 / /23+56799/0 FERGUSON & WESTON'S TEST FOR MUSCLE BALANCE. FIGURE I. This prism scale is to be held at 30 centimetres from the eyes. Each division represents the deviation of a prism of one degree to the right or left. A ten-degree prism is to be placed over the right eye with its base down- wards. The upper star is to be fixed and the row of figures just above it is focussed sharply. The point of the star will mark how many degrees of deviation are present. If the upper star is to the right it is esophoria. If the upper star is to the left it is exophoria. -/o -9 8 -7 -6 -3 -3 -6 -7 -8 -9 -/0 FIGURE 2. This scale is to be held at 30 centi- metres from the eyes, each division will then represent the deviation of a prism of one degree. A twenty-degree prism is to be placed in front of the right eye with its base towards the nose, and the right star observed. If the right star is above, there is left hyperphoria. If the right star is below, there is right hyperphoria. PUBLISHED BY FERGUSON & WESTON. (37) JACKSON'S TEST FOR MUSCLE BALANCE AT WORKING DISTANCE. The white square on a black ground is held at the working distance and looked at through a strong cylinder (20 to 40 D), held before one eye through which the white spot appears as a grey streak. To test the vertical balance the axis of the cylinder is held vertical. The streak then appears horizontal. To test the lateral balance the axis is held horizontal, the streak appears vertical. In orthophoria the streak appears to pass through the spot. Displacement of the streak the width of the spot represents heterophoria of one quarter degree when the test is held at one-third of a metre. Greater degrees of heterophoria are measured by the prism required to bring the streak upon the square, The cylinder should be held an inch or two in front of the eye, and its centre must be in the line of sight when the eyes are fixed on the white square, or the streak will not come opposite the square. PUBLISHED BY FERGUSON & WESTON. (59) The Cobalt Glass Test for Heterophoria, — BY— DR. HOWARD F. HANSELL. The value of this, (as of most tests for anomalies of the ocular muscles) de- pends upon its disturbing influence on the power of fusion and the induction of a diplopia that, without extraordinary efforts of co-ordination, is insuperable. It consists of a piece of thick glass stained with cobalt-brue, ground with par- allel surfaces and cut to fit the cell of the trial frame. All the rays of light entering such a piece of glass are absorbed, with the exception of the blue and the red which emerge unrefracted to an appreciable degree, from their original direction. The retinal image of a light, the rays of which have been modified by the cobalt glass, is therefore smaller because of the absorption of the major- ity of the constituents of white light, and not through focusing of those rays to a point, and is colored red and blue. The difference between this image and that seen by the eye before which no glass is placed is so marked that the brain unconsciously and involuntarily recognizes it as the image of another distinct and separate light and thus ceases its effort to preserve singular binocular fixation. Should both images fall on the foveae, we have single images and equilibrium or a low grade of muscular inco-ordination. Should the images not fall on corres- ponding points of each retina, we have, as indicated by the relative positions of the lights, heterophoria or heterotropia. The eye before which the cobalt glass is held is permitted by the psychic effect of change in size, color, and shape of the light, to wander into a position that is most restful for it as indicated by the change in location of the light as seen through the cobalt glass. The findings will invariably agree and are a true indication of relative directions of the visual axes, when relieved of the desire to maintain single vision, whether the glass is held before the right or the left eye. This test is inferior to some of the others because latent defects of lateral inco-ordination of two or three degrees only, or half or one degree of vertical tendencies are not manifested. Its superiority to the prism displacement test consists in the determination of heterophoria under like conditions of double foveal perception and not by a comparison of devia- tions induced by a foveal image in one, and an extra-foveal image in the other eye, since it serves to dissolve fusion without alteration of the direction of the rays of light proceeding from the test object. We are prepared to furnish the proper cobalt blue glass for Dr. Hansell's test, either in cells to fit trial frames or in square pieces to hold in the hands. FERGUSON & WESTON, Opticians, N. E. Cor. 15th and Chestnut Streets, Philadelphia. (40) Dr. S. Lewis Ziegler's Greek Cross Test Object for Muscle Balance. The Greek Cross test object was devised as a substitute for the candle or small gas flame in the Graefe test. It consists of a plain Greek cross, cut out of blackened metal, and placed over a thin strip of porcelain, behind which is stationed a gas or electric light, which is concealed by a blackened metal hood. It is used by placing the red glass over the right eye, and either P 8° base in, or P° 5 base up, as a displacing prism over the left eye. It was first devised for use'during the operation of partial tenotomy. The satisfaction in its use for operative cases has led to its adoption for all heterophoric work. It has proved itself more reliable than either the Graefe or Maddox tests. A smaller Greek cross, printed in black, on cardboard, is used as a near test object in preference to either the Graefe line and ball, the ball alone, or the single word test object. The following advantages have been found to accrue from the use of this test object: 1. The Greek cross test object is easily recognized by the patient, even under the partial blurring of vision during tenotomy. 2. The patient can accurately line up the vertical deviation by the horizontal arms of the cross, and the horizontal deviation by the vertical arms of the cross. 3. The four arms of the cross attract the attention of the eye separately, causing complete dissociation of the two images, with muscular relaxation. Hence the tendency to fuse is almost nil. 4. A series of examinations shows less variation than by other methods, which goes to prove the constancy, accuracy, and reliability of this test, and the thorough elimination of all disturbing factors. 5. The test for near, by the small black cross, is likewise more accurate than by other methods, and the error more manifest, for the same reasons that apply to the distance test. PUBLISHED BY FERGUSON & WESTON. (40 The Practical Methods for Determining the Ocular Balance. By DR. S. D. RISLEY. The ideal conditions for comfortable binocular vision are, a pair of emmetropic eyes with normal acuity of vision in each, and the physiological range of accommodation and convergence. These ideal conditions are disturbed, and abnormalities of ocular bal- ance introduced : (i) by the presence of an error of refraction ; (2) by interference with muscular innervation ; (3) by some anatomical abnormality in the muscles, e. g. in their length and direction, their origin, or their attachment to the globe. The first and most simple method for determining the presence of abnormal balance is the cover test, which usually gives important and valuable suggestions as to the character and extent of any existing defect. The method of application is important. A glare of strong light in the face of the patient should be avoided. The eyes being maintained in the primary position should be fixed upon some definite object at a distance of at least 6>n. A black card or cover should then be held in each hand by the surgeon so that either eye can be rapidly covered or exposed. Hold the cover over one steadily for a few seconds, allowing the other to fix the observed object. When the cover is re- moved, if the eyes are properly balanced, no movement will take place, the covered eye has remained in the same relative position as its fellow. The eye moves inward to fix the observed object, exophoria is suggested ; it moves outward to fix, esophoria is suggested ; it moves upward or downward to fix, hyperphoria is suggested. The covers may then be placed alternately before each eye in slow or rapid succession when the movements made in order to fix with each eye afford suggestions as above noted. In either method the degree of movement made affords also some intimation as to the degree of abnormality. The estimated degree may then be verified by placing prisms in the trial frames and repeating the test by cover until no movement of the eyes is observed in the act of fixing. The same test may then be repeated at a finite point, usually at the reading distance. The point of a pin furnishes a good test object, since to be seen accurately it calls into action the power of accommodation. More accurate measurements should then be made with some fixed form of apparatus, which will insure a perfect leveling of the glasses, prisms, etc., to be employed. Probably the best test object is'a small bright point of light before a dead black back- ground. A candle flame is too large for great accuracy. The first step is to secure some means of differentiating the image seen by each eye. This may be done in a variety of ways ; (1) by means of a dark red glass which will often develop diplopia in the presence of even minor grades of imbalance ; (2) by the Maddox Rod, or Double Prism, or a strong cylinder placed before one eye. The rod and cylinder convert a point of light into a streak, the double prism properly placed causes two images in the same eye PUBLISHED BY FERGUSON &. WESTON. (42) usually connected by a line of light. The relative position of the streak of light or double images, to the light seen by the other eye enables the observer to determine the relative position assumed by the eyes of the patient. (3) By the diplopia test. A prism of sufficient strength to cause diplopia is placed vertically before one eye to test the lateral ocular balance and horizontally, preferably with its base toward the nose for testing the vertical balance ; (4) a Maddox Rod of white glass may be placed before one eye, and of red glass before the other. If the ocular balance is perfect, the white and red streaks of light should blend both in the vertical and horizontal position. The principal involved is the same in each of these methods. If the streak, double image, or displaced image, e. g. is seen with the right eye, and the undisturbed image of the light or other test object is seen with the left eye, then, in exophoria the images will be crossed, in esophoria they will be homonomous; in right hyperphoria the dis- placed image will be too low ; in left hyperphoria, too high. The most convenient, rapid and correct method of measuring the extent of the deviation manifested by either of these methods is by means of rotary prisms, mounted in some fixed, stable apparatus, since considerable errors may be introduced by the deviation of the trial frames from a horizontal position. The findings by any of these methods are often erroneous, and the apparent errors discovered may be measurements of spastic condi- tions of the extraocular muscles associated with tonic cramp of the accommodation. No study is complete that does not include the power of abduction, adduction, and sursumduction. The power of the muscles can be measured best by a pair of rotary prisms, one before each eye, firmly fixed and level. The patient should be in a restful posture, but erect, and the eves in the primary position. The most satisfac- tory test object is a small point of light placed at 6»i before a dead black background, and free from surrounding objects. The prisms being placed with the zero point ver- tical, should be rotated slowly but steadily in the direction to secure first the abducting power. As soon as diplopia is produced, the prisms should be turned back into a neutral position, and the patient given repeated opportunity to verify the first result. The adduction and sursumduction should then be determined in exactly the same manner, the zero-point of the prism being horizontal—in measuring the power of the vertically acting muscles. In emmetropic, or in corrected and normally balanced eyes, the abduction and adduction should be approximately as 1. is to 3. i° of abduc- tion to 30 of adduction. Any considerable departure from this working basis will ordinarily reveal itself in deviation by one or all of the above described methods. The average dynamic force of the vertically acting muscles in correctly balanced eyes as measured in the manner described is 2}4° to 3^>° or 40. All errors of refraction should be carefully and totally corrected, and the ocular balance determined through the accurately centered correcting glasses. PUBLISHED BY FERGUSON & WESTON. U3) DR. JAMES WALLACE'S Table Giving the Degree of the Prism and the Angle of Rotation to Produce the Effect of two Prisms with their Bases at Right Angles. Prisms required Deg. of Prism Ang. of rotation Prjsms required Deg. of Prism Ang. of rotation Prisms required Deg. of Prism Ang. of rotation Prisms required Deg. of Prism Aug. of rotation Prisms required Deg. of Prism Ang. of rotation IXI 2X1 3X1 4X1 5X1 6x1 7x1 8x1 9x1 1 ox I 0 1.4 2. 2 3-2 4.1 5-i 6.1 7-i 8 9 10 0 / 45 63.26 7i-34 75-58 78.41 80.32 81.52 82.51 83-39 84.17 1X2 2X2 3X2 4X2 5X2 6x2 7x2 8x2 9x2 IOX2 0 2. 2 2.8 3-6 4-5 5-4 6.3 7-3 8.2 9.2 10. 2 0 / 26.34 45 56.18 63.26 68.12 7i-33 74- 3 75-57 77.27 78.18 IX3 2X3 3X3 4x3 5x3 6x3 7x3 8x3 9X3 10x3 0 3-2 3-6 4.2 5 5-8 6.7 7.6 8-5 9-5 10.4 0 / 18.26 33-42 45 53-°8 59.02 63. 26 66.47 69. 26 71-33 73-°9 IX4 2x4 3X4 4X4 5X4 6x4 7X4 8x4 9x4 IOX4 0 4.1 4-5 5 5-7 6.4 7-2 8 8.9 9.8 10.8 0 / 14.02 26-34 36.02 45 51. 20 56.18 60.14 63-25 66.01 68.10 1X5 2X5 3X5 4X5 5X5 6x5 7X5 8X5 9X5 IOX5 0 5-i 5-4 5-8 6.4 7 7.8 8.6 9.4 10.3 11. 2 0 / II. 19 2I.48 30.58 38.40 45 50. n 54-27 57-59 60.56 63-25 1x6 2x6 3x6 4x6 5x6 6x6 7x6 8x6 9x6 10x6 6.1 6-3 6-7 7.2 7-8 8.4 9.2 10 10.8 11.7 9.28 18.27 26.34 33-42 39-49 45 49-23 53- 7 56.18 59- 1 1x7 2x7 3x7 4x7 5x7 6x7 7x7 8x7 9x7 10x7 7-i 7-3 7.6 8 8.6 9.2 9.9 10.6 11.4 12.3 8. 8 15-57 23-x3 29.46 35-33 4o.37 45 48.49 52. 6 54-59 1x8 2x8 3x8 4x8 5x8 6x8 7x8 8x8 9x8 10x8 8 8.2 8-5 8-9 9.4 10 10.6 n-3 12.1 12.8 7.09 14.03 20.34 26-35 32. 1 36-53 41.11 45 40.21 51.20 IX9 2x9 3X9 4X9 5X9 6x9 7X9 8x9 9X9 IOX9 9 9.2 9-5 9.8 10.3 10.8 11.4 12.1 12.8 J3-5 6.2 1 I2-33 18.27 23-59 29.04 33-42 37-44 41-39 45 48 IXIO 2XIO 3XIO 4XIO 5XIO 6XIO 7XIO 8XIO 9XIO IOXIO 10 10. 2 10.4 10.8 11. 2 11.7 12.3 12.8 i3-5 14.1 5-43 11.42 16.51 21.50 26.35 3°-59 35-oi 38.40 42 45 If the base of a prism is rotated 360 53/ from the vertical meridian, it is 530 rj/ from the horizontal meridian. The pris- matic effect in the vertical meridian, 360 53' from the base of the prism, is ^ of the prism. The prismatic effect in the hori- zontal meridian, 530 7' from the base of the prism, is T6ff the value of the prism. The base of the prism is rotated a smaller arc for the larger effect and vice versa. The direction of the base is the direction of the perpendicular to the base. PUBLISHED BY FERGUSON 4. WESTON. Jackson: Decentering of Lenses for Prismatic Effects, with Glass having an Index of Refraction of about 1.54. Power of Lens in To obtain To obtain To obtain To obtain To obtain To obtain To obtain To olitaiu Dioptres. i° Prism 2° Prism 30 Prism 40 Prism 5° Prism 6° Prism 8° Prism io° Prism Decentie mm. Decent re mm. Decentie mm. Decentre mm. Decentre mm. Decentre mm. Decentre mm. Decentre mm. 1 D., 9.4 18.8 28.3 37-7 47.2 56-5 75-8 95-2 2 4 7 9 4 14. I 18 8 23 6 28.2 37-9 47.6 3 3 1 6 3 9.4 12 6 J5 7 18.8 25-3 3J-7 4 2 3 4 7 7-1 9 4 I I 8 I4. I 18.9 23.8 5 1 9 3 8 5-7 7 5 9 4 n-3 15-2 19. 6 1 6 3 1 4-7 6 3 7 9 9-4 12.6 15-9 7 1 3 2 7 4- 5 4 6 7 8.1 10.8 i3-5 8 1 2 2 3 3-5 4 7 5 9 7-1 9-5 11.9 9 10 1 2 1 3-1 4 2 5 2 6-3 8.4 10.5 9 1 9 2.8 3 8 4 7 5-6 7.6 2'5 11 9 1 7 2.6 3 5 4 3 5-1 6.9 8.7 12 8 1 6 2.4 3 1 3 9 4-7 6-3 7-9 J3 7 1 4 2. 2 2 9 3 6 4-3 5-8 7-3 M 7 1 3 2. 2 7 3 4 4- 5-4 6.8 15 6 1 3 1.9 2 5 3 1 3-8 5-1 6-3 16 .6 1 2 1.8 2 4 3 3-5 4-7 6. l7 .6 1 1 i-7 2 2 2 1 3-4 4-5 5-6 18 • 5 1 1.6 2 1 2 6 3-1 4.2 5-3 l9 ■ 5 1 i-5 2 2 5 3- 4- 5- 20 •5 •9 1.4 1 ■9 2 4 2.8 3.8 4-8 Transactions of the American Ophthahnoloyical Society. PUBLISHED BY FERGUSON 4 WESTON. EQUIVALENT NUMBERS OF PRISMS—JACKSON. The first column gives the number of each prism according to the new system, indicating its power of caus- ing deviation in the rays of light passing through it expressed in cent,ads. The second column gives the same deviating power expressed m prism dioptres. The third column gives the refracting angle of the prism, (its num- ber by the old system), which will cause this same deviation, when the prism has one surface perpendicular to the rays, and the glass has an index of refraction of 1.54. Deviation in Ceutrads. Deviation in Prism Dioptres Refracting Angle in Degrees. Deviation in Centrads. 9 Deviation ill Prism Dioptres Refracting Angle in Degrees. Deviation in Centrads. Deviation in Prism Dioptres • Refracting Angle in Degrees. I I ' 1 -06 9 "02 9 "39 17 17-16 16-98 2 2 ' 2*12 10 10-03 10-39 18 18-19 17-85 3 3" 3-18 11 II-03 n'37 19 19-23 18-68 4 4" 4'23 12 12 -04 12-34 20 20-26 i9"45 5 5" 5-28 13 I3-o6 13-29 25 25'53 23-42 6 6-oi 6-32 14 I4-o8 14-23 30 3° "93 26-81 7 7-01 7'35 15 15-11 15-16 40 42-38 32-18 8 8-02 8-38 16 l6-I4 16-08 5° 54-62 36'°3 Dr. Edward Jackson, Ophthalmic Review. PUBLISHED BY FERGUSON 4 WESTON. 10 Z0 18 1 6 14 12 10 8 19 17 15 13 PRISM SCALE BY DR. S. LEWIS ZIEGLER, DIRECTIONS FOR USING DR. ZIEGLER'S PRISM SCALE. The prism scale herewith shown can be used at a distance of a half meter, but a larger one for use at two meters is preferable, as the possibility of error is much less. To use the scale close one eye, and with the other look at the scale both through the prism and over it. A comparison of these two views gives the required registration. Each field must contain either the indicator singly or the numbered gradations singly ; the fields being in conjunction at the margin of the lens. Rotate the prism until the base line seen through the prism is continuous with the base line of the scale. Always keep the plane of the prism parallel with that of the scale, and on a level with it. The index line will be displaced along the scale until the indicator stands opposite the proper numbered gradation. By moving the prism up and down, along this gradation, it can be seen whether the index line accurately coincides or not. To ascertain the strength of a pair of prismatic spectacles, place the neutralizing sphero-cylinder over the spectacle lens, and proceed to take the reading of the scale. If the lenses are evenly held, it can be seen (i) whether the lens is properly neutralized, (2) whether it is accurately centered, and (3) whether the prismatic strength is correctly measured. PUBLISHED BY FERGUSON & WESTON. (47) >i Dr. Wallace's Table for Reducing Crossed Cylinder Lenses to a Sphero-Cylindrical Combination. ** Rule 1.—All crossed cylinder combinations with the same ratio, viz : + 1 to 4- 2 and + 3 to 4- 0, or 4- 1 to 4- 3 and + 2 to + 6, will have the axis of the ' resultant cylinder at the same angle, and the spherical and cylindrical lenses of one combination will be multiples or fractions of the spherical and cylin- drical lenses of the other. Rule 2.—The two cylinders may lie in any part of the arc of 180° it is easy to find the angle of the resultant cylinder by keeping it at the same angle from the weaker or stronger cylinder, as the cviinders are moved to new positions. Rule 3.—By observing tlie difference in the spherical and cylinder in passing through 10 or 15 degrees, changes of 5° in the distance between the axes may be made by dividing the difference between sphericals in IO6 into two parts or in 15° into three parts and subtracting from the higher spherical. In the same way the difference between the cylinders is to be divided into two or three parts and subtracted from the higher or added to the lower. Example : + 1 cyl. ax. 90 4- 3 cyl. ax. 15 = 4- .87 + 2.25 cyl. ax. 22 + 1 cyl. ax. 90 + 3 cyl. ax. 25 = + .75 + 2.50 cyl. ax. 32 12 25 10 + 1 cyl. ax. 90 + 3 cyl. ax. 20 = + .81 + 2.37 cyl. ax. 27 Cylinder Cylinder Spherical Cylinder Cylinder Cylinder Spherical Cylinder + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cvl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 +' 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cvl. ax. 90 + 1 cvl. ax. 90 + 1 cvl. ax. 90 + 1 cvl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cvl. ax. 90 + 1 cvl. ax. 90 + 1 cvl. ax. 90 + 1 cyl. ax. 90 + 1 cvl. ax. 90 -t 1 cvl. ax. 90 + 1 cvl. ax. 90 + 1 cvl. ax. 15 + 1 cyl. ax. 30 + 1 cyl. ax. 45 + 1 cyl. ax. 60 + 1 cyl. ax. 75 + 1.25 cyl. ax. 15 + 1.25 cyl. ax. 30 + 1.25 cvl. ax. 45 + 1.25 cyl. ax. 60 + 1.25 cyl. ax. 75 + 1.50 cvl. ax. 15 + 1.50 cyl. ax. 30 + 1.50 cyl. ax. 45 + 1.50 cyl. ax. 60 4 1.50 cyl. ax. 75 + 1.75 cyl. ax. 15 + 1.75 cyl. ax. 30 + 1.75 cyl. ax. 45 + 1.75 cyl. ax. 00 + 1.75 cyl. ax. 75 + 2 cyl. ax. 15 + 2 cyl. ax. 30 + 2 cyl. ax. 45 + 2 cyl. ax. 60 + 2 cvl. ax. 75 + 2.25 cvl. ax. 15 + 2.25 cyl. ax. 30 + 2.25 cyl. ax. 45 + .75 + .50 + .25 + .12 + '.87 + .50 + .37 + .12 + '.87 + .02 + .37 + .12 + .87 + .02 + .37 + .12 + '.87 + .02 + .37 + .12 + '.87 + .02 + .37 + .87 cyl. ax. 52% + 1. cyl. ax. 60 + 1.37 cyl. ax. 67% + 1.75 cyl. ax. 75 + 1.87 cyl. ax. 82% + .02 cyl. ax. 51 + 1.12 cyl. ax. 55 + 1.02 cyl. ax. 04 + 2. cyl. ax. 74 + 2.25 cyl. ax. 82 + .75 cyl. ax. 31 + 1.37 cyl. ax. 51 + 1.75 cyl. ax. 02 + 2.25 cyl. ax. 72 + 2.50 cyl. ax. 81 + 1. cvl. ax. 34 + 1.50 cvl. ax. 49 + 2. cyl. ax. 00 + 2.37 cyl. ax. 70 + 2.75 cyl. ax. 80 + 1.25 cyl. ax. 27 + 1.75 cyl. ax. 45 + 2.25 cyl. ax. 58 + 2.02 cyl. ax. 09 + 3. cyl. ax. 80 + 1.50 cyl. ax. 25 + 1.75 cyl. ax. 43 + 2.50 cyl. ax. 57 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 2.50 cyl. ax. 90 + 2.50 cyl. ax. 90 + 2.50 cyl. ax. 90 + 2.50 cyl. ax. 90 + 2.50 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cvl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cvl. ax. 90 + 3.50 cyl. ax. 90 + 3.50 cyl. ax. 90 + 3.50 cyl. ax. 90 + 3.50 cyl. ax. 90 + 3.50 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 1 cyl. ax. 90 + 2.25 cyl. ax. 00 + 2.25 cyl. ax. 75 + 1 cyl. ax. 15 + 1 cyl. ax. 30 + 1 cyl. ax. 45 + 1 cyl. ax. 00 + 1 cyl. ax. 75 + 2.75 cyl. ax. 15 + 2.75 cyl. ax. 30 + 2.75 cyl. ax. 45 + 2.75 cyl. ax. 60 . + 2.75 cyl. ax. 75 + 3 cyl. ax. 15 + 3 cyl. ax. 25 + 3 cyl. ax. 35 + 3 cyl. ax. 45 + 3 cyl. ax. 55 + 3 cyl. ax. 05 + 3 cyl. ax. 75 + 1 cyl. ax. 15 + 1 cyl. ax. 30 + 1 cyl. ax. 45 + 1 cyl. ax. 00 + 1 cyl. ax. 75 + 4 cyl. ax. 15 + 4 cyl. ax. 25 + 4 cyl. ax. 35 + 4 cyl. ax. 45 + .25 +' '.87 + .02 + .37 + .25 + '.87 + .02 + .37 +' '.87 + .75 + .02 + .37 + .25 + .12 ts + .50 + .25 +' '.87 + .75 + .02 + ,0 + 2.75 cyl. ax. 69 + 3.25 cyl. ax. 80 + 1.75 cvl. ax. 82 + 2.25 cyl. ax. 78 + 2.75 cyl. ax. 79 + 3.25 cyl. ax. 81 + 3.50 cyl. ax. 80 + 2. cyl. ax. 22 + 2.50 cyl. ax. 41 + 3. cyl. ax. 55 + 3.50 cyl. ax. 07 + 3.75 cyl. ax. 79 + 2.25 cyl. ax. 22 + 2.50 cyl. ax. 32 + 2.87 cyl. ax. 45 + 3.12 cyl. ax. 54 + 3.50 cyl. ax. 03 + 3.75 cvl. ax. 71 + 4.00 cvl. ax. 79 + 2.75 cvl. ax. 84 + 3.25 cyl. ax. 82 + 3.50 cyl. ax. 81 + 4. cyl. ax. 84 + 4.50 cyl. ax. 87 + 3.25 cvl. ax. 20 + 3.50 cyl. ax. 31 + 3.75 cyl. ax. 42 + 4. cyl. ax. 52 PUBLISHED BY FERGUSON & WESTON. Dr. Wallace's Table for Reducing Crossed Cylinder Lenses to a Sphero-Cylindrical Combination [Continued.] Cvlinder Cvlinder . ax. 90 . ax. 9U . ax. 90 .ax. 90 . ax. 90 . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. . ax. 90 . ax. 90 .ax. 90 .ax. 90 . ax. 90 . ax. 90 . ax. 90 + 0.00 ev + 0.00 ev + 0.00 cv + 0.00 cy + 2cy 2 cy 2 cv i cv 4cv 4cv 4cy 4cy 5 cy 5 cy 5 cy 5 cy 5 cv 7cy 7 cy 7 cv 7cy 7 cy 7ey 7cy . ax. 45 . ax. 55 .ax. 05 . ax. 75 .ax. 15 .ax. 30 . ax. 45 . ax. 00 . ax. 75 .ax. 15 .ax. 30 . ax. 45 . ax. 00 . ax. 75 . ax. 15 .ax. 30 . ax. 15 . ax. 00 . ax. 75 . ax. 15 . ax. 25 . ax. :;5 . ax. 15 .ax. 55 .ax. 05 . ax. 75 Spherical + .50 + .25 + -12 + 1.37 + 1.00 + .02 + .25 4 2.50 + 1.75 + 1.00 + .50 + .12 + 2.75 + 2.25 + 1.75 + 1.25 + .7.) + .37 + -12 ~i~ 0 cvl + 0 oO cvl + 0 75 rvl ~\~ 7 cvl 1 cvl + 1 75 CVl *i" 2 ;>0 cvl + 3 cvl + 3 50 cvl + 2 M cvl + 3 ;>0 cvl + 1 2;j cvl + b cvl + i) 50 cvl + 3 cvl + 1 :>0 cvl + 0 cvl + 7 cvl + i 7d CVl + i 7-) cvl + ;> :>0 cvl + li 7;i cvl + i 75 cvl + s 50 cvl + 9 :>U cvl + 10 cyl PUBLISHED BY FERGUSON & WESTON. (oS) +++- -+++++++- -++++++++++++++++++++++++++++++ COOO'TJOOO^OOOOOOOO. oocoor^ooo'T'^ ^ooooooooo *.X*^*^FFPFFFF*F*FFFFR^ oooooooooooooooooooooooooooooooooooooooooooooooooooooo I M II I I I II I I I I I I I I I I I I II I II I I I || I I I I I I M | | | | | | | | I I I I I I O' -' B' O' C/l V lOioicioioiancicici •b o o r. t »(1SSBPBS»SB»SS»SSSSSBP!»S!»(»»B!SS»(B»»BSSS»SS!«;!!S"S»SSS3BS y y y y y y yyyy y y y y y y y y y y y yyyy y y^y y y y y y y yy yyy y yy y y y y y y y y yyyy coooooooooooooooooooocooooooooc5oooooooc5ooooc5oooooooooo ++- +++++ IO CO Oi 3~. -1 Be IO CO 31 M C -i kjoioici^ivi to- I II I I I I I I I I I I I II II II II II I II II II I II ! 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K) + cvl . ax. hi + cvl . ax. )() + cvl . ax. to + 1 cvl . ax. 10 -B I cvl . ax. )0 - ! cvl . ax. 1(1 ! cvl . ax. 10 I cvl . ax. m ; cvl . ax. 10 ; cv . ax. 10 + i cvl . ax. 10 4 -Jcv . ax. 10 ! cvl . ax. HI + ; cvl . ax. Hi + i cvl . ax. 10 Cvlinder 5.50 cvl. ax. 5.50 cvl. ax. 5.50 cyl. ax. 5.50 cyl. ax. 5.50 cyl. ax. 0 cyl. ax. 0 cvl. ax. 0 cvl. ax. 0 cvl. ax. 0 cvl. ax. (i cyl. ax. 0 cyl. ax. 0 cvl. ax. 1 cyl. ax. 4 cyl. ax. I cyl. ax. 4 cyl. mx. 4 cyl. ax. 4 cvl. ax. 4 cyl. ax. 1 cyl. ax. 5 cyl. ax. 5 cvl. ax. Spherical .02 .50 Cylinder + -12 3. 2.75 2.50 1.75 1.25 — 1.75 cv — 4.50 cy — 7. cy — 0.75 cv — 0.50 cv — 0.25 cv — 0. cv — 5.50 cy — 5.25 cv — 5. cv 0.50 cv ■0. • 5.50 ■ 4.50 ev ■ 3.50 cv ■ 2.50 ev ■ 1.50 cv ■8. ■ 7.50 ev 1. ax. 3o .ax. 45 . ax. 5;i . ax. 00 . ax. 78 . ax. 9 . ax. 17 . ax. 20 .ax. 30 .ax. 45 .ax. 55 .ax. 07 . ax. 78 . ax. 0 . ax. 12 . ax. 17 . ax. 23 .ax. 30 .ax. 37 .ax. 40 .ax. 59 . ax. 0 . ax. 13 Cylinder 3 cv . ax. i 3 cv . ax. ! 3cv . ax. < 3 cv . ax. < 3 cv . ax. < 3 cv . ax. i 3 cv . ax. i :! ev . ax. ! 3 cv . ax. • 3 cv . ax. i 3 cv . ax. i 3 cv . ax. < 3 cv . ax. '. 3 cv . ax. '. 3 cv . ax. i 3 cv . ax. '. 3 cv . ax. • 3 cv . ax. < 3 cv . ax. < 3 cv . ax. ! 3 cv . ax. < 3 cy . ax. 1 5 cvl. 5 cyl. 5 cvl. 5 cvl. 5 cvl. 5 cvl. 7 cvl. 7 cvl. 7 cvl. 7 cvl. 7 cvl. 7 cvl. 7 cvl. 7 cvl. 8 cvl. 8 cvl. 8 cvl. 8 cvl. 8 cvl. 8 CVl. 8 CVl. 8 cvl. ax. 30 ax. 10 MX. 50 ax. 00 ax. 70 ax. 80 ax. 10 ax. 20 ax. 30 ax. 40 nx. 50 ax. 00 ux. 70 ax. 80 ax. 10 Spherical + 2.50 + 2.25 + 2. + 1.50 + 1. .50 4 2^75 -I- 2.50 I- C50 ; i. -;- .50 4 .12 : 2.50 ! T.Y>o 4 1. + .50 + -12 Cvlinder Y 0. ".0 cvl -1 7-, 75 •> 50 cvl 10. 9. 9. 50 cyl cyl cvl S. cvl 7. cyl 5. cyl cyl 11. cvl 10. 5(1 cvl 10. 9. 50 cyl cvl S. cvl 0. cyl il. -'■' cyl ax. 19 UX. 20 a x. 33 ux. 43 ax. 53 ax. 07 ax. 38 ax. 10 ax. ;>9 MX. 72 ax. 7 ax. l!) IIX. 22 IIX. 30 ax. 39 Dr. Wallace's Table for Reducing Crossed Cylinders to a Sphero-Cylindrical Combination. Rule 4.—The formula for two concave cylinders is the same as that for two convex cylinders, with minus signs instead of plus. Rule 5.—When the convex and concave cylinders are crossed, the resultant spherical has the sign of the weaker cylinder (+ or —X the resultiint cylin- der the sign of the stronger cylinder, and the axis is reversed 90°. Ex. + ley 9 o. — 2 cy. 20 = + .N7— 2.75 ax. 13. + 2 cyl. ax. 9 0 — 1 ax. 20 = — .87 = + 2.75 ax. 102. • it is unnecessary to work out more than a single series of one ratio as + 1 with 1.50: -■ 2 with— 3; + .50 with— .75 will have the same angle in the result- ant cylinder, and the spherical and cylinder of the second will be twice that of the first. The spherical and cylinder of the third will be one-half that of the first. This table combined with the rules given above will enable one to work out any combination with accuracy. In the preparation of the table the absolute refractive value of each meridian has been calculated from the elliptical curves of the rotating cylinders. PUBLISHED BY FERGUSON 4. WESTON. Coric Censes; or Periscopic Cylindrical and Spbero* Cylindrical Glasses, —BY— DR, GEORGE C. HARLAN. At a meeting of the American Ophthalmological Society, in July, 1885, I gave a description of a sphero-cylindrical lens ground on one surface only, and explained the method of its manufacture. The superiority of the periscopic form of lens, or meniscus, when high powers are required, is, I believe, universally admitted, and its use is quite general in the case of spherical glasses. This has not been the case, however, with cylinders and sphero-cylinders, as opticians gener- ally have not undertaken to grind them in this form. The lens to which I wish again to call the attention of this Society consists of crossed cylinders ground on one surface of the glass, the other side being left for any desired spherical curve. In this way a meniscus may be produced. Here, for instance, is a combination lens giving the effect of + 4. o + 2. cyl. To produce this effect crossed cylinders of 4 and 6 are required, supposing the other side to be left plain. If we wish to give the periscopic form to this glass, it can be done by making the cylinders 6 and 8 and grinding a — 2 spherical on the other surface. If a simple cylinder is needed, the spherical curve must equal that of the weaker cylinder. Note.—Dr. Wallace's table resolves all crossed cylinders with oblique axes into a sphero-cylindri- cal combination, or in other words, places the principal meridians at right angles to each other. This will be found very useful in preparing a formula for toric lenses combined with bifocal segments. PUBLISHED BY FERGUSON 4. WESTON. (52) NOTES ON RETINOSCOPY, BY Dr. James Thorington. I.—THE PRINCIPLE OF RETINOSCOPY is the finding of the point of reversal, the point where the emergent rays of light, from the eye under examination, come to a focus, or are made to come to a focus, by placing a lens in front of it. 2.—THE ROOM must be darkened, the darker the better,—all sources of light except the one in use to be excluded. 3.—THE OBSERVER need not make any note of his accommodation as in using the ophthalmoscope, but as he requires very acute vision, he should wear any necessary correcting glasses. 4.—THE PATIENT must have his accommodation thoroughly relaxed with a reliable cycloplegic. 5.—THE DISTANCE of the patient from the observer is usually one meter, and this dis- tance will be considered in the following statements. 6.—THE DIRECTION of the movement of the retinal illumination as seen in the center of the pupillary area, is a most important point in the study of retinoscopy. 7.—THE PLANE MIRROR shows the retinal illumination moving with the movement of the mirror in emmetropia, hyperopia, or myopia, if the myopia be less than one diopter, and the illumination moving opposite to the movement of the plane mirror when the myopia is more than one diopter. 8.__THE POINT OF REVERSAL for the center of the pupillary area is found when a lens (or a series of lenses) placed in front of the eye under examination, stops all ap- parent movement of the illumination in this area. 9.—A PLUS LENS is required to stop the apparent movement of the illumination, (at one meter) in emmetropia, hyperopia, and myopia of less than one diopter. io.__A MINUS LENS is required to stop all apparent movement of the illumination (at one meter) in myopia of more than one diopter. 11.__THE MERIDIAN is being refracted through which the observer passes the light. I2.__ASTIGMATISM is recognized when the illumination appears to move faster in one meridian than the meridian at right angles to it, the astigmatism will be in the meridian of slow movement. ASTIGMATISM is also recognized by the presence of a band of light extending across the pupil; this may not appear until a spheric lens has been placed before the eye. 13.—RULES FOR LENSES at one meter. Add a minus 1. D. sphere to the result obtained in the dark room in every in- stance and the result will be the emmetropic or infinity correction. 14,__All retinoscopic work should, when possible, be confirmed at the trial case before order- ing the patient's glasses. !£.__RETINOSCOPY is the most exact objective method of estimating the refraction of an eye that there is and it is the most satisfactory, when carefully executed. PUBLISHED BY FERGUSON & WESTON. (53) Subjective and Objective Methods of Measuring Refractive Errors. By JOHN T. CARPENTER, JR. All methods for the estimation of the amount of refractive errors are based upon a knowledge of the static refraction of the dioptric system of the eye. The employ- ment of a satisfactory cycloplegic is therefore a sine qua non. After the age of 55 this is often unnecessary. Methods for the measurement of errors of refraction may be classed as Subjective and Objective. In the Subjective method, we depend upon our patient's replies and determine the refractive error by means of suitable test-letters. Normal visual acuity exists when letters, subtending a visual angle of 5', with the component parts of the letter subtending a visual angle of 1/, are recognized by the patient who is placed at a convenient distance from the test card. Upon this basis of normal visual acuity are constructed the test types of Snellen and those of his many imitators. Letters are placed at 6 meters, as rays may then be regarded as practically parallel. In expressing visual acuity, we place the distance at which the patient is stationed as the numerator, and the distance at which the test-letter subtends a visual angle of 5' as the denominator of the fraction. Thus : 6-24 would mean that the patient stationed at 6 meters, read test-letters which should have been distinguished at 24 meters. PROCEDURE IN SUBJECTIVE OPTOMETRY. Hyoscyamin, Duboisin, Atropin or some reliable cycloplegic, must be instilled until complete paralysis of the ciliary muscle has been gained. The visual acuity of each eye (the other being open, but excluded from vision) must then be ascertained and recorded. Healthy emmetropic eyes, with accommodative power completely par- alyzed, should possess visual acuity = 6/6. Convex sphericals are next placed before the eye, and improvement in vision demonstrates the existence of hyperopia, as convergent rays produced by convex lenses are only brought to a focus in the short eye-ball of the hyperope. Convex spherical lenses of increasing power are introduced until visual acuity reaches its maximum and any stronger lens causes deterioration in vision. To the spherical glass producing maximum visual acuity, cylinders are now added, at first with their axes vertical, and the strongest cylinder which causes the greatest improvement in visual acuity is the measure of the astigmatism present. By rotating the cylinder 5 or 10 degrees to right or left, sharper visual acuity may sometimes be gained at one par- ticular spot which determines the axis of the astigmatism. A complete rotation of the cylinder may be necessary in order to determine its proper axis. The final result is then reached by carefully testing the increase or decrease of the combined sphero-cylinder by weak convex and concave spherical and cylindrical lenses or crossed cylinders. The same procedure is adopted in Myopia ; being careful to note the weakest concave lens producing maximum visual acuity. When much astigmatism exists, this test is supplemented by a very valuable procedure, namely : testing the visual acuity for lines drawn at right angles to each other. These are made up of 3 parallel lines of exactly equal intensity and width. PUBLISHED BY FERGUSON 4. WESTON. (54) The lines are printed on circular disks, which can be rotated so as to stand in any meridian. By having the patient fix his at- tention first upon one set of lines and placing stronger and stronger spherical lenses before the eye until these lines become sharply de- fined ; and next proceeding in the same manner with the parallel lines placed at jight angles, the result gives the correct measure- ment of the refraction in each of the principal meridians of the — astigmatic eye. The glass can then be calculated easily. Example: Lines at 9o° ||| are most distinctly seen with -! 2.50 spherical, while lines at 1800 = are best seen with + 3.50 spherical. Remembering that the lines measure the meridian at right angles to their direction, the formula for the proper correcting glass would be + 2.50 sph. O + 1.00 cyl. ax. 1800. Of great service is also the stenopaic slit, by means of which different meridians can be measured separately, and the combined lens found correcting the two principal meridians. Schemer's perforated disk, as brought forward by Thomson, the Cobalt glass, giving chromatic test; and in short, every means in our power may be needed in order to correctly solve puzzling cases. Rapid work usually means careless work. OBJECTIVE TESTS 1st and most important, I should place Retinoscopy, as giving the entire amount of refractive error, whether Hyperopia, Myopia, or Astigmatism. 2d. Direct method of Ophthalmoscope, in which a fine vascular twig in the cen- tral portion of the fundus is used, upon which to focus. It is important to remember that the strongest convex and weakest concave lens is the measure of the Hyperopia or Myopia present in the meridian at right angles to the direction of this fine retinal vessel. 3d. Ophthalmometer for detection of corneal astigmatism. Objective tests, in my opinion, are valuable as time savers, and as giving reliable in- formation independent of the "personal equation" of the patient at least, (if not of the ophthalmic surgeon making the test). No objective test can supplant the more tedious but more practical subjective methods, and if I were compelled to adopt one method it would be that of subjective optometry with test-lenses. The most satisfactory routine in the measurement of refractive errors I believe to be the following : 1. Testing visual acuity at 6 meters. 2. Testing punctum proximum and punctum remotum with small type. 3. Testing muscular balance at 6 meters, and in accommodation at ^ ctm. 4. Ophthalmoscope—Direct method. 5. Ophthalmometer for Corneal Astigmatism. 6. Retinoscopy and the Direct Method of Ophthalmoscope, to be used after in- stillation of a Cycloplegic, preferably Hyoscyamin Sulphate, gr. y^—3i or Duboisin Sul- phate, gr. }(—m. This gives the total error present, whether Hyperopia, Myopia, or Astigmatism—both corneal and lenticular. This correction should serve as a basis from which to try, in the careful and methodical manner, above described the sub- jective method with test-lenses in the trial case. For final proof the sphero-cylindrical correction should be proved by Retinoscopy. PUBLISHED BY FERGUSON & WESTON. (55) The Detection of Simulated Monocular Blindness. By DR. WM. CAMPBELL POSEY. It is at times necessary to be able to prove that vision, which is claimed to be lost or extremely defective in one eye, is really good. This happens in the army when recruits demand exemption from service on the plea of bad sight, in medico-legal cases, when damages are sued for upon account of professed loss of sight from injury, and in hysteria. As the individuals making this claim are intelligent and oftentimes familiarize them- selves with many of the tests use'd for the detection of their simulation, it is necessary that the examiner should have a variety of procedures at his command, with which he is perfectly familiar, to outwit them, and to show conclusively that the organ which they would have rated below the standard, is actually a useful member, possessing a satisfactory degree of acuity. In the performance of the tests, it is always wise to give the person under exami- nation the impression that his tale is believed, and that the procedures which are being carried out are simply for the purpose of ascertaining the condition and the degree of visual acuity in the sound eye. Although there are many devices for the detection of malingering, but three of the more valuable will be detailed here. THE PRISM TEST. The subject is seated before a lighted candle, and a prism of eight degrees, base up or down having been placed before the sound eye, he is requested to regard the flame at five meters distance. If double images are acknowledged, the proof of his simulation is accomplished, for one of the images must belong to the supposedly defec- tive eye. This test has been modified by Al. von Graefe in a most ingenious manner. The sound eye is made to see double by holding the apex of a prism before half of its pupil. One image is thus obtained through the unobstructed half of the pupil, and the second through the other half by the deflection of the rays by the prism. The patient should at once, without hesitation, acknowledge seeing double images. The edge of the prism is then raised or depressed sufficiently to cover the entire pupil, so that but one image is perceived by the sound eye, although on a different vertical plane. If the patient now sees double, there is conclusive proof that the avowed defective eye must be act- ing, and the fraud is detected. If the ordinary Snellen type be substituted for the candle flame, it is possible to obtain the degree of visual acuity in the pretended bad eye by producing double im- ages of the different lines on the chart in the same manner as in the candle test. As the bringing of the apex of the prism before but a part of the pupil is always attended with some difficulty, and the patient apt to become cognizant of what is be- PUBLISHED BY FERGUSON & WESTON. (56) ing attempted, Baudry has recently introduced a device by means ot which these diffi- culties are entirely overcome. He employs a triangular prism, a section of a right angled triangle, and uses the base of the prism to deflect the light instead of its apex. This prism is divided into two parts by horizontal sections and cemented at their bases to the centres of the parallel aspects of the same thicknesses. In its entirety, the lens represents a fragment of polished glass divided into three distinct parts in juxtaposition to one another by their ground surfaces. The prism is contained in a round metallic box, with central apertures in each of its surfaces. By means of a simple mechanism it is possible to bring the bodies of the prismatic sectors or their bases before the sound eye, and single or double vision obtained at will, without the possibility of the subject being cognizant what portion of the prism he is looking through. To avoid the difference in tint between the true image and the virtual one pro- duced by the passage of the rays through the prism, a red glass is placed before the candle flame. This serves to neutralize the decomposition of light produced by the prism and makes the real and virtual images identical. HARLAN'S TEST. By this method, the sound eye is secretly excluded by placing a lens of such high degree before it, that its far point falls within a few inches of the eye, whilst a lens of such weak power is placed before the pretended defective eye, that its visual acuity is not interfered with. Practically, the refraction of both eyes having been ascertained by the ophthalmoscope, the subject is seated five meters away from the ordinary test charts. Whilst both eyes are open, a high convex spherical lens (plus sixteen diopters) is slipped into a trial frame before the sound eye and a lens which about corrects the refraction of the bad eye is placed before it. If any of the letters on the chart are now read, this must have been accomplished by the avowed defective eye, as the vision in the sound eye had been excluded by the high spherical lens. An effort should now be made to convince the patient of the degree of the visual acuity in his supposedly defective eye by asking him to read with the lens still in posi- tion, but with his bad eye closed. SNELLEN'S TEST. In the application of this test, Snellen has taken advantage of the law of comple- mentary colors, employing a test card with alternate red and green letters. The patient is seated before the card and told to regard the chart through a pair of spectacles which are so constructed that a red glass is placed before one eye and a green one before the other. The eye looking through the red glass can only perceive the red letters as °reen is the complementary color of red, while the eye looking through the green glass can only see the green letters for the same reason. If a man who is blind in one eye looks at this chart through these glasses, he will see only the red or the green letters according as the red or green glass is before the sound eve. If both red and green letters are read, then it is obvious that both eyes are functioning. PUBLISHED BY FERGUSON 4. WESTON. (57) AN ADJUSTABLE LAMP OR LIGHT SCREEN. ESPECIALLY ADAPTED TO THE SHADOW TEST. To obtain the best results in examination of the eye by artificial light, it is essential that the size and shape of the light source shall be easily adjustable, and general illumination of the room reduced to the minimum. This is peculiarly true of the modern and more exact methods of applying the shadow test, in which accuracy depends very largely upon the reduction in the area of both light and mirror surface. The illustrations show a device which meets all the requirements out-lined above. The main vertical barrel of the screen is made of asbestos, and completely covers the glass chimney, and is furnished inside with springs, which clasp the latter gently, holding it in position. The lateral openings are arranged so as to be present opposite the greatest illuminating power. On one side, four circular openings, respectively, 5, 7^, 10 and 25 millimeters in diameter, have been arranged in a revolving disc which is accurately halted at the desired point by a spring clip. These fulfill all possible requirements of skiascopy, while the largest is quite sufficient for most ophthalmo- scopic work. Directly opposite the above openings is one which exposes practically the en- tire flame, while its projecting border prevents needless radiation of light. Two shutters are hinged to fit over this opening, one being blank to prevent the escape of light when using the opposite side, while the other is fenestrated and backed by a sheet of thin porcelain glass. The latter plan secures a clear white illuminated image corresponding to the opening in the metal, and one which, owing to the absence of marked diffusion at its margin, forms a most satisfactory object for distant muscle testing. For this purpose we have adopted the simple Greek cross as suggested by Dr. Ziegler, but smaller, and find it satisfactory. The diameter of the arms is 10 millimeters, and the total diameter of the cross is 50 millimeters. This lamp screen can be used with either the Welsbach or Argand gas burner. Price, $3.50. FERGUSON & WESTON, Opticians, Chestnut and 15th Sts., = = Philadelphia, Pa. PUBLISHED BY FERGUSON 4 WESTON (58) The Ophthalmometer and its Clinical Use. By DR. E. W. STEVENS. The Ophthalmometer of Javal and Schiotz, consists of a telescope mounted upon a tripod, which can be moved laterally, or backward or forward to obtain the proper focus. By means of an elevating screw, attached to one foot of the tripod, the tele- scope is adjusted for the level of the observed eye. The telescope contains a double refracting prism between two convex lenses. The eye to be measured is placed at the principal focus of the first lens. At the principal focus of the second lens there is formed an inverted image of the erect image, formed by reflection on the cornea. At this point there is spider line sight for which the eye piece of the instrument is focused. A graduated arc is fixed upon the tube of the telescope so as to rotate with the tube about its axis. This arc is provided with a long pointer, which indicates upon the large disk of the instrument, the number of degrees the arc and telescope have been rotated. To this graduated arc is also attached two white enameled sights or mires mounted on a background of black velvet. One mire, a right angled parallelogram in shape, is clamped at 200 on the arc. The other mire is cut in a series of steps, each of which is calculated to represent one dioptre of corneal refraction, or a power equal to a cyl- inder of one dioptre. This stepped mire is arranged to slide upon the graduated arc. When properly adjusted, the images of the mires, doubled by the prism in the telescope, are seen reflected on the cornea of the observed eye, as shown in (Fig. 2). Fig. 2. A large disk, upon the periphery of which degrees are painted in inverted charac- ters, numbered from o° to 1800, stands just behind the arc which carries the mires. These numerals appear erect and in correct position when reversed by reflection on the cornea, and viewed through the inverting telescope. The special purpose of the large disk is to enable the observer to read the degrees of the meridians of greatest and least curvature directly from the reflection on the cornea. To this end each mire is furnished with a short white pointer. The lar°-e disk and pointers had no place in the original model of the Ophthal- mometer, the meridians of greatest and least curvature being read from a stationary graduated circle facing the observer, just back of the graduated arc. An index mov- ing on this circle gave the position of the axes. In some modern instruments this same mechanism has been adopted. PUBLISHED BY FERGUSON 4 WESTON. L59J HOW TO VERIFY THE INSTRUMENT. To students beginning the use of the Ophthalmometer, as well as to those who wish to verify the exactness of their instrument, the artificial astigmatic cornea will be of great service. This cornea is made of polished metal and mounted on a small disk, so as to rotate within a graduated circle. An arrow engraved on the movable disk points out the meridian of greatest curvature. The known value of the astigmatism of the artificial cornea affords the means of controlling the accuracy of the measurement. The use of the Ophthalmometer requires a close attention to detail, and a rapid execution of several operations which can only be acquired by practice. The first requisite is a good light, which must be behind the patient, so that the disk and mires are fully illuminated. The examiner should first adjust the telescope by rotating the ocular until the cross-lines are brought clearly into view. The ocular should then be turned slowly to the left, so as to draw it out to the greatest extent compatible with seeing the cross lines sharply defined. The telescope should be turned so that the long pointer is below and at zero. The patient is now seated before the instrument in an easy position, with his chin resting on the chin rest, and his forehead pressed against the forehead rest. His eyes should be widely open and exactly on the same level. This position is indispensable for ascertaining the precise axes of the principal meridians. One eye is now covered by the small shade, and the observer sights along the telescope through the notch above it at the patient's eyebrow. Then sighting through the tube, he moves the instrument forward or backward, and raises or lowers it by the thumb screw until the eye is brought into the field of the telescope, and a distinct image of the mires and disk is seen on the cornea. We now see pictured upon the cornea of the observed eye twro overlap- ping images of the large disk. In the oval space thus formed are seen the two mires, to which we confine our attention. The observer now slides the mire at his right along the arc until its reflection touches the reflection of the stationary mire, and notes whether the two guide-lines which bisect the mires are in a continuous line. When the eye under examination is astigmatic, we see that in certain positions of the arc these guide-lines do not lie in a continuous line. To bring them into line we rotate the telescope so that the long pointer moves from o° toward 1350. If the lines do not become continuous when 1350 is reached, the rotation proceeds no further in this di- rection, but the pointer is turned back to o°, and then toward 45°, but never beyond 450. With regular astigmatism the lines always become continuous within 450 of the zero. When we bring them into line, we move the slipped mire along the arc until its image is seen exactly in contact with that of the other mire (Fig. 2). PUBLISHED BY FERGUSON & WESTON. (60) The instrument should now be carefully focused, and in doing this it should be drawn as far away from .the observed eye as possible without impairing the definition of the mires. The double image of the disk, which lies in a plane a little behind the mires, will then appear slightly blurred. Having noted the angle marked by the long pointer, we next rotate the telescope with its arc 900. In executing this movement, it is generally best to rotate from left to right, so that the stepped mire can always be reached through one of the openings in the disk. If the images of the mires now overlap (Fig. 3) for example, two steps in this second position, with the long pointer at 8o°, there is astigmatism of two dioptres with the rule, and is recorded 2.00 D. cyl. ax. 8o°, as (Fig. 3). Fig. 3. If on the other hand, the images of the mires in the second position, instead of overlapping, are seen to recede from each other, there is astigmatism against the rule. In this case, in order to ascertain the exact number of steps or dioptres to which the separation of the mires is equivalent, they are approximated by moving the sliding mire until the reflections touch, and the telescope is then rotated back to the- primary position. The mires will now overlap and the amount of astigmatism can be read off just as in astigmatism with the rule. The upper surface of the arc carrying the mires is graduated to show dioptres of refraction. It does not measure the Hypermetropia or Myopia of the eye, but indi- cates the corneal curvature. The total refraction of at least one corneal meridian should be given, and preferably, the one of least refraction. The rectangular mire being clamped at 200, we obtain the required value in dioptres for the meridian for which the arc is set by adding 200 to the reading of the stepped mire. For example, if we find in the right eye one dioptre of astigmatism with the rule, at 750, and the right hand mire at 230 on the arc, the refraction may be recorded O. D. 43.00 D = D. cyl. ax. 75 with the rule. If so desired, the astigmatism can be read from the graduated arc, by bringing the images of the mires into exact contact in both principal meridians. At times the Ophthalmometer indicates that the principal meridians of the cornea are not at right angles to each other. Thus one meridian may be at 1800 and the other at 8o°, in this case, both numbers should be noted, and where there is Hyper- metropia the axis of the cylinder will be 8o°, and when there is Myopia, 1800. CLINICAL VALUE. The Ophthalmometer measures only the astigmatism of the anterior surface of the cornea and this differs from the total subjective astigmatism of the eye in so far as the former is affected by a, astigmatism of the posterior surface of the cornea, and b, lental astigmatism. Moreover, the Ophthalmometer reading assumes for the correcting cylin- PUBLISHED BY FERGUSON 4 WESTON. (61) drical glass a position in actual contact with the cornea, and in the higher grades of astigmatism, the distance from the cornea, at which the glass is worn, must be taken into account. According to Bull and Cibret, the value of the astigmatism of the posterior sur- face of the cornea is —0.75 D. In fact, when we measure an eye, which, even under a mydriatic shows no subjective astigmatism, the Ophthalmometer usually records a corneal astigmatism of 0.50 D. too. 75 according to the rule. On the other hand, when the instrument shows no astigmatism of the anterior surface of the cornea, there is generally found a subjective astigmatism of o. 75 D. or 0.50 D. against the rule. The normal emetropic eye, therefore, seems to show a slight astigmatism with the rule, of the anterior surface of the cornea, and this is neutralized by an internal astigmatism of the same grade against the rule. Numerous observations, however, have shown that this internal astigmatism, while on the average equal to —0.75 D., may be either greater or less than this by one dioptre. In this case, we have probably another factor, viz : astigmatism of the lens. Of lental astigmatism, like that of the posterior surface of the cornea, we know little or nothing, since we only assume it to be present when we find a marked differ- ence between the total subjective astigmatism and that of the anterior surface of the cornea. It is not possible, therefore, at the present time, to lay down any empirical formula by means of which the total astigmatism may be calculated from measurements of the corneal curvature as made by the Ophthalmometer. In many cases the total astigma- tism differs but little from the corneal astigmatism, while in astigmatism against the rule and in high grades of direct astigmatism (over 3.00 D.) the total astigmatism is gen- erally higher than that indicated by the Ophthalmometer. (62) Interval of o. 12 D. Number of Lens in Dioptrics Interval of 0.25 D. Interval of 0.50 D. Interval of 1 D. Interval of 2 D. Focal Distance in Millimetres Focal Distance in Inches Nearest Corresponding Lens iu Old System O. 12 8000 314.96 O.25 4000 157-48 144 °-37 2666 104.99 96 0.50 2000 78-74 72 0.62 1600 62.99 60 °-75 .*333 52-5 48 0.87 1143 44.99. 42 1.00 1000 39-37 40 1.12 888 34-99 3^ J-25 800 3i-5 3° I.50 666 26.22 26 !-75 57i 22.48 22 2.00 500 19.69 20 2.25 444 -17.48 18 2.50 400 15-75 16 2.75 363 I4-31 15 or 14 3.00 333 13.12 J3 3-25 308 12.11 12 3-5° 285 11.25 11 3-75 266 10.49 4.00 250 9.84 10 4-25 235 9. 26 9 4-5° 222 8-74 4-75 210 8.29 5.00 200 7.87 8 5-5o 182 7.16 7 6.00 166 6-54 6.50 154 6.06 6 7.00 U3 5-63 7-5° J33 5-25 8.00 125 4.92 5 9.00 111 4-37 4-5 10.00 100 3-94 4 11.00 91 3-58 3-5° 12 83 3-27 3-25 r3 77 3-03 3 M 7i 2.8 2-75 15 66 2.64 16 62 2.44 2-5 17 59 2.32 18 55 2.17 2.2.5 20 50 1.97 2 22 45 1.79 i-75 In the old system the lenses are ground with a radius of curvature in Paris inches. The focal length is almost exactly the same number of English inches as the radius of curvature is of French inches. PUBLISHED BY FERGUSON 4 WESTON. (63) ; A!'-J: J ;-■; *:. '-: -|"V> Wm-MM #M m mmm, mmm m Umti mm, m mm Wim titfiiP