THE USE A N I) ABUSE OF1 8 P E C TACLESs ZE3~5T CHAS. M. CARLETON, M. D. THE USE AND ABUSE OF SPECTACLES. THE ANNUAL DISSERTATION READ BEFORE THE CONVENTION OF THE jVUttital §or ictii, AT HARTFORD, MAY 27, 1869. By CHARLES M. CARLETON, M. D. OF NORWICH. [Reprinted from the Proceedings of the Society.] NEW HAVEN: PEINTED BY TUTTLE, MOEEHOUSE & TAYLOK. 1869. ERRATA. Page 8, line 16, for opthalmometre read ophthalmometre. “ 15, line 6, for P2 read presbyopia. “ “ lines 29 and 36, also page 16, line 6, for opthalmoscope read oph- thalmoscope. Page 20, line 35, for covergent read convergent. THE USE AND ABUSE OF SPECTACLES. There are few subjects connected with the practice of medi- cine and surgery more important or less understood, by the mass of the profession, than the selection of spectacles. Our knowl- edge of the affections of the eye, arising from the anomalies of re- fraction and accommodation, has been greatly increased during the last few years. Many diseases which were formerly considered in- curable, are now found to be perfectly amenable to treatment. This change is due chiefly to the researches of Helmholtz, Yon Graefe, Donders, Bowman, Liebreich, Wecker, and a few others. Their zeal has been untiring, and the world, as well as the profes- sion, owes them an endless debt of gratitude. I will cite one class of cases familiar to all; that of over-worked students. How frequently do they consult us, in regard to eyes apparently healthy, complaining that they can read only for a few minutes at a time without everything becoming blurred. “ The letters dance before their eyes,’’ and, if they persist in their work, an intolerable nervous pain through the orbits and temples obliges them to close their eyes and rest. They anxiously ask if they are in danger of becoming blind. This question was, formerly, very frequently answered in the affirmative. The patient was con- demned to wear green glasses or a shade, to go out of doors only after sundown, and on no account allowed to use his eyes for read- ing or any fine work for a period of six months or a year. The disease was variously called Irritability of the Retina, Retinitis, Choroiditis, Incipient Amaurosis, &c., &c. Donders has demon- strated beyond a question that this troublesome affection (Asthen- opia) is, in nearly all cases, due to Ilypermetropia and easily cured by the proper use of glasses, without loss of time to the patient while under treatment. 4 Spectacles should in all cases be selected by an oculist, or at least by a physician possessed of specific knowledge on the subject. A case of trial glasses, consisting of a complete set of convex and concave spherical and cylindrical lenses, prisms, tinted glasses, and stenopaic apparatus, is an indispensable requisite. No jeweler or mere optician should ever be entrusted with so delicate and im- portant a task. An injudicious selection frequently ruins eyes that might by proper treatment be rendered useful. To make a proper selection of glasses in any and all cases, it is necessary that the practitioner should thoroughly understand: 1. The properties of optical lenses. 2. The eye as an optical instrument. 3. The ano- malies of refraction and accommodation of the eye. Let us con- sider these in their numerical order. TUB PROPERTIES OP OPTICAL LENSES. The lenses in most general use as aids to vision are the spherical biconvex and biconcave, with the radii of curvature of the two surfaces equal. Rays of light passing through the centre of either of these lenses are not deflected. Parallel rays, emanating from an object at an infinite distance* falling upon a biconvex lens are united at a certain point behind the lens (A, Fig. 1,) which point is called the focal point or principal focus. Fig. 1. The focal distance of simple biconvex and biconcave lenses is nearly equal to the radius of curvature of the lens. The fact that convex and concave lenses of equal power exactly neutralize each other, furnishes us with an easy method of determining the power of any given lens. Thus if the lens be convex, we neutralize it with a concave lens from the trial case, and the number of the concave glass will give that of the convex. * An object is considered, by oculists, to be at an infinite distance when the rays emanating from it fall upon the eye so nearly parallel that the divergence is imperceptible, which obtains at a distance of about eighteen or twenty feet. 5 Convergent rays are brought to a focus at a point (A. Fig. 2,) between the principal focus and the lens. Fig. 2. Divergent rays are brought to a focus at a greater distance from the lens than the principal focus, unless they emanate from an ob- ject situated at the same or a less distance from the lens than the principal focus, when the lens will only have the power of render- ing them parallel or less divergent. (See Fig. 2.) Rays em- anating from an object at twice the focal distance from the lens, are brought to a focus at the same distance on the other side of the lens. Rays emanating from an object situated at B (Fig. 1,) are brought to a focus at C. If the position of the object be changed to C, rays emanating from it will be brought to a focus at B. Any two points thus dependent upon each other, are called conjugate foci. Biconcave lenses render parallel rays divergent. On leaving the lens they assume a direction as if they emanated from a point nearer the lens. Parallel rays passing through a biconcave lens of six inch focus appear to emanate from a point six inches in front of the lens. This point, (Fig. 3, a) where the deflected rays, if prolonged backwards through the lens, would intersect each other, is called the negative virtual focus. It is an imagi- nary one being situated on the same side of the lens as the object. Fig. 3. Divergent rays, emanating from an object at a finite distance, are rendered more divergent, and have their imaginary focus nearer the lens than the principal focus. 6 Beside the glasses described above we have the concavo-convex or positive meniscus, convexo-concave or negative meniscus, the cylindrical convex and concave lenses, prisms, and tinted glasses- The plano-convex and plano-concave should never be used for spec- tacles, as, for an equal degree of power, they have more aberration than biconvex and biconcave glasses. The menisci (periscopic glasses) have the least aberration for very oblique rays. There- fore objects viewed obliquely through them are less distorted than when seen under similar circumstances through any other glass. For this reason they are, in most cases, to be preferred to the sim- ple biconvex and biconcave lenses, except where very high pow- er is required, when their greater weight is a disadvantage. All the lenses described thus far are spherical lenses ; i. e. they are segments of spheres and refract equally all rays which fall upon them in all the planes of the segment. Besides these it is fre- quently necessary to have recourse to cylindrical lenses; i. e. lenses which are segments of a cylinder and which refract those rays strongest, which fall upon them in a plane at right angles to the axis of cylindrical curvature. The refraction grows less and less towards the cylindrical axis, at which point none occurs. In certain forms of impaired vision, prisms are indicated rather than lenses. It is sometimes necessary to combine the two. The action of the prism is to refract all rays passing through it towards its base. Tinted glasses are used for modifying the light, in cases where ordinary daylight cannot be endured. Green glasses were form- erly recommended on the supposition that the red rays of the so- lar spectrum were those which irritated the retina. It is now a settled fact that not the red but the orange rays have this effect. Blue excludes the orange rays, and is, therefore, the proper color to be employed. Furthermore as blue occupies a more excentric position in the solar spectrum, it makes less impression upon the retina. Smoke-glasses should never be used, as they diminish the whole volume of light, and thereby render the image less distinct. Goggles and eye-protectors are much too frequently used. The former over heat the eye and should never be worn except when, soon after a severe operation, the eye is inflamed and peculiarly susceptible to cold. The latter possess the same disadvantage, but in a less degree. In most cases where anything of this character is required, the light or medium blue curved eye protectors (coquettes) are the best. It is sometimes necessary to combine the blue tint with a refracting power. If the power required be low, the 7 lens may be cut from tinted glass, but in high powers the varying thickness of the lens causes a considerable difference in the tint in the centre and at the edges of the glass, which confuses the vision. In such cases Mr. Laurence, of London, recommends the joining of very thin plates of tinted glass, by means of Canada balsam, to the backs of plano-concave and convex lenses. THE EYE AS AX OPTICAL INSTRUMENT. The eye may be regarded as a camera obscura with a concave screen, the retina, upon which is formed a diminished and inverted image of the object. The dioptric system of the eye consists of the cornea, aqueous humor, crystalline lens and vitreous humor; conjointly they act as a biconvex lens, and bring parallel rays, in the normal eye, to a focus upon the retina. The cornea and aqueous humor may be considered as presenting only one refracting surface, on account of the parallelism of the two surfaces of the cornea, and the fact that the two media possess very nearly the same refractive power. The re- fraction of the vitreous humor is nearly the same as that of the aqueous. The lens is by far the most powerful refracting medium in the eye, without it parallel rays would not be brought to a focus upon the retina but behind it. The optic axis is an imaginary line (a b Fig. 4) drawn from the centre of the cornea to a point lying midway between the optical disc and the macula lutea. Fig. 4. The visual line is an imaginary line drawn from the object to the macula lutea (Fig. 4 c, ') thus found, bear the same relation to each other as the real points r and p, as their distances are diminished in an equal ratio. The range of accommodation is, therefore, easily found by the formula If r' lies at 6 inches and p' at J APR 1 1111 3 inches, -r= = -. ’ A 3 6 6 Besides the absolute range of accommodation described above, which exists when each eye is examined separately, it is neceessary to distinguish two other ranges, the binocular and the relative. The binocular range is suificiently explained by its name. The relative range is the degree of accommodation which exists while the convergence of the visual lines remains in a fixed state. ANOMALIES OF REFRACTION AND ACCOMMODATION. By refraction of the eye is understood its refraction while at rest, independent of muscular action. Its degree is ascertained to a nicety by an examination made while the muscles of accommo- dation are paralyzed. In a state of rest the eye is adjusted for its farthest point. Accommodation is the voluntary action whereby the eye be- comes adjusted to a nearer point than when at rest. “Refraction is dependent on the anatomical condition of the component parts of the eye; accommodation, on the contrary, de- pends upon the physiological action of muscles.”—Donders. The refraction of the eye is considered normal, when, the eye being in a state of i*est, parallel rays are united exactly on the anterior surface of the layer of rods and cones of the retina. Such an eye is called emmetropic. Its far point lies at infinite distance. (Fig. 7). Fig. 7. The eye may deviate from the emmetropic condition in two ways. Its principal focus may lie in front of the retina, or behind it. In the former case myopia exists, and divergent rays only will be 13 united upon theN retina (Fig. 8). Parallel rays are brought to a focus in front of the retina. Circles of diffusion {d d) are formed upon the retina, and vision is consequently indistinct. Fig. 8. In the latter case hypermetropia exists and only convergent rays are brought to a focus upon the retina (Fig. 9). Parallel rays are brought to a focus behind the retina, and circles of diffusion (d d) are formed upon the retina. Fig. 9. It will be seen from this that myopia and hypermetropia are exact- ly opposite conditions of the eye. The refraction may vary in the different meridians of the same eye. It may be emmetropic in the horizontal meridian, and myo- pic or hypermetropic in the vertical, and vice versa, or differences in the degree of the same anomaly may exist. This asymmetry is termed astigmatism. The anomalies of accommodation are dependent upon the follow- ing conditionsLoss of the lens, weakness, paralysis, or spasm of the ciliary muscles. Deficiency in the range of accommodation dependent upon either of the before cited causes, if extensive, should, as far as pos- sible, be remedied by glasses. Presbyopia is the diminution of the power of accommodation dependent on advanced life. In it the near point recedes farther and farther from the eye with increasing years. In pure presbyo- pia the far point is unaffected. Quite late in life, however, a slight degree of hypermetropia is acquired from the flattening of the lens. 14 Presbyopia is chiefly dependent upon changes in the structure of the lens, which in old age becomes more firm, resisting in a greater or less degree the action of the ciliary muscle. The retrocession of the near point commences as early as the tenth year, but is not usually recognized until about the fortieth year. An eye is considered presbyopic as soon as the near point has receded to a greater distance from the eye than 8 inches. Presbyopia may, therefore, co-exist with myopia of less than -J, but it manifests itself later in life. Presbyopia co-exists with hypermet- ropia in cases where, the hypermetropia having been corrected by means of glasses, the near point lies at a greater distance than 8 inches. In pure presbyopia the normal acuteness of vision and normal range of accommodation may in all cases be restored by means of proper convex glasses. The range of accommodation in presbyo- pia is determined by the same method as in emmetropia. The opinion is very general that the use of convex glasses should be deferred as long as possible. This is a very grave error, and I am forced to believe that pride often contributes largely to the en- tertainment of this opinion. The overtaxing of the accommoda- tion in the endeavor to see small objects hastens the progress of the affection, and at the same time wearies the patient unnecessa- rily. There can be no question of the propriety of furnishing pa- tients with suitable glasses as soon as they are in the slightest de- gree annoyed or inconvenienced by presbyopia. We often see cases where at 50, 60, and occasionally even at 70 yeai’s of age, a person is able to read at a distance of 10 or 12 inches without the aid of glasses. Such people always consider themselves lucky excep- tions to the general rule, and usually attribute it to their good judgment in the management of their eyes, more particularly in never having indulged in the use of glasses. In point of fact such people owe their immunity from the use of glasses to being slight- ly myopic, as may be proved by requesting them to read No. xx Snellen at 20 feet. They will not be able to do so except with the assistance of concave glasses of to Myopia may always be diagnosed in cases where spectacles have not been required for dis- tinct vision of near objects at or soon after the fortieth year. Don- ders, after treating Of this subject, says: “ The more I investigate the subject, the more fully I am convinced that at a given time of life the range of accommodation is an almost law-determined quan- tity.” I must differ with him so far as to claim that presbyopia 15 progresses much more rapidly when the eye is overtasked by poor artificial light. The degree of presbyopia is easily determined after having once decided upon a definite distance (8 inches) as its commencing point. Thus if the presbyopic near point (p2) lies 16 inches P2=£—an(l ought, ceteris paribus, to be corrected by con. vex glasses of 16 inch focus. In practice, however, these glasses are found to be somewhat too strong, for, owing to the increased con- vergence of the optic axes, they will bring the near point closer to the eye than 8 inches. The weakest glasses that will enable the pa* tientto read, easily, No. 1 Snellen at 12 inches are usually suffi* cient, in cases where no hypermetropia exists, and even these may not be tolerated at first. If the range of accommodation be good p may usually be brought to 8 inches, but if it be much diminished, p must not lie nearer than 10 or 12 inches. MYOPIA. In myopia the far point is more or less approximated to the eye. Parallel rays are not united upon the retina but in front of it, con- sequently each pencil of rays forms a circle of diffusion on the retina (Fig. 8, d d), and distant vision is rendered indistinct. Ob- jects situated at a definite finite distance only will be distinctly seen* It was formerly supposed that myopia was dependent upon an in- crease in the convexity of the cornea. This is now known to be erroneous. Indeed it has been found that, as a rule, the cornea is less convex in myopic than in emmetropic eyes. The most frequent cause of myopia is an abnormal increase in the length of the eyeball in its antero-posterior axis. It. is often attended with posterior staphyloma, which should always be sus- pected and sought for, by means of the binocular opthalmoscope, in cases where the existing myopia exceeds as its presence is of the greatest moment to the patient. The manner of finding the far point has been already explained. It only remains necessary for me now to briefly explain the method of ascertaining the existence and approximate degree of myopia, in cases where the statements of the patient are not trustworthy. This is accomplished by means of the opthalmoscope. On examining a myopic eye in the erect image, (with the mirror merely) if the observer fixes his attention upon the optic disc or retinal vessels and moves his head in any direction the image will appear to move in the opposite direction. In emmetropia the image remains fixed, and in hypermetropia it moves in the same direction as the observer’s head. To obtain an 16 erect image of a myopic eye it is necessary to place a concave lens behind the mirror and bring it within the focal distance of the ob- served eye. The lens renders the convergent rays from the myo- pic eye parallel, and the fundus of the eye is seen as from an infi- nite distance. The opthalmoscope reveals other peculiarities in the myopic eye, but the limits of this dissertation will not allow of their consider- ation. Myopia is sometimes confounded with amblyopia (weakness of vision) as persons affected with amblyopia habitually bring small objects near to the eye in order to obtain larger and more clearly defined retinal images. The diagnosis between the two is easy. In amblyopia the patient is unable to distinguish very small ob- jects at any distance. Moreover, vision is not improved by con- cave glasses as is the case in myopia, but rendered less distinct, as the glasses diminish the size of the retinal images. Myopia is a disease peculiar to civilization. It is most frequent in the higher and literary circles, and rarely met with in sailors or agriculturists. The principal cause of the disease is the excess- ive use of the eyes for near objects, conjoined with insufficient light, and a stooping posture, which is so commonly assumed by students. The same causes make the disease progressive. In look- ing at near objects the optic axes are strongly converged. This causes increased pressure upon the eye, through the medium of the recti muscles, which, if long continued, results in congestion of its inner tunics and increased pressure of the ocular fluids. Bulging of the posterior pole, (that pai*t unlike the rest of the eye receiving no support from the muscles) and consequent lengthening of the antero-posterior axis of the eye, is the final result of this pressure. It is also probable that after the eye has been so constantly accom- modated for near objects, that the ciliary muscle may become more or less permanently contracted, and that the lens losing a part of its elasticity, remains abnormally convex. Myopia is hereditary to a great extent, and this hereditary principle is accumulative. The popular belief that myopia diminishes with increasing age, and more surely where the use of glasses is not indulged in, is er- roneous. We have already shown that myopia consists in the ap- proximation of r to the eye, and that the effect of age is to di- minish the range of accommodation by removing p farther from the eye. As the causes which give rise to myopia are equally fa- vorable to its farther development, by removing the principal of 17 these causes, (the convergence of the optic axes) we necessarily re- tard the progress of the disease. This end is accomplished by the judcious use of biconcave lenses or negative menisci, which render parallel rays divergent. In the selection of glasses for myopes, the weakest that will correct the myopia should be chosen. Too strong glasses are often productive of much injury. If r lies at 7 inches M—| and should, theoretically, be corrected by glasses of —f but, as the con- vergence of the optic axes prevents the eye from accommodating itself for its far point, the apparent myopia is greater than the real, and concave 7 is too strong. To ascertain the exact power required, give the patient concave 8 and request him to read No. XX at 20 feet. Let us suppose that he can do so, but that the letters appear indistinct. Concave 60 placed before the spectacles renders the ob- ject less distinct, while convex 60 placed before the spectacles ren" ders the letters clear and well defined. Convex 50 impairs the vis- ion. From this we know that the glass is yet too strong by To find the proper power we must deduct convex 60 from concave 8, thus : “7 nearly; remembering the rule to give the weakest glass that will correct the myopia, he is furnished with concave 9£, and we find that neither positive nor negative glasses placed be- fore it make any improvement. It is often necessary to furnish patients with glasses for reading at a distance of 18 inches or 2 feet. In cases wdiere the myopia is considerable or the accommodation poor, it is generally best to fur- nish them with glasses that bring r to that distance, rather than to wholly neutralize the myopia, which would greatly decrease the size of the retinal images. Suppose a patient requires concave 9 for distance, for objects at 18 inches he will require —i+TV—tV, and for reading at 12 inches he will require ——uV- When the myopia is slight, the range of accommodation good, and the eye otherwise healthy, neutralizing glasses may be worn both for distant and near objects. Donders thinks this is even desirable and that it greatly retards the progress of the disease, particularly in youth. He says: “ When persons with moderate degrees of myopia have in youth accustomed themselves to the use of neutralizing spec- tacles, the eyes are in all respects similar to emmetropic eyes, and the myopia is, under such circumstances, remarkably little progres- sive. I am acquainted with numerous examples of this, even among those of my friends who have passed their lives in study. 18 Glasses of tV adopted at seventeen years of age, are often still sufficient at forty-five years, both for seeing acutely at a distance and for ordinary close work. Not until the age at which emme- tropes need convex spectacles, and often even some years later, do the neutralizing spectacles become rather too strong for close work, and it is desirable to procure somewhat weaker ones, which, with the narrower pupil peculiar to that time of life, are now nearly sufficient for distance also. In order to obtain all the advantages of concave glasses the myope must begin early with them. If the myopia amounts only to a fourth or a third of the range of accommodation, we may immediately wholly neutralize it. If it amounts to more we must usually begin with weaker glasses, and replace them at the end of six months with stronger ones.”* To test the range of accommodation in a myope, glasses should be given him which exactly neutralize the myopia (No. XX being dis- tinguished at 20 feet), with these r will lie at qo . Then ascertain how near he can read No. 1 with ease. Let us suppose that he reads No. 1 at 5 inches by the formula JH we obtain It is important in myopia that each eye be examined separately as well as the two together, for there occasionally exists a marked difference in the degree of myopia in the two eyes, which may de- mand glasses of different foci. In such cases, if the difference be considerable, it is not usually advisable to furnish each eye with the glass that exactly neutralizes its myopia, for as a rule such specta- cles render vision confused, on account of the difference in the size of the two retinal images. In a few cases the patient, after a little practice, is enabled to see clearly with them, and then their use is admissable and even advantageous, as it enables him to estimate distances, f If the patient be annoyed by the difference in the size of the two images, each eye must be furnished with the glass appro- priate for the least myopic eye; or the difference of refraction in the two may be partially neutralized by furnishing the least affected eye with its appropriate glass, and the other with one of a somewhat higher power. Thus if the myopia of one eye equals and the other £ the glasses may be respectively — tV and — |- or - The circles of diffusion are thereby diminished, and a cer- tain degree of binocular vision secured. It has been proposed, in * Donders p. 421. f It is a singular fact, that when seen by one eye only, a die from which medals are struck, appears in relief like the perfected medal. 19 such cases, when the sight of the two eyes is equally good, to fur- nish each eye with the glass which lies midway between the two degrees of myopia. Thus, myopia of one eye being and of the other it would be advised to prescribe — £ for both eyes, but such spectacles wrnuld evidently be of no use to either eye. HYPERMETEOPIA. Hypermetropia is the exact opposite of myopia. In it the re- fractive power of the eye is too low, or the optic axis too short, therefore, when the eye is in a state of rest parallel rays are united behind the retina and circles of diffusion are formed upon the re- tina (Fig. 9, ). Compound hypermetropic astigmatism (IIAh) requires convex spherico-cylindrical glasses, thus: HAh composed of H^b+Ahy is corrected by 2bsOysC. Mixed astigmatism (Amh and Ahm) requires bi-cylindrical glasses, having one convex and one concave surface, the axes of the two forming a right angle thus: Amh composed of Myb-f-Hgb, is corrected by and — T\jc with their axes at right angles, (written s\c And Ahm composed of by Tbc f The foregoing examples explain the method of correcting, at once, both the astigmatism and ametropia. In other words, of converting the eye into an em- metropic one. This is not always desirable, for whilst the correc- tion of astigmatism always improves vision, the use of very strong glasses interferes with the combined action of the ciliary and in- ternal recti muscles in the effort of accommodation as well as greatly to affect the size of the retinal images. If it be desired to correct the astigmatism and retain a definite degree of myopia it is simply necessary to deduct the desired degree of myopia from the reflective power of the two principal meridians, and then cor- rect the remaining ametropia. Examples: E exists in the principal h meridian, and M=Tb in the v. We wish to obtain De- * As the lens is placed about | inch in front of the nodal point of the eye, it is necessary, except where the degree of aberration is very slight, to deduct $ inch from the focal distance of concave lenses, and add i inch to that of convex lenses. f For the sake of simplicity the correction proceeding from the distance be- tween the lens and the nodal point of the eye will be omitted in the examples of compound and mixed astigmatism. 27 ducting MjV from the refraction of each meridian, we have in h, E—HjV) and in v, Mxb—This is corrected by 'Ac T—sV- In K M=2V, in v, We desire Deducting we obtain in h, — in y, MXV— M^V; corrected by In h, in v, II=XV; we desire MXV, deducting MXV, we obtain in h, H£ —in y, H^— MX corrected by In y, M=XV in h, II—2b, we de- sire M=5V> "by deduction we obtain in y, MXV—in h, H2Vr—MXV—HtV which is corrected by x\yC f ~21