BERKELEY LIBRARY UNIVEBSHT Of CALIPOtMIA /D6: THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA GIVEN WITH LOVE TO THE OPTOMETRY LIBRARY BY MONROE I. HIRSCH, O.D., Ph.D. ESSENTIALS REFRACTION Sp Thomas G. Atkinson, M. D. SS!l CHICAGO G. P. ENGELHARD al witli pathological dis- eases of the eye, it has been thought wise to include a chapter briefly describing those ocular diseases wliieli are intimately connected with disturbances of vision, and for whieli, therefor.', the refractionist is frequently first consulted. Thus warned, the refractionist can, by a very reasonable exercise of care, readily detect and identify these conditions, and promptly refer tluin to the oculist for appropriate treatment. Special attention is given to the use of the oi.htlial- moscope and retinoscope. These instniments have for many years en'oyed a deserved popularity among En- ropoati refractionists in tlie estimation and correction of refractional errors, and the author believes they are destined to attain equally general favor in this country. The illustrations are from a series of entirely new and original drawings, designed with a view of eluci- dating those points which the author's experience has demonstrated to be essential points, most happily demonstrable by means of diagrams, but which, unfor- tunately, are not as a rule made the subjects of illus- trations in text books of refraction. The present edition represents practically an entire rewriting of the book, and the addition of sections on optical principles and hygiene of the eye, thus making a complete manual on all that pertains to the art and science of refraction. Chicago, January 2, 1914. CONTENTS. Chapter I Light 9 Chapter II Optics — Visibility ID Chapter 11 1 Tii e Eye 35 Chapter lY Eefkactiox of the Eye. . . 47 Chapter Y Lenses 57 Chapter YI Accommodation and Convergence G3 Chapter Yll Retinoscopy 77 Chapter YIII Ophthalmoscopy 97 Chapter IX Correction of Hyperme- TROPIA 113 Chapter X Correction of Myopia 121 Chapter XI Correction of Astigma- tism 127 Chai)ti'r XIT Practical Instructions.. 141 Clia})ter XI 1 1 Strabismus and Imbal- ance 153 Chapter XTY Asthenopia 1(>9 Chapter XY Diseases of the Eye Con- nected WITH Disturb- ance OF YisioN 179 Chapter XYF ErrTixo the Classes 191 Chanter X \' 1 1 11 vcue XE of tue Eye 215 CHAPTER I. T.IOHT. Nature and Source of Light. Lit/hi is a I'oi'iii (•[' j»liysical i'iu'i\av, which, at-t- in^- upon the retina ol' the ('>i\ produces in the brain the sensation of vision. The same word, Light, is used in physiology to designate the sensation thus produced. This, however, is not the optical significance of the term. Genekatiox of Light. — The commonest modes of generation of light are (1) the chemical process of combustion, and (vM the nioh'cular activity of friction. SouKCKS. — The chief source ot light is the sun, in which probably chemical and molecuhir activ- ity both take i^ai't. The light generated by the sun is called Natural light. Other common sources of light are lamps, candles, gas (cond)us- tion), and latterly electric light (friction). Light thus generated is called Artiiicial light. XATrHK.-^The exact nature of light, like that of other forms of physical energy, is not as yet understood. We are able to recognize and study it onlv tlirough its effects upon the material uu^vlia which it intiuences, or whidi influence it. hi a general way it is understood to consist of an oscillatory vil)ration of the panicles of ether. TuANSA.nssiox. — The utilization of light re- quires its transmission from one point to another 10 REFRACTIOX in si:»ace through suitaljle media. This is accom- plished bv the vi1)rations communicating them- selves to adjoining particles of th(- >ame medium or to those of another medium. It sliould be understood, of course, that actually light vibrations are never communicated from one medium to another, since light vibrations are per- tinent only to ether. What actually happens is that the vibrations are communicated from the? ether in the interspaces of one medium to the ether in the interspaces of another medium. But for working convenience we say that they are communicated from one medium to another. Transparency and Opacity. — Media or bodies which permit of this transmission of light vibrations through their substances are called transparent. Those which do not are called opaque. Transparent and opaque are, of course, relative terms, simply denoting the comparative capacity of different media for transmitting light vibra- tions. Probably no form of matter is either ab- solutely transparent or absolutely opaque. Every medium of greater density is more or less opaque as compared witli one of less density. Tliat is to say, the light vibrations of a rarer medium, are never completely communicated to a denser medium, some of them ])eing turned back by the denser into tlie rarer medium. . Transmission — Absorption — Eeflectiox. — Of those vibrations which are communicated to a LIGHT 11 modiniii, ^nmo pass throiiiTli it and arc re-com- imiiiii-att'd to anotlier luediuin; others exhaust themselves upon the substance of the medium and are transfoi'ined into other forms of energy. The f(U"mc'r are said to be tninsniifh'd, the hitter ah- sorhciJ. \)\ the medium. Tliose which are turned back by a denser medium into a rarer medium arc said to be reflected. Every medium — i. e., every form of matter — absorbs light vibrations to a more or less extent. Dynamics of Light. The dynamics of light inchule a consideration of the modes of franstnisslon , relocHy. force, and effects upon matter, of its vibrations. Method of Transmission. — Light vi])rations al-e supposed to travel in the form of waves; that is to say, the path of the transmitted vii)ration is marked by an alternate expansion and contrac- tion, or, to speak more correctly, an alternate rari- fication and condensation, of the medium, ulti- niiilcly due. of course, to the alternate repulsion and attraction of its atoms. Velocity. — Light vibrations are estimated to travel through space — i. e., through luminous ether — at a speed of 186,300 miles per second. This, however, is an average estimate, since even ether offers some resistance to their ])assage, and therefore their velocity progressively decreasres during transmission. The velocity of light vibrations varies directly 12 REFRACTION as the force of their propulsion (impetus) and inv.crsoly as the density of the medium. Force. — Xo satisfactory basis has been deter- mined for the computation of the force or impe- tus of light vibrations. Hence in practical op- tics eacli source of liglit is taken as an independ- ent standard, and the second factor, viz., the Illustrating- the different oscillatory Avave lengths. c()m})arative densities of the media through which the vibrations pass, is the only factor regarded in tlie determination of their relative velocity. The denser the medium the more slowly the vi- brations travel. OsciLLAToia' Velocity. — In addition to the ve- locity of their transmission, light vibrations have a lateral or oscillatory velocity, dependent upon tlic size of tlie vibratoiT wave — i. e., upon the range of repulsion and attraction communicated to tlie particles of ether. The larger the wave — i. e., the greater the range of this repulsion and attraction — tlu* less the oscillatory velocity. Effects of Light Upon Matter. Outride of the already mentioned fact that light \il)rations propagate themselves through different forms of matter at varying velocities and under LIGHT i:j van'ing ooiulition.^, iiotliiiig is known of any pnrol}' (Ivnaniic effects produced by them upon objective media wliivli is of any parlicular \aliie in a study of optics. 1Mie only effects which con- cern optics are those which light vibrations pro- duce upon the retina and are sul)jectively inter- preted by the eye. IntfIxsity. — The relative transmission-velocity of liglit vil)rations produces an effect upon the retina which the l)i'ain interprets as comparative intensity of light. There are, as will presently be seen, other conditions of the ocular apparatus itself which intluence intensity, but so far as the vibrations themselves are concerned, the more ra[)idly thev ai'c traveling when they strike the retina the more intense the sensation of light pro- duced, it is for this reason, among others, that IHustrating- the resultant straight path of tlie lig:ht vibrations, constituting the ray. light received from a near point appears more intense than that I'lom a fai' point. Tliis cor- responds to the loudness of sound. Color. — The relative oscillatory velocity — in other words, the relative wave-length — of light vibrations is responsible, through its effects upon the retina, for sensations of color. The more rapid -these oscillations — i. e.. the shorter the waves — • the higher the color. The shortest perceptible light-waves gives the sensation of violet; the long- 1 4 REFRACTION est, that of deep red. This corresponds to pitch in sound. Geometries of Light. Closely allied to the dynamics of light, ])ut technically distinct from them^ are its geometric relations, upon which the whole system of op- tics, so far as it relates to refraction, is based. Linear Propagatiox. — Foremost of these geo- metric postulates is the well-known axiom that light yibrations are propagated in a straight line, and cannot be made to trayel in any other kind of course. This is what makes it necessary for an object to be in our uninterrupted line of yision in order that we may see it. Rays. — For optical purposes, therefore, the yi- brations themselyes are not regarded as such. The imaginary straight lines in which they trayel are regarded as the units of light, and are called rays. A combination of rays, representing the passage of seyeral yibrations of ditferent wayc-lengths^ is called a penciL DiyERGEXCE OF Eays. — Kays of light leaying an object, whether reflected or generated by the object, are projected in a diyergent manner and in all ayailable directions, and doubtless continue to diyerge as long as they remain in the same medium. Infinite Rays. — At a certain distance from their origin, the angle of diyergence of those rays which come within our range of yision is so slight that it is impossible to show that they are not LIGHT 15 parallel, and for optical purposes they are then regarded as parallel. Experience has shown this distance to l)e six meters or over. Kays, there- fore, M'liicli oriuiiiale six meters or more from the Showing how the angle of reflection CBP is equal to the angle of incidence ABP. observer are said to come from infinity, are called infinite rcn/s^ and are regarded as parallel. Finite I^ays. — Rays which proceed from an object less than six meters from the observer are called finite rays, and are divergent. Reflection. — As previously stated, when rays of light strike the surface of a denser nuMlium not all of them are communicated to the new medium, some of them being turned back into the rarer medium. These are said to be reflected. The re- flecting power of a medium is proportionate to the smoothness of its surface. Angle of Incidence and Reflection. — The angle which a ray striking such a surface makes witli the pci'pondieular of tlio >urface is called the 16 REFRACTIOX angle of incidence. The angle which the same ray makes with the same perpendicular after re- flection is called the anole of reflection. Illustrating' how a ray AB, upon entering a denser medium, is bent toward the perpendicular, as BC, and iipon entering a rarer medium is bent away from the perpendicular, as CD. Laavs of Ekflection. — Eeflection takes place in accordance with two geometric laws, viz. : 1. The angle of incidence is equal to the angle of reflection. 2. The incident and reflected rays are hoth in the same plane, which is perpendicular to the reflecting surface. Eefractiox. — "When a ray of light passes from one medium into another of different density, if the surface of the medium into which it passes is perpendicular to the ])nth of the ray it con- tinues to travel in the same straight line. If, however, the surface of the receiving me- dium is not perpendicular to the ray, the latter, upon entering it. is hent or deflected from its LIGHT 17 course. This bending of a ray is called Refrac- tion. If it passes into a denser medium it is bent toward the perpendicular of the surface; if into a pf I I. Illustrates the index of refraction. AB represents the incident ray entering- the surface of the refracting medium; B C the refracted ray. XY is the sine of the ang-le XBY made by the incident ray with the per- pendicular PP'. X'Y' is the sine of the angle X'BY' made by the refracted ray with the same perpendicu- lar. The ratio between XY and X'Y' is the index of refraction of the two media. rarer medium it is bent away from that perpen- dicular. Index of Refraction. — Naturally, the rela- tion of the angle of incidence — i. e., the angle which a ray striking such a surface makes with the perpendicular — to the angle of refraction — 18 REFRACTION i. e., the angle which the same ray makes with the same perpendicular after refraction — is not uniform, as in the case of reflection, but varies with the comparative densities of the respective media. For optical purposes we estimate the degree of refraction by comparing the sine of the angle of incidence with the sine of the angle of refrac- tion, and the ratio between these two geometric quantities is called the index of refraction of one medium as compared with the other. For working convenience we regard air as the standard medium, and the ratio between the sines of the angles which a ray of light makes with the perpendicular before and after passing from air into a given medium is said to be the index of refraction of that medium. The index is plus or minus according as the ratio is in favor of or at the expense of the angle of refraction. Example : A ray of light passing from air into water and refracted by the water. The sine of the angle of incidence is 1.333 times greater than the sine of the angle of refraction. There- fore the index of refraction of water is said to be -f 1.333. N. B. — In optics, so far as they relate to re- fraction of eyes, we never have to do with any re- fracting medium of less density than air. Hence the index of refraction of those media which con- cern us is always plus, and no attention need be paid to minus indices. CHAPTER II. OPTICS— VISIBILITY. The visibility of an object is clue to (1) its capacity for reflecting part, but not all, of the rays of light which strike its surface, and (2) its ability to change the dynamic qualities of those rays which it reflects. An object which either transmits, or absorbs, or reflects all of the rays which it receives is not visible as an object. Showing- how (vision being- wholly due to reflected rays) an object which transmits all the rays and re- flects none of them is invisible to the eye. An object which perfectly transmits all of the rays which strike its surface is absolutely invis- ibio. An object which completely absorbs all of the rays which reach it is seen simply as an area of shadow. 20 REFRACTION An object which uniformly reflects all of the rays which strike its surface is seen sheerly as an area of light, similar in all respects to the source of its illumination. MIRROR Showing how an object like a mirror, which reflects all the rays is itself invisible but appears to the eye in all respects as the original source of light. Those substances which we usually regard as being quite transparent (air, crystal glass, water, etc.) perfectly transmit some of the rays and uniformly reflect others, so that we both "look through them'^ and also see them as an area of pure light. OPTICS — VISIBILITY 21 An object which is visible in detail absorbs part of the light rays which strike its surface and reflects others, according to the various form and character of its surface. Those rays which Showing- how an object which transmits some rays and reflects some is visible to the eye as an individual image. it reflects are changed in velocity, wave-length, etc., also in accordance with the varied surface of the object^ and effects are produced upon the retina corresponding to these changes. The net sum of these effects constitutes what is known as the retinal image, from which the brain judges of the identity of the object. The more nicely balanced the absorption and reflection of light, the more clearly the object is 22 REFRACTION seen. In ordinary sunlight there is too much reflection; this is why we see things in much clearer detail just after sun-down. Eeflected Image. — All retinal images are tliereforo reflected images. However, when the light is reflected directly from the object to the retina we say that we see the object itself. It is manifest that an object can be thus seen only when it is in an uninterrupted straight line with the retina. When the light from an object is inter- cepted by another surface, and by it reflected to the retina we speak of the retinal image being a reflected image. By this means an object may become visible which is not in the direct line of vision. Reflection of Light. A body with a highly and imiformly polished surface, which does not transmit light, reflects practically all of the rays which strike its sur- face. When the light reflected by such a surface comes from a source of pure light, it is seen, as already stated, as an area of pure light, similar to the source of illumination. AVhen, on the other hand, the light so reflected reaches the polished surface from another object, it is reflected in all respects in the same condi- tion as it was received from the original object, and makes precisely the same effect upon the retina as thougli roeeived by the eye from tlie OPTICS — VISIBILITY 23 original object. In other words, the mirror gives a' reflected image of the object. Projection. — Since light always travels in a straight line, the brain is only able to project light rays — that is, to refer them to an origin, in a straight line. Hence the image of an object seen in a mirror does not appear to the brain to APPARENT POSITION OF LIGHT Illustrating the apparent position of a reflected image. he located at the original object but on a straight line projected from the retina through the point at which the rays strike the mirror. But the brain judges of distance by the dy- namic qualities of the rays which reach the retina, and these have-not been changed by the process of reflection. 'Fherefore the distance at which the image a{)}X3ars to be located is the distance which tlio rays have actually traveled— namely. 24 REFRACTION the distance from the object to the mirror plus the distance from the mirror to the eye. The apparent location of the reflected image from a plane mirror, therefore, is in a straight Showing how parallel rays are focused by a con- cave mirror at a point F, midway between the surface S and the optical centre C. The distance SF is the focal length of the mirror. line from the eye through the reflection point on the mirror, as far beyond the mirror as the ob- ject is from the reflection point. Concave Reflection of Light. A concave surface is to be regarded as made up of a number of plane surfaces inclined to- ward each other. The optical center of a concave mirror is the OPTICS — VISIBILITY 25 center of the sphere of whiclj the concave surface is a segment. Principal Focus — Focal Length. — Parallel rays, falling upon a concave mirror, are reflected Illustrating conjugate foci, F and' F'. If tlie light be at F the reflected image will focus at F', but if the light be at F' the image will focus at F. as convergent rays which meet at a point on the axis midway between the surface of the mirror and the optical center. This point is called the principal focus of the mirror, and the distance from the surface to this point is called the focal length of the mirror. Con/ersely it follows that rays originating at 26 REFRACTION the focal point of a concave mirror are reflected as parallel rays from the surface of the mirror. It also follows that rays which originate at the optical center of a concave mirror are reflected back from the mirror in the same lines, and the object is its own image. Com JUGATE Foci. — If the origin of the rays be at a point within the optical center (but not with- in the principal focus) the reflected rays will con- verge at a point an equal angular distance witli- Illustrating a virtual focus F. The light at L pro- jected on a concave mirror will appear from any point between A and B to be at F. out the center. N"ow if the point of convergence outside the optical center be made the point of origin the former point of origin inside the op- tical center becomes the point of convergence of the reflected rays. These two points have there- fore a reciprocal relation to each other, and are called conjugate foci. Virtual Focus^ — Virtual Image. — If the rays originate at a point inside the principal focus of a concave mirror, by the laws of reflection the OPTICS — VISIBILITY 27 rays are reflected from the surface as divergent rays, and never meet. As explained above, an eye in the path of these divergent rays will in- terpret them as coming from a point in a straight ^^. --^d- Illustrating how a real reflected image becomes in- verted. line from the eye through the point of reflection on the mirror, and as far behind the mirror as the originating point is from the surface. The'point at which tlie image thus appears to be located is called the virtml focus, and the image thus seen a virtual image. A concave mirror therefore gives.two kinds o± image or no image at all, according to the loca- tion of the object. If within the principal focus, it gives an erect virtual image, because the rays are reflected di- vergently and never meet. If at the optical center, there is no image at all, because the rays are reflected so as to make the object its own image. 28 REFRACTION If outside the optical center^ it gives a real inverted image, because the rays are reflected so as to meet at a point within the center, and have therefore crossed before they reach the eye. Refraction of Light. Every body or substance which transmits light exercises more or less power of refraction — i. e., it deflects the ray more or less from its original course, provided the ray strikes its surface at '/ '> ^y>^ Illustrating how a virtual reflected image remains erect. other than a perpendicular angle. By this means, as well as by reflection, objects may be rendered visible which are not in an uninterrupted straight line with the eye. A familiar example of this may be experienced by placing a coin in the bottom of an empty bowl and moving away until the side of the bowl just hides the coin from view, then having someone pour water into the bowl, whereupon the coin will again come into sight, owing to the refrac- OPTICS — VISIBILITY 29 tion by the water of the rays proceeding from the surface of the coin. Degrees of Eefk action. — The degree of re- fraction exercised by a medium depends upon (1) its relative density, and (2) the angle at which the ray strikes its surface. Spherical Refraction, Principal Axis. — When the surface of the re- fracting medium is spherical — i. e., the segment INCIDENT RAY AXIAL n A V INCIDENT RAY Illustrating the principal focal point, F, and the principal focal distance, BF. in convex refraction. Showing' point where the incident rays meet the axial ray after passing into a denser medium. AB axial ray, BP, focal distance. F, focal point, where rays actually meet (positive). Rule does not apply where the incident rays enter the medium very far from the axial ray. of a sphere — it is mathematically apparent that one, and only one, of a group of parallel or di- vergent rays which enter it will do so at right angles to its surface. Ths one ray will continue its course through the new medium in the same 30 REFRACTION straight line it had before. It is called the prin- cipal ray, and its course the principal axis. Principal Focal Point and Distance. — AMien the surface of the medium is convex, the rest of the rays, being bent toward the perpendicu- lar of the surface, will, if continued through the inustrating the principal focal point and principal focal distance, BF, in concave refraction. Showing point where the incident rays are projected backward so as apparently to meet the axial ray at F. AB, axial ray; Bt, focal distance; F, focal point where rays appear to meet (negative). medium long enough, meet at a point on the prin- cipal axis. When the surface of the medium is concave, the rest of the rays, being bent to- ward the perpendicular^ are diverged, and will never meet, but the eye, receiving these rays, pro- jects them to an imaginary point outside the me- dium at which they would geometrically meet. This point in each case, where the rays meet or seem to me, is called the principal focal point, and the distance between it and the refracting surface is called the principal focal distance. OPTICS VISIBILITY 31 Positive and Negative. — In convex refraction the principal focal pointy where the refracted rays actually meet, is said to be positive; in concave refraction, where they are projected to meet, it is said to be negative. Spherical Abberation. — If, however, we trace the course of rays which are very oblique to tlie Illustrating' spherical aberration. Showing that when the incident rays enter the denser medium too far from the axial rays they do not reach a focus. axial ray, or which strike the refracting surface far from its axis, we find tliat they do not meet the axial ray all at the same point. This lack of exact reunion of all the rays is known as spher- ical ahheration, a very annoying fault in optical instruments designed for very accurate work. The eye has this fault to a small degree, but not enough to interfere with vision. Principal Posterior Focus. — Rays emanat- ing from a point at an infinite distance from the 32 REFRACTION refracting surface strike the surface with so little divergence as to be practically parallel to each other, and if the refracting inedium be denser, they are rendered convergent by refraction. The point where these refracted rays met is called the principal posterior focus, and the distance be- tween it and the refracting surface the principal posterior distance. Principal Anterior Focus. — At a certain point of nearness of the luminous point to the re- Showing- the formation of an inverted image by rays passing- through a convex spherical medium. fracting surface, the rays are so divergent that the refracting medium can no longer render them convergent, but only parallel. These refracted rays will therefore never meet, and the point of their reunion is at infinity. The point from which such rays proceed is called the principal anterior focus, and its distance from the refracting sur- face the anterior focal length. Formation of Images. — From every point of an object there proceeds one ray which is not bent from its course by a convex refracting medium — OPTICS — VISIBILITY 33 viz.^ tlu- rfT> Aiiich strikes the refracting surface perpendicular! }'. Such rays arc called rays of direction. These rays of direction meet and in- tersect at the center of curvature of the surface, or the optic center. All other rays proceeding from the various points of the object are refract- ed so as to meet their respective axial rays at the principal posterior focus. But this posterior focuSj as we liave seen, is further from the re- fracting surface than the length of the radius, therefore the focused image of the various points of the object are formed beyond the point of in- tersection of the axial rays, hence the image of the object is an inverted one. Circles of Diffusion. — An image is clearly defined only at the plane of the focal reunion of the rays. In, any plane anterior to this the rays have not yet united; and posterior to this, the rays have united, crossed, and again diverged. A screen placed either in front of or behind the plane of focal reunion therefore receives a blurred image, because every point of the object is repre- sented^ not by a corresponding single point, but by a circle of diffused rays. The size of these circles increases as the screen is moved forward or backward. As we shall presently see, this is the principle of spherical errors of refraction in the eve. CHAPTER III THE EYE. For the purposes of this work the eye need be considered only in its capacity as an optical in- stnmiont. and in so far as its structure has to Rectus Nerve Rectus Muscle"^^^ Sc^ev* Illustrating' the anatomy of the eye. do with the phenomenon of refraction and its accessory functions. The Anatomic Construction of the Eye. The i'ljcbaU is formed of the segments of two hollow spheres^ of different size and convexity, the smaller and more convex of which is set into the larger and less convex as a crystal watch glass is let into its case. 36 REFRACTION The Cornea. — The larger spherical segment consists externally of the sclera, which forms five- sixths of the entire eyeball, including all of the posterior portion and that part which is inside the bony orbit. Into the anterior portion of this segment is inserted the smaller segment, the cornea. The Iris. — At the posterior part of the cornea, where it is set into the sclera, inside the globe, is suspended the circular curtain, called the iris, which forms the pupil of the eye. It is the pig- mentation of the iris which gives color to the eye. The Pupil. — In the center of the iris is a cir- cular aperture through which light passes in and out of the eye. This aperture is called the pupil. To the observer under ordinary conditions the pupil appears black because, as we shall presently see, it is impossible for the observers eye, under ordinary conditions, to receive and focus any of the rays proceeding from another's eye. The iris is furnished with two sets of muscles, one running circularly or concentrically, the other in a radiating direction. Contraction of the first group draws the iris centripetally inward, lessen- ing the size of the pupil ; contraction of the latter group draws the curtain centrifugally outward, increasing the size of the pupil. The Lens. — Immediately behind the iris is the crystalline lens, a double convex lens whose pos- terior surface is more convex than its anterior, THE EYE 37 with an elastic capsule, and filled with trans- parent, colorless fibres. The Chambers. — The space between the cornea and the iris is called the anterior chamber, and that between the iris and the lens the posterior chamber. (Some anatomists deny the existence of a posterior chamber.) The Hujmors. — These chambers communicate with each other, and are filled with a transparent, colorless fluid of a slightly greater density than water, called the aqueous humor. The remainder of the interior of the sphere, comprising four- fifths of the entire globe, is filled with a trans- parent, jelly-like substance, of about the same refractive density as glass, called the vitreous humor. The Eetina. — Spread out upon the internal posterior surface of the ball is the retina, the sensitive membrane which receives the rays of light, after their passage through the foregoing structures, as an inverted image of the object from which the rays proceeded. The eye may, therefore, be regarded as a cam- era, of which the cornea, crystalline, and vitreous are the lenses, the iris the shutters, and the retina the sensitive plate. The perception and interpre^ tation of the image are functions of the brain, and^ belong to a study of physiology and neurology. The Ciliary. — Around the crystalline lens is the circular muscle called the ciliary muscle, and around the extremities of the lens are the sus- 38 REFBACTION pensorj ligaments, which limit the convexity of the lens. These ligaments are held in place by the choroid, the internal lining coat of the sclera. When the ciliary muscle contracts, it draws the choroid forward and releases the suspensory liga- ments, whereupon the elasticity of the lens changes its shape to one of greater convexity. This has an important influence upon the refraction of the eye, as will presently be seen. The Yellow Spot. — Exactly in the center of the retina, corresponding to the visual axis of the eye, is a small vascular spot, called the macula lutea or yellow spot. This is the spot where the sense of vision is most perfect, and forms the center of the focusing system. In the center of the yellow spot is a minute depression, called the fovea centralis, which is the most sensitive point in this sensitive spot. The Disc. — About an eighth of an inch to the inner side of the yellow spot is the place where the optic nerve enters the retina. This is marked by a slight protuberance, and as it is entirely devoid of sensitiveness it is commonly called the blind spot. In optical language it is also called the disc of the eye, and the fact of its non- sensi- tiveness is taken advantage of in oplithalmoscopic examination to focus the light upon this point. Axes and Points. — There are various axes of the eye — i. e., imaginary lines drawn through the eyeball in different directions, and various points situated at different sites on the axes, which are THE EYE 39 important in the study of the optics of the eye, and they should be carefully studied and thor- oughly comprehended by the practitioner of optics. Since the eyeball consists of segments of two different sized spheres, there are two spherical systems for which to estimate these axes and cardinal points. However, the horizontal axes of the two systems are identical, and the cardinal points of both systems are situated on this hori- zontal axis. Furthermore, the respective points for the two systems are found to be so near to- gether that for practical purposes they are re- garded as being identical. There are, then^ really six cardinal points of the eye, but practically only three. 1. The 'prin- cipal pointy situated on the horizontal axis two mm. behind the cornea. 2. The nodal point, situ- ated on the same axis seven mm. behind the cor- nea. 3. The principal focus, situated on the same axis where it cuts the retina. The principal point is the point where the hori- zontal axis is cut by the mean of the refracting planes (in this case the mean of the curvature of the eye). The nodal point is the center of the refracting system of the eye, corresponding to the optical center of a sphere already described, and the rays which pass through this point are not refracted, being rays of direction. A luminous point placed above the ])rincipal axis of the eye forms its image 40 REFRACTION on the retina below that axis, and vice versa, hence the retina gets an inverted image of the object. Showing that the further forward the nodal point is (as in myopia) the larger the size of the inverted image on the retina. The principal rays from these various luminous points which make up the object cross at the nodal point. In the eye whose refraction is normal the nodal point is about 7 mm. behind the cornea, but its Showing that the further back the nodal point is (as in hypermetropia) the smaller the size of the in- verted image on the retina. location of course varies with the curvature of the eye. It will readily be seen that the further forward the nodal point is, where the principal rays cross, the larger ^nll be the inverted image on the retina; the further back it is, the smaller the image. THE EYE 41 The principal focus is that point on the prin- cipal (horizontal) axis where the rays refracted by the combined systems of the eye are brought to a focus. In the normal eye it is of course located at the retina. The axes of the eye, with which refraction is concerned, are (1) the principal axis, (2) the op- tic axis, and (3) the visual axis. The principal axis, as already indicated, is an Showing- the principal points of the eye. imaginary line drawn through the center of the eyeball dividing it horizontally into two equal parts. The optic axis is an imaginary line drawn through the center of the cornea and the nodal point to the inner side of the yellow spot. (This is practically identical with the principal axis.) The visual axis is an imaginary line drawn from the object looked at through the nodal point to the yellow spot. 42 REFRACTION In addition to the above axes and cardinal points, there are the axes of rotation of the eye- ball, imaginary lines around \\liich the eyeball rotates by means of the recti and oblique muscles (these axes are vertical, horizontal and oblique), and the point of the center of rotation. inustratinpT the necessity for a larger dimension of the object, the further away it is, in order to con- form to the visual angle. Angle Alpha. — The angle formed at the nodal point by the optic and visual axes is called the angle alpha. In a normal eye it is about 5°. Angle Gamma. — The angle made by the visual axis and a line drawn from the object through the center of rotation is called the angle gamma. The Visual Angle. — The angle made by two lines drawn from the extreme boundaries of the object looked at tlirough the nodal point is called tlie visual angle. The minimum size of this angle in order that the brain may interpret the image is 1°. That is to say, two luminous points sep- arated by an angular distance of less than 1° would be perceived by the brain as only one luminous point. Acuity of Vision. — The ability to distinguish THE EYE 43 two separate luminous points at a given visual angle is^ of course, ultimately a function of the brain, and has nothing to do with refraction. It is known as acuity of vision. It is, however, habitually made use of in testing refraction; for if a patient in whom there is no reason to sus- pect any defect of the brain is unable to obtain a clear image of an object at a distance corre- sponding to the normal minimum \isual angle, it must be because the condition of his refraction is such tliat the rays from that object do not fall upon liis retina at the proper visual angle. Any correction, therefore, which enables him to dis- tinguish such an object at the minimum angular distance has manifestly rendered liis refraction normal. The most familiar and useful applica- Illustrates the construction of the test type to con- form to the visual ang-le at a g-iven distance. This angle must not be less than 1°. tion of one to the other is the test-type cards that are employed to test distant vision. Test Types. — These types are constructed on the principle of the visual angle. On these test cards each letter is so constructed that at its proper distance (e. g , ^o. G type at G meters) the mini- mum distance between its parts is not less than 1°. 44 EEFRACTION Muscles of the Eye. ExTRixsic Muscles. — In addition to the ciliary muscle and those of the iris, which are called the intrinsic muscles, the eyeball is furnished with four pairs of quite good sized muscles, attached Showing- the muscles of the eye in situ. to the outer coat, which are called the extrinsic muscles, and which serve to rotate the eyeball on different axes and in different directions. Each eyeball has one of these six pairs, as follows: Siiperior Rectus, attached to the upper part of the eyeball/ which rotates it upward. Inferior Rectus, attached to the lower part of the eyeball, which rotates it do^vnward. Internal Rectus, attached to the inner side of the eyeball, which rotates it horizontally inward. THE EYE 45 External Rectus, attached to the outer side of the ball^ which rotates it horizontally outward. Superior Ohlique, attached to the outer and upper side of the ball, which rotates it outward and upward. Inferior Ohlique, attached to the inner and lower side of the ball, which rotates it inward and downward. Muscular Mechanism. — These muscles act in pairs, with and against each other, and the force exerted by any pair is, in a normal healthy per- son, equally divided between the two eyes. The internal recti act together, pulling both eyes hori- zontally inward with exactly equal effect, and are opposed by the external recti, which also act to- gether pulling the two eyes equally outward. The superior oblique act together in a similar fashion, and are opposed by the inferior oblique. It must not be supposed that these muscles are ordinarily at rest, only those being in action which are re- quired for a given movement of the eye. On the contrary, all of the muscles are constantly in a state of contraction ; when the eyeball is still, the force exerted by each pair is exactly equal, so that no one set overcomes another; when it is desired to move the eyeball, a little more nervous energy is put into the proper pair of muscles, so that their action overcomes that of the rest to just the extent necessary to execute the required movement. It is important to bear this in mind, as will be seen when we come to study muscular insufficiency. CHAPTER IV. REFKACTIOISr OF THE EYE. The eye is a refracting instrument, with a con- vex surface, of such density as compared with air that, in a normal eye in a state of rest, paral- lel rays are exactly focused on the retina at the back of the eyeball. This must not be taken to mean, as the ordi- narily employed illustrations would seem to in- lllustrating how axial rays and their divergents are focused on the retina in the normal eye at rest. The divergents have become parallel with the axials by the time they enter the eye. dicate, that rays of light reaching the eye from distant objects at infinity all fall upon its sur- face in a horizontal direction, so that only one horizontal ray — namely, that one which travels along the principal axis of the eye — is unrefract- ed. It has already been shown how that rays of direction proceed from all points of a distant object, and pass through the nodal point unre- fracted. It is the diverging rays accessory to these ravs of direction which, by the time they 48 REFRACTION reach the surface of the e3'e, are practically paral- lel and are refracted by the media of the eye so as to meet their respective rays of direction at the retina. Index of Refraction. — As a matter of fact the refractive system of the eye is a compound The normal or emmetropic eye. one, made up of three separate media of different densities (the aqueous humor, the vitreous hu- mor, and the lens), and three separate refracting surfaces of different convexities (the surfaces of the cornea, lens, and vitreous). However, for optical purposes it is convenient to regard the eye as a single refractive system, whose net re- fractive effect is the resultant of the various parts. This net index of refraction of the eye is about 1.4, but naturally it varies, DiOPTRiSM. — The refractive mechanism of the eye is called its dioptric system, and the retina, REFRACTION OF THE EYE 49 in a normal eye, is situated exactly at the prin- cipal focal point of the dioptric system. Emmetrqpia. — This condition is called emme- tropia; an emmetropic eye is one whose refrac- tion is normal. Ametropia. — When the retina is not at the The hyperopic, or short eye. principal focal point, but is either beyond or with- in it, so that parallel rays come to a focus either in front of or behind the retina, the eye is said to be Ametropia An ametropic eye is one whose refraction is abnormal. HYPERMETRoriA. — ^Whcu the retina is situated within the principal focal point, so that parallel rays come to a focus behind tlic retina, the eye is said to be Hypermetropic or Hyperopic. Myopia. — When the retina is situated beyond ' the principal focal point, so that parallel rays 50 REFRACTION come to focus in front of the retina, the eye is said to be Myopic. Astigmatism. — When the refractifig surface of the eye is irregular, so that all the rays do not focus at one point, the eye is said to be Astig- matic. The myopic, or long- eye. Anisometropia. — tWhen the eyes are both ame- tropic, but differently affected — i. e., when one eye is myopic and the other hypermetropic, or when one is astigmatic and the other spherically abberated, the condition is called Anisometropia. It is highly important in testing refraction to test each eye separately, excluding the other from vision meanwhile. Errors of Refraction. The four most common errors of the eye which the refractionist is called upon to correct are: 1. Hypermetropia or Hyperopia. 2. Myopia. 3. Astigmatism. 4. Presbyopia. REFRACTION OF THE EYE 51 Hypermetropia is that condition of the eye in which parallel rays are focused behind the retina. A A. ^:.---^— P Hyponnetropic or Short Eye. — Parallel rays of light, AA, from distance focusing- behind retina at F. Dotted lines — rays of light from near object — focus still farther behind retina at P. In other words the antero-posterior diameter of the eyeball, the distance from the cornea to the retina, is too short in proportion to the refracting Hypermetropic eye corrected by a convex lens, which hastens refraction of the rays, and thus brings the focal point forward. AA, rays. P, focal point. power of the eye, and the principal focal point is back of the retina. In order to coiTect this error the eye has to be furnished with a spher- 52 REFRACTION ical lens wliicli will increase eye refraction and hasten focusing of all the rays. In other words, the refraction of the eye must be assisted by a Myoiiif or Long- Kye. — ParaUel rays of lig-lit, AA, focused too soon at P. Dotted lines show object nearer the eye focused farther back. lens of the same curvature as the eye itself — namely, a convex lens. Convex lenses are there- fore called plus lenses, because they add to the refractive power of the eye. Myopia is that condition of the eye in which parallel rays are focused in front of the retina. The antero-posterior diameter of the eyeball is Showing: the myopic eye corrected by a concave lens, which delays refraction of tiie rays and there- fore puts the focal point further back. too long in proportion to the refractive power of the eye, and the principal focal point is in front of the retina. Its correction requires a REFRACTION OF THE EYE Oo spherical lens which will lessen eye refraction and delay focusing of all the rays. That is to say, the refraction of the eye must be antag- onized by a lens of opposite curvature to that of fjiQ eye — namely, a concave lens. Concave lenses are therefore called minus lenses, because they detract from the refractive power of the eye. Astigmatism signifies a condition of the eye in which the curvature of the cornea is not the same in all its meridians, one or two being more or less 1 1 1 ' . i M 1 i 1 1 1 1 1 TT 4U-- t=H>nUiltj - . ^ — ii 1 [ --^P^ Tilnstrates astigmatism, where the rays entering too soon. convex than the rest, so that while the rays from the normal meridians are properly focused, those from the defective meridians form a line of un- focused rays, either in front of or behind the retina, and a diffused indistinct image results. As the name implies, it is impossil)lc to focus all the rays at a point. Chief Meridians.— The most convex and tlie least convex meridians are always precisely at right angles to each other. If one is vertical the other is horizontal: if one is at 20 the other is at 110. These are called the chief meridians, and 54 REFRACTION are tlie meridians which must be estimated and corrected. The eye is normally a little astigmatic^ the vertical meridian being naturally a trifle more convex than the horizontal, but as long as the difference is not sufficient to interfere wth clear vision it is regarded as normal and not corrected. In pathological astigmatism the relative con- Illustrates the correction of the foregoing, by a concave cylinder with its axis at right angles to the defective meridian. The raj'^s entering the cylinder at right angles to its axis are refracted and their focal point carried back to coincide with that of the normal meridian. vexity of the meridians is usually the same as in normal^ — i. e., the vertical and horizontal meridians are usually the two chief meridians, and the vertical is usually the more convex, and the astigmatism is then said to be "with the rule." But of course there are frequent excep- tions to this rule,, in which case the astigmatism is said to be "against the rule." Presbyopia is really a phase of hypermetropia, but the name is used to indicate that form of hypermetropia which depends upon the effects of age upon the crystalline lens, hardening it and REFRACTION OF THE EYE 55 thus preventing the ciliary muscle from perform- ing its accommodating function. Presbyopia is arbitrarily said to begin when the patient^s near point has receded to 22 cm. This usually occurs at about 45 years of age^ and increases about 1 D. for every subsequent five years. Its correction is, of course, accomplished by convex lenses. CHAPTER V. LENSES. Lenses rire nowadays usually cut out of crown glass, pebbles being very expensive, hard to grind, and possessing no particular advantages over glass. They are of twojdnds as to curva ture — convex and concave. Convex Snrfnee Shows how rays entering- a convex lens, and being- bent toward the perpendicular, converge. Note that the focal point is where the refracted rays actually meet (positive). In pursuance of the principle that rays of light entering a denser jnediujn. are bent toward the perpendicular of the r efrac ting_surface^ rays which enter a convex lens, other than the ray which traverses the principal axis^ are bent toward each other — i. e., so as to converge — while those which enter a concave lens, other than along the prin- cipal axis, are bent away from eacli other — i. e., so as to diverge. The focal poin t oJ__aJlens is the point at which parallel rays, refracted by the lens, are brought to a focus. 58 BEFEACTION The focal point of a convex lens is the point at which the refracted rays actually come to a focus^ and is called positive. The focal point of a concave lens is the point at which the refracted rays would meet if pro- jected backward on the concave side of the lens, and is called negative. This is, of course, pre- cisely the opposite to the focal point of a con- vex or concave mirror for reflected rays. Fo cal Point (Kegafive) Shows how rays entering a concave lens, and being bent toward the perpendicular, diverge. Note that the focal point is where the refracted rays would meet if projected backward on the concave side of the lens (negative). The focal length of a lens is the distance be- tween the refracting surface and the focal point. Lenses are also spherical — i. e., segments of a sphere — and cylindrical — i. e., segments of a cyl- inder. There are convex and concave spherical s and convex and concave cylindricals. Spherical lenses refract all rays except one (the principal axis), and therefore converge them all toward or diverge them all away from a point, the resulting path of the rays being cone-shaped. LENSES 59 Cylindrical lenses refract only those rays which strike them at right angles to their axis; those which strike them in the same line as their axis are perpendicular to the refracting surface and therefore pass through without any change of di- rection. Cylindrical lenses therefore converge or Showing how rays which enter a cylinder paraHel to its axis are unchanged, while those which enter at right angles to the axis are refracted. diverge a straight line of rays toward or away from' a point, the resulting path of the refracted rays being fan-shaped. DioPTRiSM. — The refracting power of a lens is called its dioptrism. Inasmuch as the density of the glass is the same in all lenses, this dioptrism is dependent upon the curvature of their surfaces — the greater the convexity or concavity, the greater their dioptrism. A lens having a focal length of 1 meter — i. e., which brings parallel rays to a focus at a dis- tance of 1 meter from its surface — is taken as a GO REFRACTION standard^ and its strength designated as 1 diop- ter, or 1 D. The dioptrism increases in inverse geometrical ratio to the focal length. Manifestly a lens whose focal length is half a meter has twice the curva- ture, and therefore twice the dioptrism, of one whose focal len.gth is 1 meter. Therefore a lens whose focal length is half a meter is 2 D, a quar- ter of a meter 4 D, etc. The diopter of every lens and its focal length are given in the trial case. The lenses are graded, '2545 6 Shows different methods of grinding- spherical lenses. as a rule, in fractions of 0.25 of a diopter, which are quite convenient to work with. Gkindinct of Lenses. — Spherical lens es are ground in the following manner: ' 1. Plano-convex. 2. Bi-convex. 3. Converging concavo-convex. 4. Plano-concave. 5. Bi-concave. 6. Diverging concavo-convex. CoNCAVQ-CoNVEx Lenses. — It will, of course, LENSES 61 be seen that in the concavo-convex form of lens (by far the most commonly used, by the way, of all the different forms) the net effect of the lens depends upon the relative cur vatu re of the enter- ing and merging surface. If both entering and emergin.o- surfaces were equally curved, the ray would be refracted upon entering, and exactly neutralized again when it emerged, only being displaced laterally. If the entering surface is more curved than the emerging, the net result is that of a convex lens; if the emerging surface has the greater curvature, the net effect is that of concavity. Cylindrical leni iLs arc alwa ys_ gi'ound with the reverse side piano, except when they are com- bined with spherical lenses for the correction of compound and mixed astigmatism, in which case the spherical correction is ground on the reverse side. CHAPTER VI. ACCOMMODATION AND CONVERGENCE. The Eye at Rest. — When the noi-mal eye is at rest, parallel rays— i. e., rays that originate six meters or more from the observer — are focused on the retina; hence in this condition objects at this distance are, so far as refraction is con- cerned, clearly seen. inustrates how divergent (finite) rays entering the normal eye at rest are focused back of tlie retina. It is evident that under the same condition of refraction divergent rays — i. e., rays that pro- ceed from a less distance than six meters — do not focus on the normal retina, but are carried beyond it and come to a focus behind it. Hence objects at a distance of less than six meters are not clearly seen by the normal eye in a state of rest. Accommodation.— In order that the eye may clearly see" objects at a finite distance the refrac- tive power of the eye must be increased. This is accomplished by the contraction of the ciliary 64 REFRACTION muscle, drawing forward the choroid and releas- ing the suspensor}^ ligament of the lens, whose elasticity then changes its shape so as to increase the convexity of the refracting surface. This power of changing the focus of the eye is called accommodation. Far Point. — Tlie point from which rays of light will focust upon the retina of an eye whose lUustrates how the eye accommodates itself to focus divergent rays on the retina. Note the increased convexity of the lens (produced by contraction of the ciliary muscle). ciliary muscle is relaxed to its fullest possible- ex- tent is called its far point, or punctum remotum. Inasmuch as the normal eye, with its ciliary completely relaxed, is adapted for focusing in- finite — i. e., parallel — rays, it follows that the far point of a normal eye is at infinity — i. e., at six meters and beyond. It must be remembered that in the matter of accommodation any point beyond six meters is equally at infinity. The normal relaxed eye is, so far as its accommodation is concerned, adapted for an image of an object six miles off equally as ACCOMMODATION AND COVERGENCE 6o well as for that of an object six meters distant. Wliat difference there is in the vision of two such objects is due to other factors, such as in- tensity, visual angle, etc. Near Point. — The point from which rays of light will focus upon the retina of an eye whose ciliary muscle is contracted to' its fullest possible degree is called its near point, or punctum proxi- mum. The near point is usually ascertained by meas- uring- the least distance at which the subject can read, wi th each, eye _separately, the reading type No. 1 whiclTiFconstructed according to the visual angle. Or it may be found by placing in front of the eye a piece of card pierced with two small holes, not farther apart than the diameter of the pupil (the other eye being excluded meantime), and directing the patient to look at a small object, say a pin head, which is gradually moved nearer to the eye. As long as the accommodation is able to focus the rays from the pin which pass through the holes in the card, they will form a single image on the retina; but as soon as the near point is passed, so that the rays become too divergent for accommodation to focus, they will form two sep- arate images and the eye will see two pins. Range of Accommodation. — The distance be- tween the far and near points of an eye is called its range of accommodation. 66 REFRACTION * Amplitude of Accommodation. — The mus- cular and nervous energy necessary to change the eye from its far to its near point i.s called its amplitude of accommodation. Lens Measurement of Accommodation. — It has already been seen that the act of accommoda- tion consists in a muscular increase in the con- NEAR POINT Illustrating- how a convex lens which renders x'ays from the near point equivalent to those from the far point, focusing- thein on the retina is the measure of the accommodation of the eye. vexity of the crystalline lens, by which refraction of the finite (divergent) rays is increased and fo- cusing hastened. If the refraction of these rays be assisted by a convex glass lens held before the eye, the ac- commodative effort of the eye will be spared to that extent; and it is possible to find a convex glass lens which will spare the eye its accommo- dative effort altogether, so that by the help of such a lens rays from its near point will focus on the retina without any accommodation at all, just as though they came from the far point. It \\ill readily be seen that the dioptric strength of ACCOMMODATION AND COVERGENCE 67 the convex lens which accomplishes this is the exact measure of the entire amplitude of accom- modation of the eye. Conversed, the distance from the eye to its near point is the focal length of the convex lens whose dioptric strength corresponds with the amplitude of accommodation. XoRMAL Amplitude of Accommodation. — Ordinarily the range and amplitude of accommo- dation are about the same for all healthy eyes at a given age. The following table shows the ampli- tude at various ages: 10 years 14 D. 15 years 12 D. 20 years 10 D. 30 years 7 D. 40 years 4.5 D. 50 yeai^ 2.5 D. 60 years 1 D. 75 years The reason for the decrease of amplitude as age advances is that the capsule of the crystal- line lens loses its elasticity, so that, although the ciliary contracts and releases the lens from the suspensory ligament^ it is no longer able to as- sume so convex a form. It is probable, too, that with advancing age the ciliary muscle itself be- comes thus capable of contraction. Absolute and Binocular. — The amount of accommodation which one eye can exert when the other is shut out of vision is called absolute ac- commodation. That which both eyes together can 68 REFRACTION exert is called hinocular accommodation. The lat- ter is a little more than the former, so that in testing for glasses, after testing each e3^e sep- arately it will usually be found that both eyes to- gether can take a little stronger convex and a lit- tle weaker concave correction than each eye sepa- ratelv calls for. - YpIIow Spot Yellow Spot Rectus Muscle Illustrates how the eyes at rest have their yellow spots adjusted for parallel rays and need no converg- ence. Convergence. Convergence is a necessary element in near binocular vision, and is, therefore, intimately as- sociated with accommodation. It is, however, quite independent of it, and paralysis of one does not affect the other. Convergence is the power of directing the visual axes of the two eyes toward a point nearer than infinity. In order to get singleness of binocular vision the image formed on the central parts of ACCOMMODATION AND COVERGENCE 69 the retina must exactly coincide in the two eyes, or there will be double vision. For objects at infinity the visual axes of the eyes at rest are adjusted to accomplish this; but for objects with- in a finite distance it is necessary to direct the yellow spot in each eye toward the same point. This is done by converging — i. e., pulling the eyes inward by means of the internal recti muscles. Point Illustrates the pulling of the «y^s inward (converg- ence) by the internal recti to direct the yellow spots toward the same near point. Range of Convergence is the extent of adduc- tion and abduction capable of being accomplished by the internal and external recti respectively. The former is known as positive convergence, the latter as negative convergence. Amplitude of Convergence is the amount of muscular and nervous energy necessary to change the eye from extreme adduction to extreme ab- duction, or the reverse, the former being known as amplitude of positive convergence, the latter amplitude of negative convergence. 70 REFRACTION Prism Tests. — The amplitude of convergence is measured by means of prisms, of which every trial case contains a sufficient supply. According to the laws of refraction already enunciated, prisms deflect rays of light toward their base, both upon their entering and leaving Illustrates how a ray is bent toward the base both on entering- and on emerging from a prism. the medium. Upon striking the surface of the prism, i. e., upon entering a denser medium, the ray is bent toward the perpendicular, which is toward the base. Upon passing out of the prisms into the air again, i. e., upon entering a rarer medium, it is bent away from the perpendicular, and that is also toward the base. The optical effect of a prism is, of course, to displace the image away from the base and to- ward the apex of the prism, the object appearing to be in a straight line from the eye through the point of convergence, and as far from the e3^e as the light has actually traveled. It will readily appear that a prism placed be- fore the eve with its base outward will focus the ACCOMMODATION AND COVERGENCE 71 central rays on the outer side of the yellow spot of that eye, while in the uncovered eye they will fall upon the yellow spot, thus producing double vi^^ion. To overcome this, the internal rectus Illustrates how a prism, base out, focuses a ray on the outer side of the yellow spot. must contract and pull the eye inward until the images on both retinae coincide as to position. This compensation^ however, is not accomplished solely by one eye, inasmuch as one rectus cannot act independently of its mate. Therefore, in- stead of the rectus of the prismed eye pulling that 72- REFRACTION eye inward until the deflected rays focus on its yellow spot, both eyes are pulled inward until the image falls upon corresponding retinal points midway between the yellow spot and that of prismatic deflection^ thus dividing the effect of prism equally between the two muscles. A prism of given strength held before one eye is equal in effect to two prisms of half that strength held before both eyes. Illustrating- the metric angle made l)y various visual axes, M'E M"E M"'E, with the central line MH. Note that the parallel lines AE. A'E' (eye at rest) make no ang-le at all, while BE, B'E' (eyes divergent) make negative angles. ECH and E'CH which are measured by prolonging the central line and axes to meet be- hind the eyes at C. The same conditions apply to the external mus- cles under the influence of prisms with their base inward. Measure of Convergence. — Prisms are num- bered according to the degree of their angle, and the angle of the prism which can be overcome by the recti muscles, i. e., the strongest prism with which single vision can be miantained is the measure of the amplitude of convergence. The amount of deviation, measured in angles, ACCOMMODATION AND COVERGENCE 73 produced in both eyes by a given prism or pair of prisms, is equal to half the total prismatic angle; and as the deviation is equally divided, between the two eyes the deviation of each eye is equal to one-fourth of the total prismatic angle. Thus, a prism of 8°, or two prisms of 4°, will produce a total deviation of 4°, or a deviation of 2° in each eye. For example, a prism of 8°, base out, before the right eye, will cause the central rays to focus 4° to the outer side of the yellow spot in that eye. In order to overcome the effect of this prism, and obtain single vision, the internal recti muscles will pull each eye inward 2°, so that the rays will focus on each retina 2° outside the yellow spot. Metric Angle. — The degree of convergence is sometimes expressed in meters, the metric angle being the angle formed by the visual axis witli the median line, i. e., an imaginary line drawn perpendicular to the base-line joining the points of rotations of the two eyes midway between them. A metre is taken as the unit of this me- dian line, the metric angle of convergence for a point one metre away being called 1, half a metre away 2, two metres away i/^, and so on. Wlien the object is at infinity the visual axis and the median line are of course parallel and the metric angle is then infinity or zero. Far Point. — ^The distance for which the con- vergence is adapted when the angle of converg- ence is at its minimum is called the fai' point, or 74 REFRACTION punctum remotum, of convergence. Theoretically this point is infinity, but actually the visual axes of most eyes in this state of rest are divergent, so that the metric angle is a negative one. In this case the distance of the far point is found by projecting the lines of the visual axes backwards until they meet behind the eye. Near Point. — When the metric angle is at its maximum, i. e., when the eyes are converged to their utmost capacity, the distance for which they are then adapted is called the near point, or punc- tum proximum, of convergence. Amplitude of positive convergence is meas- ured by the strongest prism, base out, with which single vision can be maintained. This indicates the greatest amount of deviation of which the internal recti muscles are capable. A normal pair of eyes should be able to overcome one prism of 20 to 30 deg. or two prisms of 10 to 15 deg. each. Amplitude of negative convergence is meas- ured by the strongest prism, base in, with which single vision can be maintained. This indicates the greatest amount of deviation which the exter- nal recti are capable of producing. Normally the eyes can overcome one prism of 6 to 8 deg. or two of 3 to 4 deg. each. Ahsolute negative convergence, i. e., in which the metric angle is negative, is measured by the strongest prism, base in, with which single vision can be maintained of an object at six meters. acco:mmodation and covergence 75 Insufficiency of Convergence. — Anything short of the ahove-named normal degrees of ampli- tude indicates weakness or imbalance of the ex- trinsic ocular muscles^ which must be further in- vestigated and dealt with as laid down in the chapter on that subject. Ratio of Convergence and Accommodation. Convergence and accommodation normally in- crease and decrease in mutual ratio, since the nearer an object is, the more one has to converge to see it, and the more accommodation one has to use. Indeed, the simple metric and dioptric system already explained of expressing degrees of ac- commodation and convergence enables us to ob- serve a very uniform ratio between the two func- tions. Thus when vision is adapted for an object at 1 meter distance the convergence, as expressed by the metric angle, is 1, and the accommodation, as expressed in diopters, is 1 D. For an object at infinity convergence and accommodation are both infinity. For an object at 2 meters convergence is 1/2 and accommodation 0.50 D. At half a meter convergence is 2 and accommodation 2 D. The amplitude of convergence is, however, nor- mally a little greater than that of accommodation, both near and far. Below is a table showing the metric angle for various distances and the actual values of the metric angle in degrees of the circle. The table is based on an average distance between the centers of rotation of the two eyes of 6.4 cm. 76 REFBACTIO:^ Distance of the Object The Metrical Value Expressed from the Eyes. Angle. in Degrees. 1 metre 1 1° 50' 50 cm 2 3° 40' 33 cm 3 5° 30' 25 cm 4 7° 20' 20 cnn 5 9° 10' 16 cm 6 11° 14 cm 7 12° 50' 12 cm 8 14° 40' 11 cm 9 16° 30' 10 cm 10 18° 20' 9 cm 11 20° 10' 8 cm 12 22° 7.5 cm 13 23° 50' 7 cm 14 25° 40' 6.5 cm 15 27° 30' 6 cm 16 29° 20' 5.5 cm 18 33° 5 cm 20 36° 40' It is readily seen that in cases of hyperme- tropia and myopia the direct ratio between ac- commodation and convergence is disturbed, the hypermetrope using less convergence and more ac- commodation, and the myope more convergence and less accommodation. This will be more clear- h^ apparent when we come to deal with these er- rors of refraction. CHAPTER VII. EETINOSCOPY. Retinoscopy, skiascopy, or the shadow test, has long been a popiilnr method of estimating and The lieiiiioscope. correcting refraction among European ophthal- mologists, and is rapidly gaining favor in this coimtrv. Xo refractionist should allow a case 78 REFRACTION to pass out of his hands without subjecting it to this valuable test. It has the distinct advan- tages of being entirely objective in character, i. e., not at all dependent upon what the patient may or may not see or think he sees, simple to carry out, and accurate in results. The retiiioscope itself is a very plain instru- ment, consisting simply of a round mirror about ONE METER T^ight over patient's head and the observer with mirror at one-meter distance. 3 cm. in diameter, mounted on a stem handle, and having a sight hole through the center of the mir- ror. There are several different makes of retino- scopes on the market, any one of which is serv- iceable; but for reasons which are elsewhere ex- plained, it is best to use a concave mirror of 25 cm. focal length. The principle of the retinoscope is that it per- mits the rays emerging from the pupil of the subject's eye to be intercepted in their path by the observers eye, a thing which, as already ex- RETINOSCOPY 79 plained, cannot be done without some such con- trivance. The light thrown into the subject's pupil from the mirror of the retinoscope natural- ly returns again by the same path, and is inter- cepted by the mirror. But the sight hole enables the observer to place his own eye right in this Illustrates the kind of lamp for retinoscopy. path of the returned rays, and thus to receive them upon his own retina. The examination should take place in a dark room, especially until the operator is practiced and experienced, so that the illumination of the 80 REFRACTION patient's pupil may stand out in bold relief from the surrounding darkness, and the observation thus be rendered easier. The best possible light, in the writer's judg- ment, is an electric bulb of at least 32 C. P., and Illustrates the technic of retinoscopy. cylindrical chimney, but gas or an oil frame will do very well. TOiichever light is used, however, it should have a cylindrical opaque cover with a circular bull's-eye aperture, and over this aper- ture a diaphragm with a circular hole 10 mm. in diameter for the light to emerge. The Z?(yr/ir should be placed five or six inches UETINOSCOPY 81 in front and to the left side of the observer, fac- ing so that its rays Avill fall upon the mirror when held before the observer's right eye, but will leave his other eye in the shadow. The nearer the source of light is to the mirror, consistent with the conditions already described, the better, as the illumination will then be the more brilliant. The observer seats himself or stands about one meter from the patient. The source of light, tlie observer's eye, and the patient's eye should, if possible, be all in the same horizontal plane; the more nearly this rule is observed the simpler will be the operation and the more reliable the observation. The observer now applies his right eye to the sight hole (at the back of the mirror, of course) and throws the reflected light from the lamp into the patient's pupil. This is at first quite a diffi- cult feat, but can be accomplished by first finding the bright disc of reflected light, and then slowly and carefully bringing it, by appropriate manipu- lation of the mirror, onto the patient's face, and finally onto his pupil. The modern electric retin- oscope, fitted with its own electric light, does away with this difficulty. The observer's eye being directly in the path of the rays as they are reflected back from the patient's pupil, the fundus of the patient's eye is seen as a red glare; this is called the fundus reflex. S'2 ){i:m{A( I'loN The Shadow. — Xow tlic operator rotates his head, toorether witli tlio jiiirror, very sliojhtlv to and fro^ from one side to tlie other, meantime carefully watehiiig Ihe patient's pupil, and a shadow will be seen to appear alternately from each side of [lie pupil and iiiove across the illu- minated area. This shadow moves either in the same direction as the mirror is rotated, or in the opposite direction. In technical language, it moves either with or against the mirror. As a matter of fact, of course, it is not the shadow that moves at all, but the illuminated image of the light thrown by the mirror, but as the shadow follows the image and is more readily seen than the light-image, we give our attention to the movements of the shadow. Eeasox of ^Movements. — The movement of the shadow with or against the mirror depends upon whether the rays of light that emerge from the patient's pupil have met and crossed before they reach the observer's eye, or not. If they have not yet crossed, it is manifest that the image seen by the observer will be a "virtual erect'^ image which will move in just tlie opposite direc- tion from the luirror — on })reeisely the same prin- ciple that the image is seen to move through a convex lens, as explained in a former chapter. If, on the other hand, the rays have crossed by the time they reach the observer's eye, the image will be an inverted and real one, formed between UETINOSCOPY 83 tlie patient's eye and the mirror^ which will move in the same direction a^ the mirror. ONE METER Illustrates how parallel emerging rays from an em- metropic eye never meet, so that when they reach the retinoscope they are still uncrossed, causing the shadow to move against the mirror. Point of Eeversal. — The point in front of the patient's eye where the rays emerging from the pupil meet and cross, and where the image Showing how parallel emerging rays from an emme- tropic eye are brought to a focus at 1 meter by a convex lens of 1 D. bringing the point of reversal exactly at the retinoscope. therefore changes from a virtual erect image to an inverted real (mo, as evidenced hy the change 84 REFRACTION in its moA'cments in relation to those of the mir- ror, is the point for which we seek in rctinoseopy. Or, to speak more correctly, we seek to bring the emerging rays to a crossing point at a certain lixed distance from the patient's eye, and by the h)i. owing- how divergent emerging rays fi. ni a li5'per- opic eye are still uncrossed when they reach the retinoscope, causing the shadows to move against the mirror. nature and strength of tlie lens which we are obliged to use to do this we judge of the refrac- tion of the patient's eye. Emmetropic Point of Eeversal. — It is man- ifest that rays of light emerging from a normal emmetropic eye in a s1?ate of rest are parallel, and will therefore never meet and cross. It is also plain, from a consideration of what has already been said about dioptrism^ that a convex lens of 1 D placed immediately before the patient's eye, will bring these parallel rays to a focus just 1 meter in front of the eye. Therefore, if the observer's eye be just one meter (or actually jnst a trifle more than one RETINOSCOPY 85 meter, to allow for the distance between the pa- tient's retina and the lens), from the patient's eye, and no lens be held before the latter, the rays will still bo ])arallel when they reach the observer, and ilie shadow will move against the mirror. Showing- how divergent emerging- rays from a hyper- opia eye are brouglit to a focus at 1 meter by a con- vex lens as mucli stronger than 1 D. as the rays are divergent. But if a convex lens of 1 D be mounted before the patient's eye, tlie rays will be brought to focus and cross at 1 meter^ i. e., where the ob- server's eye is situated, so that if he move just a trifle nearer the shadow will move against, and a trifle further away it will move witli, the mirror. Hypermetropic Point of Keversal. — Rays of light emerging from a hypermetropic eye in a state of rest are divergent, and will therefore never meet and cross. And in this case it will require a convex lens of more than 1 D. to bring these divergent rays to a focus at 1 meter. It will, in fact, need a convex lens precisely as much 86 EEFRACTION in excess of 1 D. as the patient's eye is hyper- metropic, in order to bring the point of reversal at 1 meter where the observer's eve is located. Illustrates how convergent rays from a slightly my- opic ey.e are still uncrossed ^vhen they reach the retin- oscope, causing the shaflo\v to move against tlie mir- ror. Observe that tliese rays would meet at a point within infinity. Myopic Point of Eeversal. — Rays of light emerging from a myopic eye in a state of rest are convergent, and will meet at a point somewhere Sliowing hoAv sligiitly convergent rays from the slightly myopic eye are broviglit to a focus at 1 m?ter by a convex lens as much less than 1 D. as the rays are convergent. inside of infinity, i. e., somewhere within six meters of the patient's eye. If the degree of mj^opia present is just 1 D. the emerging rays will meet at just 1 meter, i. e., just where the ob- HETINOSfOl'Y 87 servers e3'e is situated; if it is less than 1 1). they w ill ineet at some point further than 1 meter, and will, tlierefore, be still uncrossed when they reach the observer; if it be more than 1 D. the i-ays Avill have met and crossed before they reach the observer, and the shadow will therefore moye with the mirror. In the first instance (myopia of just 1 D.), no lens of any kind will be necessary to bring the Showing- how convergent rays from an eye that is myopic more than 1 D. have met and crossed before tliey reach the retinoscope, causing the shadow to move with the mirror. point of reversal at 1 meter; in the second in- stance (myopia of less than 1 D.) it will still need a convex lens to bring the point of reversal at 1 meter but it will require a convex lens to of less than 1 D. — just as much less as the eye is myopic; in the third instance (myopia of more tlian 1 D.) it will be necessary to delay the meet- ing of the rays in order to brin^" the point of re- versal at 1 meter, and hence a concave lens will be required of just as much strength as the myopia is in excess of 1 D. 88 REFRACTION Interpretation of Shadow. — If, then, wit!) the observer's eye at one meter from the patient's, the shadow is seen to move against the mirror, the patient's eye is either emmetropic (rays par- allel), or hypermetropic (rays divergent), or less than 1 D. myopic (rays convergent but meeting Showing how convergent rays are brought to a focus at 1 meter by a concave lens of dioptric strength etiuivalent to the excess of myopia over 1 D. In this illustration the dotted line represents the convergence which would, unaided, bring the p. of r. at 1 meter, i.e., 1 D. of myopia. A 0.50 D. concave lens render the actual rays equal to this, hence there is 1.50 D. myopia in the eye. at further than 1 meter) . If they move with the mirror, the eye is myopic more than 1 D. (rays convergent and crossing in less than 1 meter). Emmetropia. — ^ow if a convex lens of 1 D. before the patient's eye bring the point of reversal just where the observer's eye is, so that by mov- ing a trifle backward or forward the shadow moves respectively with and against the mirror, we know that the rays emerging from the eye are parallel, and the refraction of the eye is normal. Hypermetropia. — If, with a convex lens of 1 D., the shadow still move against the mirror, we RETINOSCOPY 89 know that the 1 D. lens has not succeeded in bring- ing the rays to a crossing point at 1 meter, and that they must therefore be divergent rays, i. e., the eye is hypermetropic. We then try stranger convex lenses until we find one which just re- verses the movement of the shadow. The strength Showing- how convergent rays from an eye that is myopic just 1 D. came to a focus at just 1 meter, bringing the p. of r. exactly at the retinoscope. The myopic effect of the eye is just equivalent to that of a 1 D. convex lens (see above). of this lens in excess of 1 D. is the measure of the divergence of the rays, and therefore of the hyper- nietropia of the eye. ^Iyofia. — H on the other hand, the shadow is seen to move against the mirror, and a convex lens of 1 D. completely reverse the movement, we know tliat the 1 D. convex lens has brought the rays to a crossing point nearer than 1 meter, and that they must therefore be convergent rays, i. c., the eye is myopic, but less than 1 D. myopic. We then try weaker and weaker convex lenses until we find one that just reverses the movement of the shadow. The strength of this lens less than 00 REFRACTION 1 D. is the measure of the convergence of tlie rays, and therefore of the myopia of the eye. If, Avith the observer's eye at 1 meter, the image is indistinct, and by moving a triflle backward and forward the shadow is seen to move respectively with' t^nd against the mirror, we know that the rays emerging from the eye are just convergent enough to reach their crossing point at 1 meter; i. e., the eye is just 1 D. myopic. If, however, with the observers eye at 1 meter, the shadow is at once seen to move with the mir- ror, we know that the emerging rays have already met and crossed at some point nearer than 1 me- ter; i. e., that the eye is more than 1 D. myopic. In that case it is necessary to delay their meeting in order to bring the point of reversal forward to 1 meter, and we tlierefore, beginning with the weakest, try successively stronger concave lenses until we find the one that just reverses the move- ment of the shadow. The strength of this lens tells how much in excess of 1 D. the eye is myopic. Examples. 1. Shadow Is Seen to Move Against the Mirror. — Rays are either parallel, divergent, or so slightly convergent that they have not yet met. Eye is either emmetropic, hyperopic, or slightly myopic. Convex lens of 1 D. just brings the point of reversal to the observer's eye, so that by moving slightly backward and forward the shadow is made to move respectively with and against the mirror. RETINOSCOPY 9 1 The rays are parallel, and the refraction of the eye normal. 2. Shadow Is Seen to ]\[ovi-: x\gainst the Mirror. — With convex lens of 1 L). they still move against. Emerging rays are divergent; eye is hypermetropic. Convex lens of 2.50 D. just brings the point of reversal to observer's eye. The eye is hypermetropic 1.50 D. and a convex lens of that strength will correct the hypermetropia. 3. Shadow Is Indistinct With the Naked Eye. — By moving a trifle; backward and forward the observer sees it move respectively with and against the mirror. The point of reversal is ex- actly at observer's eye. Emerging rays are just sufficiently convergent to meet at one meter ; hence the eye is just 1 D. myopic. 4. ^ Shadow Is Seen to Move Against the Mirror.— With convex lens of 1 D. it moves de- cidedly with the mirror. Emerging rays are con- vergent, not sufficiently so to meet unaided at 1 meter, but sufficiently so that the 1 D. convex lens brings them to focus sooner than 1 meter. Convex lens of 0.50 D. just brings the point of reversal at observer's eye. The eye is myopic 0.50 less than 1 D., i. c., 0.50 D. and a concave lens of that strengili will correct the myopi'a. ^5. Shadow Is at Once Seen to Move with THE Mirror. — Emerging rays have already crossed; they are convergent to a greater degree than 1 D. and the eye is myopic more than 1 D. It is necessary to delay their meeting to bring for- 92 REFRACTION ward the crossing point. A concave lens of 1.2o D. just brings the point of reversal at observers eye. The e3'e is myopic 1.25 more than 1 D., i. e.^, 2.25 D., and a concave lens of that strength will correct the myopia. Practical Points. Position- of Light. — The best position for the light will be found to be, as already described, about six inches in front and to the left of the observer, so that the rays may cross obliquely in front of his left eye. leaving it in shadow, and fall obliquely upon the mirror held before liis right eye. The observer's right eye is used for ex- amination of both of the patient's eyes. Some operators, however, prefer to have the light a little behind and above the patient's head, because it gives less annoyance to the unused ^ye. It is a mere matter of taste, but manifestly, he nearer the source of light is to the mirror, the more brilliant will be the illumination of the pa- tient's pupil. Concentration of Light. — It should be re- membered that it is the patient's pupil, and the pupil only, that is under examination. The move- ments of the shadow on the face and on the cornea are valueless and only serve to mislead. Hence, within certain limits, the smaller and more con- centrated the light that is thrown into the eye the better. The average observer will find that a 10 mm. hole in the diaphragm of the lamp will UETINOSCOPY 93 o-ive him the best area of illumination at one meter. The power of the light must be at least ?j2 candle power; 64 c. p.-is better. Movements of Mirkok.— For the same reason, namely, that the pupil is the only part to be ex- amined, the tilting of the head and mirror from side to side to produce the movements of the shadow shonld be very slight, so as not to get the illumined disc outside the pupil. When the point of reversal has almost been found, many operators then change to a smaller hole in the diaphragm, so as to get a still more concentrated illumination for determining the last few degrees of error. Focal Length of Mirror. — It is important that the observer bo situated further from the pa- tient than the focal length of the mirror, else it is evident that the rays of light thrown by the mirror into the patient's eye will not have crossed be- fore they enter it, the image made on the pa- tient's retina will be an erect instead of an in- verted one, and the whole process will be reversed. This can be ensured by employing either a plane mirror, or one of not more than 25 cm. focal length f(U- one meter -vork. •Six Meter Method. — Some operators prefer to work at infinity, i. e., to place themselves at six meters from the patientV eye. This method has the advantage that no a iificial forcing of the point of reversal has to b« effected in order to make a normal starting poini", as in the one meter niclhofl. Witli llio mirror a- six meters emerging 94 REFRACTION parallel and divergent rays have not met by the time they reach the observer^ and convergent rays have always crossed; hence the shadow always moves against in emnietropia and liypcropia^ and with in any degree of myopia. The convex lens that reverses the movements- of the shadow is the straight measure of the hypermetropia and the concave lens that accom- plishes the same effect is the straight measure of tlie myopia. The method has many disadvan- tages, however, chief among which are the diffi- culty in arranging the light satisfactorily, and the inconvenience of being obliged to walk to and fro every time it is necessary to change the lens at the patient's eyes. Computing the Correction. — With the one meter method it is only necessary to remember that from the result obtained, i. e., from the lens by which the point of reversal was found, -[- 1 I^- must be subtracted, that being the refraction nec- essary to find the normal point. Thus if the net lens refraction used to find the point of reversal be — 2 D., then the necessary correction is — 2 D. less -f- 1 D., which is — 3D. If the net re- fraction used be -f- 2 D-? f^^^i^ ^^^^ proper correc- tion is + 2 D. less + 1 D., which is + 1. D. Edges of Shadow. — The clearer defined the edges of the shadow, and the quicker it moves across the field, the lower the degree of refractive error. Ill-defined, slowly-moving shadows denote n high degree of ametropia. Note how the shadow RE'liNOSCOi'Y 1)5 becomes sharper and moves more rapidly as the correcting Ions is more and more nearly ap- proached. Retinoscopy in Astigmatism. Althoiigli the estimation of spherical errors of refraction is the principal" object of skiaskopic Illustrating- why the shadow in retinoscopy is oblique in astigmatism, viz., because the image is oval shaped. examination, these are not the only forms of error that this test enables ns to detect and measure. Astigmatism also shows up under the shadow test with more or less definiteness. Oblique Edges. — If the eye is astigmatic the edges of tlie shadow made by rotating the mirror from side to side are usually oblique, no matter how the mirror is tilted. This is due to the oval shape of tlie image u])on the retina. If then the edges appear oMiciue it may 1)0 at once concluded that the eye is astigmatic. Method of Estoiatiox. — If the oblique-edged shadow moves against tlie miiTor the astigmatism 96 BEFEACTION is hypermetropic; if with the mirror_, it is myopic. The mirror must then be rotated at right angles and parallel to the two edges respectively, and csicb. meridian corrected separately^, the same as in hypermetropic and myopia. More detailed in- structions for this will be given when we come to consider astigmatism. Varying Velocity. — Sometimes, however, the edges of the shadow are vertical, and move hori- zontally, and the only way we are led to suspect astigmatism is because one edge is more distinct and moves more quickly than the other. For this reason we must never be content in performing skiaskopy to simply find the point of reversal by one angular rotation of the mirror only, or we may overlook an astigmatism. The point of re- versal having been found by rotating the mirror liorizontally, it should then be tested by rotating the mirror vertically, i. c., on a horizontal axis, and at one or two other angles, in order to make sure that the refraction is the same in all meri- dians. We shall refer to this whole matter at more length in a future chapter on Astigmatism. CHAPTER VIII. OPHTHALMOSCOPY. Ophthalmoscopy is considerably more difficult to carry out than retinoscopy, and it is question- able whether as valuable or as accurate informa- tion is obtained from it. Nevertheless, it has its place in refraction, and should be practiced in every case, especially as by its means a patho- logical condition of the eye is often detected wliich would otherwise escape attention. The ophthalmoscope is in principle a simple in- strument, being essentially nothing more than a small plane mirror, similar to a retinoscope, but a little smaller, mounted on a short stem handle, and provided with a central sight hole. However,- the various modern makes of ophthalmoscope are fitted witli a series of convex and concave lenses which are made to revolve by means of a wheel at the back of the mirror, so tliat any diopter lens, plus or minus, can be wheeled in front of the observer's eye, and thus obviate the rather cumbersome necessity of luounting lenses in the spectacle frame during this examination. For the satisfactory accomplishment of ophthal- moscopy it is ahnost indispensable to employ atropin, or a convex k'us, for the reflection from tlie mirror is exceedingly bright (the instrument being lu>ld very close to the eye), and complete relaxation of the acconinuxlation is a])solute]y essential to correct I'csults. d8 REFRACTION Ophthalmoscopy is divifled into two general methods, the (lirect and indirect. The Ophthalmoscope. Direct Ophthalmoscopy. The Principle.— Tl;e essential principle of OPHTHALMOSCOPY 99 oplitlialmoscop}' is the same as that of rctinoscopy, namel}-, the bringing to focus of tlie rays that emerge from ilie patient's eye. However, in letinoscopy the object of the test is to bring them to focus at a certain known distance in front of the patient's eye, and as tliis distance (one OISbKRVEK S KYK I'ATIENT's EYK Illustrates huw, in direct opthal, parallel rays from a normal eye at rest are focused on tlie retina of the observer's normal relaxed eye. meter) is too great to permit of the observer get- ting an intelligible image of the patient's retina upon his own, even when the rays have been properly focused, he is obliged to depend upon the movements of the image and its shadow to tell when the focus has heen found. In direct ophthalmoscopy the object is to bring the rays that emerge from the patient's eye directly to a focus on the retina of the observer by rendering Jjotli the observer's eye and the patient's eye perfectly normal as to refraction and relaxed, as to acconiniodation ; and the evi- dence that tin's has been accomplished is in the perception by the observer of a clear image of the patient's retina or fundus. 100 REFRACTION If the patient^s eye is normal in refraction and completely relaxed as to accommodation, then the rays that emerge from it are parallel; and if the observer's eye is similarly normal and relaxed, then it is adapted to receive and focus parallel rays; hence the details of the patient's fundus will be plainly pictured on the abservers retina, and clearly seen by him. We start out by taking care that the observer's eye is normal in refraction and relaxed as to ac- commodation; we then see that the patient's eye is relaxed as to accommodation, either by paralyz- ing the accommodation or having him relax it; then the only remaining factor in the equation is the patient's refraction — if this be normal the two opposing eyes will be identically alike, rays omero-inff from the one will be accuratelv focused on the other, and the observer will see the fundus clearly ; but if it be abnormal, so that rays emerg- ing from the patient's e3^e are either divergent or convergent, then in order to focus them on the observer's retina a convex or concave lens will be required equal to the degree of divergence or con- vergence in question, i. e., equal to the hyperopia or the myopia of the patient's eye. Technic of Direct ]\ri:TTiOD. — In the direct method, the light, which shonld l)e similar to that used in retinoscopy, is placed at the side of tlie patient's h(>ad, on tlie same side as the eye to be examined, about on. a level Avith his temple, so that no direct rays fall on his eye. If the opera- OPHTHALMOSCOPY 101 tor liavu any error of refraction he must correct it with }3roper glasses. The operator seats him- sc'ir i 111 mediately facing the patient^, on the same side as the eye under observation^ and uses the same relative eye hims(>lf. That is, in examining IHustrales the technic of direct oplithalmoscopy llie patient's right eye, he sits on his right side, and uses his own right eye. The observer's un- used eye should remain open during tlic observa- tion. The Oj)cral(ir, liolding llic iniri'or about 1-") cm. from tlie patient's eye, and relaxing his own ac- commodation, applies his eye to the sight hole 102 REFRACTION and throws tlie rclleclecl light into tho patient's pupil. As soon as the red fundus reflex is ob- tained, the operator, with his eve at the sight hole, gradually approaches nearer and nearer to the patient's eye, taking care to keep his own ac- commodation relaxed and to keep the light focused on the patient's pupil, until he is quite close to the ])atient's eye. Observation of Fundus. — If the patient's re- fraction is normal, the details of the fundus, i. e., the blood vessels, etc., should now be plainly seen, for if the patient's eye is completely relaxed, the rays of light leaving it are parallel, and if the observers. eye is similarly relaxed it is in a posi- tion to receive and focus parallel rays, hence the details of the patient's fundus should be plainly pictured on the observer's retina. Possible Errors. — If at the first attempt the fundus is not clearly seen, the observer should try two or three times, in order to make sure that it is not his own accommodation or his technic at fault. The operation is one which re- quires considerable practice and cannot be done hurriedly, but the results from a scientific point of view are well worth trying for. Beginners find it especially hard to keep their accommodation relaxed. This can most easily be done by imagin- ing the patient's fundus to be away at the back of his head^ «and gazing into it with that impres- sion. OlMrrilALMOSCOl'Y 103 Use of the Ophthalmoscope. In AiMETKorjA.— When quite conviuwd that his own technic or accommodation is not at fault, ;in(l the imago of the patient's fundus still re- jiiai us indistinct, tlie only conclusion for the ob- server to come to is tliat tlie rays emerging from \ho patient's eye are not parallel — they are either (li\i"rgcnt (liypcrniefroi)ia) or convergent (myopia) CONVEX LENS 0B8ER VEn'S EYE PATIENTS EVC IHustrates how, in direct ophthalmoscopy, divergent rays from the hyperopic eye are focused on the ob- server's retina (i. e., rendered equivalent to parallel rays) by a convex lens equal to the degree of di- vergence. and it will be necessary to interpose a lens, con- vex or concave as the case may be, whose dioptric strength just equals the degree of divergence or f onvcrgence of the patient's rays, in order to focus lliein on the observers retina. Whetlier this lens shall be convex or concave, and of what strength, can only l)e ascertained ])y trying. Ty llvi'i:i{Mi:'n:()i'i.\. — The observer should first wheel a weak convex lens in fi-ont of the sight liole of the instrument, and look again. If this helps, he should wheel successively stronger con- 104 REFRACTION vex lenses in front until he finds the strongest convex lens that gives him a clear view of the details of the fundus. This indicates that the patient is hypermetropic, i. e., the rays emerging from his eye are divergent, and the lens which focused them on the observer's retina, i. e., which OBSEB VERS EVE- lUustrates how convergent rays from a myopic eye are focused on the observer's retina (1. e., rendered equivalent to parallel rays) by a concave lens equal to the degree of convergence (myopia). made them equivalent to parallel emerging rays, is the measure and correction of the hyperme- tropia. In" Myopia. — If a weak convex lens makes the fundus still more indistinct, the observer should then try a concave lens, and if this helps he should wheel successively stronger concave lenses before the sight hole until he finds the weakest concave lens which gives a clear view of the fundus. This indicates that the patient is myopic, i. e., that the rays emerging from his eye are converg- ent, and the lens wliich focused them on the ob- server's retina, i. e., which made them equivalent to parallel emerging rays, is the measure and cor- rection of the myopia. OPHTHALMOSCOPY 105 In Astigmatism. — As in retinoscopy, so in di- rect ophthalmoscopy, astigmatism may be detected v.'ith some degree of definiteness. If the patient's eye is astigmatic, it is evident that the details of the fundus will be distinctly visible in one me- ridian while they are indistinct or invisible in another. In this case the operator finds the strongest convex or the weakest concave spherical lens which enables him to gain a clear view of the most indistinct meridian, and this spherical lens is the measure of the convex or concave cylinder which will correct the astigmatism. For reasons already explained, the axis of the correcting cylinder must be placed at right angles to the defective meridian. More detailed instruc- tions for this ratlier difficult operation will be found in tlic chapter on astigmatism. Observer's Kefraction. — The results of the direct metliod of ophtlialnioscopy, while not equal to those of retinoscopy, are yet very valuable and accurate when the process is properly carried out, and like retinoscopy, it has the advantage of being quite objective in character. Naturally, however, the success of tlie examination depends entirely ii])(tn llic observer liiiusclf. 'Vho fii'st thing lie Dinst be careful of is tliat liis nw n refraction is normal. If therefore he has any error of refrac- tion it is imperative that lie obtain and wear dur- ing the observation a proper correction of his own ametropia. Or, if he prefers to work without his 106 REFRACTION own correction, he must at least know just what his own error is, and make allowance for it in esti- mating the result of his observation, adding it to or subtracting it from the lens power needed to focus the patient's retina on his own, as the case may be. Observer's Accommodation. — The other prin- cipal factor in the success of tlie examination is that the observei-'s accommodation shall be re- laxed during the observation, else of course par- allel rays from the patient's eye will not focus on his retina. This is by far the most difficult item in the whole procedure, and is usually only acquired by long practice. Some operators find it helpful to wear an opaque disc or a strong- convex lens over the unused eye during exam- ination, so as to induce relaxation of the eye they are using; if this is done, however, it is nec- essary to use a very thin, closely-fitting mounting frame, so as not to interfere witli tlie hohling of the ophthalmoscope at the active eye. Allowance for Atropin or Lens. — If atropin or a convex lens has been employed to paralyze the patient's accommodation, an allowance of 1 D. must be made for the former in estimating the final result, or of the actual strength of the latter. Indirect Ophthalmoscopy. The Indirect MctJtod of ophthalmoscopy is still more diflicult of accomplishment, and more meager in results than the direct method. The Principle of the indirect method more OPHTHALMOSCOPY 107 nearly rot^einbk'S that of retiiioscopy ; that is to sav, it consists in an artificial forcin^r of the focus- Sliowing how, in indirect ophthalmoscopy, i)aralh'l rays from an emmetroi)ic eye are focused by the objec- tive lens the same distance beyond it, no matter how far from the eye the objective is held, line A B equal C D. (Image remains same size.) ing of the rays which emerge from the patient's e3-e. By means of a strong convex lens held close in front of the patient's eye the emerging rays are made to meet and cross a' very short distance in Showing how divergent rays, in indirect ophthalmos- copy from the hyperopic eye are focused further be- yond the objective the further away from the eye the objective is held. C D greater than A B. (Image gets smaller and smaller.) front of the eye, and the observer's eye is held sufficiently far from the patient that he receives 108 REFRACTION a very enlarged picture of a very small area of the patient's fundus. As it is necessary that this small area be definitely circumscribed, the disc is chosen as the point of observation. As the emerging rays have crossed long before thev reach the observer's eye, he sees a re-in- Showing how, in indirect ophthalmoscopy, converg- ent rays from a myopic eye are focused less and less beyond the objective, the further from the eye the ob- jective is held, C D less than A B. (Imag-e gets bigger and bigger.) verted, that is, an aerial image, formed between the lens and his eye. This aerial image must now be regarded as the object at which the observer is looking. Now, if the rays emerging from the patient's eye are parallel (emmetropia), no matter at what distance the convex lens is held, these parallel rays will focus at the same distance from the lens, namely, at its principal focal distance, and the size of the image will bo the same. If the emerging rays are divergent (livper- metropia), the further away the lens is moved from the point of emergence the more divergent the rays will be when they striken the lens, the OPHTHALMOSCOPY 109 longer they will take to focus, the image will be formed further and further beyond the lens, and will consequently be smaller and smaller. On the other hand, if the emerging rays are convergent (myopia), the further away the lens is moved from the point of emergence the more convergent the rays will be when they strike the ^^^SJ'^s Illustrates the technic in indirect ophthalmoscopy. lens, the more quickly they will focus, the image will be formed less and less distance beyond the lens, and will therefore be larger and larger. Technic of Indirect Method, — The light and general arrangement are the same as in the di- rect method. In this case, however, the ophthal- moscope, with the observer's 63^6 at the sight hole, is maintained at about 33 cm. from the patient's eye, and the observer can use the same eye in examining both those of the patient. A strong 110 REFRACTION convex lens is then held by the observer's unoc- cupied hand close against the patient's eye^ in sucli a way as not to obstruct the light from the lamp to the mirror. One of these strong convex lenses is included in the ophthalmoscope case, but the writer has usually found it too strong, and prefers a 9 D. or 12 D. from the trial case. View of the Disc. — As already intimated, in this method of examination it is not sufficient to get a view of the fundus of the eye. Having thrown the light into the pupil through the strong convex lens (called the objective), and obtained the red fundus reflex, the mirror must be focused so as to reveal the disc of the e3^e. This re- quires considerable practice, and can at first only be accomplished by simply shifting the mirror until the disc comes into view. The observer will know when he has focused the disc by the fact that the red fundus reflex will change to a glisten- ing buff color. This image of the disc (which, being viewed through a couatx lens, is a virtual not a real image), must be kept steadily in view and the objective lens slowly withdrawn in a perfectly straight line, away from the patient's eye toward tlie observer's. Use of the Ophthalmoscope (Indirect Method). In Emmetropia. — If upon withdrawal of the objective the image of the disc remains the same size, the eye is normal. OPHTHALMOSCOPY 111 In Hypermetropia. — If the image diminish in size^ tlie eye is hxpermotropic. The greater the diminution the higher the degree of hyperme- tropia. Convex lenses shoukl tlien he wheeled be- fore the siglit liole nntil the strongest one is found wliicli will cause the image to remain sta- tionary in size on withdrawing the objective. This is the measure of the liypcrmetropia. In Myopia.— If the image increases in size, the eye is myopic; the greater the increase the higher tlie degree of myopia. Concave lenses must then be wdieeled before tlie sight liole until the weak- est is found wliicli will neutralize the change in the size of the image. 'I'll is is the measure and .correction of the myopia. In AsTJGMATis:\r. — If the image increase or di- minish 'in size in one direction only, the eye is astigmatic in the meridian at riglit angles to the direction in which the image changes. If it di- minish the astigmatism is hypermetropic, if it increase it is myopic. And that spherical lens, wheeled before the sight hole, Avhicli neutralizes it is the measure of the cylinder which, when placed with its axis at right ansfles to the defect- ive meridian, w'ill correct the astigmatism. The carrying out of this method of ophthal- moscopy is exceedingly difTicult, and requires long and careful practice. In the writers judgment it is neitlier so reliable nor so valuable as rhinos- copy. CHAPTER IX. CORRECTION OF HYPERMETROPIA. llypermetropia has already been described as that condition of the eye wherein parallel rays are bronglit to focus behind the retina. In other words, the antero-posterior diameter of the eye is too short in ])roportion to the refracting power V Showing- the effect of chromatic refraction in the hvperopic eye. The light lines represent the blue rays AA, the heavy lines the red rays BB. The former are nearer to focus at the retina than the latter, giv- ing a blue centre, surrounded by the red. of the eye, and the principal focal point is situ- ated back of the retina. Chromatic Test. — In the trial case, if it be a complete one, will be found what is termed a chromatic lens. This lens is a combination o^ cobalt lenses so arranged as to suppress all of the rays passing through them except the violet an. the red. The test is based upon the physical la\^ that the liigli-velocity waves, i. e., the violet, art refracted to a greater degree llinn tlie low-veloc ity, 1. 0., the red. 114 REFRACTION Now if the refraction of the eye be normal, the difference in the refraction of the two types of rays will not be sufficiently marked to produce any sensible chromatic aberration^ and the patient will, upon looking at a flame through the lenS;, see an ordinary white flame. If, however, his eye is hyperopic, the violet rays have not yet come to a focus by the time they reach the retina, the red rays are still further from focusing, hence the ap- pearance of the flame is a violet center with- a red margin. The effect of the cobalt upon the vio- let rays is to make them appear blue; so that, as a matter of fact, the hypermetropic eye sees a red margin ajound a blue center. In other refractive errors the chromatic lens gives other appearances whieh will be described under their proper cap- tions. The value of this test is of course limited to the mere detection of the error; it affords no basis for its estimation or correction. Distant Type Test. — The principle of the dis- tant type has already been explained. It con- sists of uniform black letters, so constructed that their horizontal and vertical dimensions subtends, at the distance at which they are designed to be used, the minimum visual angle. That is to say, two lines drawn from the extreme boundaries of the letters marked 6, at a distance of 6 meters from the eye, through the nodal point, will make with each other an angle of 1 deg., which, as previously explained, is the minimum angular dis- tance at which two points can be distinguished. CORKECTIOJN OF llYrEKiMli^TKOri.V 115 It cannot be too insistently borne in mind that, while we avail ourselves of this principle of acuity of vision for making refraction tests, the actual faculty of visual acuteness is a function of the brain, differing in different persons even when the refraction is uniformly normal, and it is not the visual acuity which we are testing, but the ability of the eye to focus on its retina the parallel rays which proceed from the test type. We make use of the minimum visual angle simply because it affords the most delieate test of refractive ability. If the refraction of the eye is normal, then, it ought to be able to distinguish clearly distant type jSTo. 6 at a distance of 6 meters, because rays proceeding from this distance are parallel when they reach the eye, and the eye at rest is normally able to focus parallel rays on its retina. But a hypermetropic eye can also read this type at this distance, because, although in a state of rest it is unable to focus parallel rays, it can do so by using some of its accommodation. How then shall we differentiate between the normal and the hypermetropic eye? By a very simple and logical test. Tlie normal eye, reading No. 6 type at G meters, lias its accommodation completely re- laxed, i. c., it has assumed the least convex form of wliicli it is capable. Thprefore if wc now mount a woak convex Umis before it, thereby has- tening the focusing of the ra^s a little, the normal eye Ijas no ineans of adjusting itself to the new focus, and its vision is blurred. But the hyper- 116 BEFK ACTION metropic eye^, reading No. G type at 6 meters, is employing some of its accommodation; hence it we place a weak convex lens before it, thereby hastening the focusing and bringing the focal point forward a little, it can, by relaxing its ac- commodation, still read the type quite well ; indeed it does so with a feeling of greater easiness than before. The rule, then, is that if the vision is impaired by a weak convex lens the eye is normal, if not it is hypermetropic. Measure of Hyperopia. — Now if we keep try- ing stronger and stronger convex lenses until we find the one beyond which any further increase in strength blurs the vision, we have found the convex lens which renders the rays equivalent to parallel ones, and puts the eye in the position of a normal eye. This lens, then, is the measure of the hyperopia of the eye, and will be the proper correction. Gradual Relaxation. — Practically, however, the best procedure, after once ascertaining by a weak convex lens that the eye is hyperopic, is to at once over-correct the error with a strong con- vex lens, and then gradually reduce its strength by mounting concave lens before it, until the pa- tient can just read No. 6 at 6 meters. The net convex correction then before the eye is the measure of its hyperopia. The advantage of this plan over that of trying successively stronger con- vex lenses is thnt it first completely relaxes the cil- COKUECTION OF llYPEK.METKOriA 117 iary iiiuscle, and then gradually finds the point where is begins to contract, which is more relia- ble than gradually inducing it to relax. Example. — For example, suppose -\-4: D. over- corrects the hypermetropia and blurs the vision. Beginning witli — 1 D., put up in front of the -]-4 D. lens a series of successively stronger con- cave lenses until No. 6 can be read at six meters. Suppose that — 2 D. makes this possible. Then the correction is +4 D. less —2 D., that is, +2 I). Retinoscopy IX Hypermetropia. — Under re- linoscopy the rays emerging from the patient's eye are divergent, and will not have met by the time they reach the observer's eye at one meter, hence the shadow will move against the mirror. Xeither would they have met had they been parallel, i. e., if the patient's eye had been normal. But if they were parallel a convex lens of 1 D. before the patient's eye would bring them to focus at one meter, i. e., at the observer's eye, and give the point of reversal at that point ; whereas if they are divergent, a convex lens of 1 D. will not be suiticient to focus them at the observer's eye. The rule, then, is that if the shadow move against the mirror, and a convex lens of 1 D. does not give (lie point of reversal, the eye is hyperopic. We must tlicn try successively strong- er convex lenses until we find the one which brings the point of reversal at one meter, and the strengtli of til is lens in excess of 1 D. is the measure of divergence of the emerging rays, i. e., of the hy- 118 KEFRACTIOX peropia of the eye, and the necessary correction. Example. — The shadow is seen, to move against the mirror. A convex lens of 1 D. still leaves it moving against. A convex lens of 2.25 D. just brings the point of reversal at the observer's eye, so that by moving slightly backward or forward it is made to move with and against. Then -|-2.25 D. less +1 D., or -|-1.25 D., is the measure of the hyperopia and will correct it. Direct Ophthalmoscopy in Hypermetropia. — Under the direct method of ophthalmoscopy the rays emerging from the patient's e3^e are diver- gent, and the normal unaccommodated eye of the observer cannot focus them upon the retina, and therefore cannot get a clear view of the patient's fundus. If a convex lens wheeled in front of the sight hole improves the image we know that the emerging rays are divergent, i. e., the patient is hyperopic. and the strongest convex lens which enables the observer to get a clear view of the patient's fundus, i. e., which renders the diver- gent emerging rays equivalent to parallel rays, is of course the measure of their divergence, i. e., of the hyperopia of the eye, and its correction. Example. — Under direct ophthalmoscopy, with no lens at all, the observer cannot ^ee the fundus of the patient's eye. A weak convex lens improves it, and a -|-2 D. is the strongest lens with which a clear view is obtainable. Then the pa- tient has 2 D. of hyperopia and a +2 D. lens is his proper correction. {■()i!i;i;( rrox OF iiyimikmki koima 11^ Indirect OriiTiiALMOscoPY in Hypermetro- PiA. — ^IJnder tlic indirect method, the rays from the patient's eye, being divergent, will strike the ob- jective lens at a more and more divergent angle the further away the objective is held from the patient's eye, and will therefore be brought to focus further and further beyond the focal length of the objective lens, making a smaller and smaller image. We now mount before the patient's eye suc- cessively stronger convex lenses until we find the strongest one which renders the image the same size, no matter at what distance the objective is held; that is, the size of the image neither in- creases nor diminishes as we withdraw the ob- jective. This means that the convex lense in question has rendered the emerging rays just par- allel, and is therefore the measure of their di- vergence, i. e., of the hyperopia of the eye, and will correct it. Example. — Under indirect ()})litlialmoscopy the image of the lens is found to diminish in size as we withdraw the objective. Under successive- Iv stronger convex lenses mounted before the eye tlie diminishing becomes less and less, until with a convex lens of 2 D. the image remains the same size as we withdraw the objective. Then the eye has 2 D. of hyperopia, and a +2 TX lens is its proper correction. CHAPTER X. COKRECTIOX OF MYOPIA. Myopia^ as already explained, is that condition of the eye in which parallel rays are brought to focus in front of the retina. In other words, the antero-posterior diameter of tlie eye-l)all is too A B- A B Showing the effect of chromatic refraction in my- opia. The blue rays (light lines AA) have focused and crossed by the time they reach the retina, while the red rays (heavy lines BB) have just come to fo- cus, making a red centre surrounded by blue. long in proportion to tlie refracting power of the eye, and the principal focal point is situated in front of tlie retina. Chho:\[atic Test. — In the niyojur, or long eye, the violet ra3's, being refracted more quickly than the red rays, have already come to a focus anil have crossed by tlie time tlicy reach the retina; the red rays, Ixung U'ss aiVected by refraction, liave either not yet met or have just come to a focus of the retina. Hence llie apjX'arance of tlie flame is that of a violet (or blue) margin annind a red center. This appearance is diagnostic of (lie my- opic eye. 122 HKH{A( TIOX Distant Type Test for Myopia. — The myopic eye is unable to distinguish the letters of N^o. 6 test type at 6 meters, because the parallel rays which proceed from those letters are focused be- fore they reach the retina, and as the patient's eye is already in a state of rest, i. e., it has as- sumed the least convex form of which it is capable, it has no further means of adjusting itself to the too-far-forward focus. It is true that an astigmatic eye is also unable to distinguish No. 6 type at 6 meters, because of its inability to focus the parallel rays which enter it along its defective meridian, and it is some- what difficult to differentiate at once between a simply myopic and an astigmatic eye. In astig- matism, however, the ability to decipher the type is as a rule not so complete as in myopia; the patient is generally able to read the type, with an effort. And if, in addition, the spokes of the as- tigmatic chart appear all black alike to him, then it is pretty safe to proceed as if the error were simple myopia. But the tests for astigmatism, hereafter to be described, should in every case be fully applied. Lenses foh ]\Iyopia. — A convex lens held be- fore the myopic eye of course renders tlie vision still less distinct, because it still further hastens tbe focusing of the parallel rays, and therefore makes the circle of diffusion produced by the crossed rays on the retina still larger in area. A weak concave lens, on the other hand, improves the rORHK( I ION Ol MVoriA 123 vision, because it delays (lie focussing of the rays, thus lessening the area of the circle of dif- fusion. We then try successively stronger con- cave lenses, until we find the weakest concave lens with which the patient can clearly decipher No. 6 type at 6 meters. This is the lens which renders the rays, as they strike the myopic eye, equivalent to i)ai'allel rays entering a normal eye, and is therefore the measure of tlic myopia of the eye, and will be the proper correction. Eetinoscopy in Myopia. — Under retinoscopy the rays emerging from the patient's eye are con- vergent. If the degree of myopia is a high one, i. e., if the emerging rays are very convergent, they will have met and crossed before they reach the observer's eye at one meter distance; but if the degree of myopia is a low one, i. e., if the emerg- ing rays are only slightly convergent, tliey will not have met by the time they reach the observ- ers eye, but will meet before reaching infinity ((> meters). In the latter case, i. e., in low degrees of myo- pia, the shadow will l)e seen to move against the mirror, the same as in cnimetropia and hyperopia, but it will fake a convex lens of less dioptric strength to get the ])()iiit of reversal a I 1 meter than is required to bring parallel rays to a focus at that point; it will, in fnct, require a convex- lens just as much less than 1 I), as the rays are convergent, hence the amount that the reversing 124 KEFR ACTION lens is in default of 1 D. is the measure of the myopia of the eve. In high degrees of myopia, the shadow will at once be seen to move with the mirror, because the emerging rays have alreadv crossed by the time they reach the observer's eye at one meter. It will now take a concave lens to bring the point of reversal at this distance; that is, the focussing of the rays must be hindered, and brought for- ward to the 1 meter point. We therefore find the vreakest concave lenst which just reverses the shadow. [N'ow we know that if the emerging ravs had just met at 1 meter the eye would be just 1 D. myopic. The concave lens, therefore, which delays the focussing so that they meet at just 1 meter is the measure of the convergence of the emerging rays in excess of 1 D. Hence the strength of this concave lens, added to 1 D., is the total measure of the myopia of the eye, and will be its proper correction. Examples. — Shadow is seen to move against the mirror. A 0.25 lens just reverses the shadow movement. Then the eye is myopic to the degree that 0.25 D. is less than 1 D., i. e., it is myopic 0.75 D., and a —0.75 D. lens will correct the myopia. Shadow is seen at once to move with the mir- ror. A — 1.00 D. lens just reverses the move- ment. Then the eye is myopic to the degree that 1 D. is in excess of 1 D., i. e., it is myopic 2 D , and a — 2 D. lens will correct ii COKRKCTION OF MYOl'lA 125 Direct Ophthalmoscopy in Myopia. — Under tlie direct nietliod of ophthalmoscopy the rays emerging from the patient's eye are convergent, and the normal unaccommodated eye of the ob- server cannot focus them upon his • etina, hence cannot get a clear view of the patient's fundus. A convex lens wheeled before the sight hole makes matters worse, as it converges the rays still more. We therefore try concave lenses, and the weakest concave lens which enables the observer to get a clear view of the patient's fundus, i. e., which renders the convergent emerging rays equivalent to parallel rays, is of course the measure of their convergence, i. e., of the myopia of the eye, and its correction. Example. — Under direct ophthalmoscopy, with no lens at all, the observer cannot see the details of the patient/s fundus. A weak convex lens makes is still more indistinct, but a weak concave lens improves it, Mid — 1.50 D. is the weakest con- cave lens with which a clear view is obtainable. Then the patient has 1.50 D. of myopia, and a — 1.50 D. lens is his proper correction. Indirect Ophthalmoscopy in Myopia. — Un- der the indirect method, the rays from the pa- tient's eye, being convergent, will strike the ob- jective lens at a more and more convergent angle the further away the objective is held from the pa- tient's eye, and will therefore be brought to focus nearer and nearer within the focal length of the objective lens, making a larger and larger image. 12G KEl'KACTION We now mount before the patient's eye succes- sively stronger concave lenses until we find the weakest one which renders the image the same size at whatever distance the objective is held; that is. the size of the image neither decreases nor in- creases as we withdraw the objective. This means that the concave lens in question has rendered the emerging rays, just parallel; it is therefore the measure of their convergence, i. e., of the myopia of the eye, and will correct it. Example. — Under indirect ophthalmoscopy the image of the disc is found to increase in size as we withdraw the objective. Under successively stronger colicave lenses the enlarging is seen to be less and less, until with a concave lens of 1.50 1). the image remains the same size as the ob- jective is withdrav/n. Then the eye has 1.50 D. of myopia, and a — 1.50 D. lens is its proper cor- rection. CHAPTER XI. CORKECTION OF ASTIGMATISM. Astigmatism is by far the most troublesome to estimate and correct of all the commoner refrac- tive errors of the eye. It has already been de- scribed as that condition of tlie eye in which the curvature of the cornea is not the same in all of its meridians, the result being that while the rays which enter the normal meridians are prop- erly focussed on the retina, those which enter tlirough the defective meridians form a line of unfocussed rays, either in front of or behind the retina, and a diffused indistinct image is seen. Chief Meridians. — As we have already seen, the meridians of greatest and lease refraction are practically always at right angles to each other, and are called the chief meridians. Prismatic Test. — In order to carry out the chromatic test for astigmatism it is necessary to place the white flame behind a screen having a small circular hole about G mm. in diameter im- mediately in front of the light,, so that not too many rays may enter the eye. The chromatic lens is then mounted before the eye. Of the rays which pass through- the meridian of greatest re- fraction the bhu} rays come to focus first, while of those which enter the meridian of least re- fraction the red rays come to focus quickest, and as these two meridians are at right augles to each other the two colors appear drawn out at right 128 KEFKACTION angles corresponding to the chief meridians. This test only applies in high degrees of astigmatism. Test Type in Astigmatism. — It is manifest that the rays of light proceeding from the distant type cannot be correctly focussed upon the retina by both of the chief meridians at once. If one is adapted to focus the ra3^s, the other is not. Hence the distant type cannot be seen with distinctness by an! astigmatic eye. This inability of the eye to see the distant type clearly is not in itself a proof of astigmatism^ for we have seen that it pertains to the myopic eye also. But if the dis- tant type can be read clearly it is sufficient evi- dence that there is no astigmatism. Wheel Test ix Astigmatism. — We have al- ready seen that the simplest method of detecting the deficiency of one of the meridians of the eye is to test the ability of the eye to discern a series of black lines arranged to correspond with the ocular meridians. We therefore instruct the pa- tient to look at the wheel chart and pick out the spokes that appear to him the faintest. These spokes represent the meridian at right angles to the defective meridian of the eye. JN'ow it is evi- dent that whatever spherical lens will focus the rays in this meridian, i. e., bring out the faint lines clear and l)lack, will be the measure of the character and degree of defect in the meridian. We therefore instruct the patient to look at the wheel and pick out the spokes that appear the faintest to him. Begin wit1i a weak convex spher- CORRECTION OF ASTIGMATISM 129 ical lens and see if it improves these faint lines. If it does, mount a strong convex spherical lens_, as in the nicisiircmcnt oL' liypormetropia, and gradually nMlucr it with coiujne lenses, until the spokes of the wheel which were faintest to the naked eye are clear and distinct. The convex splierical correction necessary to accomplish this is the measure of the convex cylinder which, with its axis at right angles to the defective meridian, w ill correct tlie astigmatism. If a convex lens does not improve the weakest lines, but ])lurs them, then try a concave spher- ical lens until the weakest is found which brings the faintest lines out clear and black. This lens is the measure of the concave cylinder which, with its axis perpendicular to tlie defective merid- ian, will correct the astigmatism. As this method is applicable only to simple astigmatism, i. e., astigmatism in which only one meridian is defective, it may be impossible to get any results from it, in which case the observer must proceed to other methods. The Stenopaic Slit ix AsTiGMATis:\r. — In the trial case will be found on opaque disc wn'th n narrow slit in it. This is called the stenopaic slit. The effect of this disc, when mounted before the eye, is to cut out all the rays of light except those which enter along the line of the slit, and liciice, ])y turning the disc so as to make the slit coincidi' with the various meridians of the eye, all I'ays will b(^ exchided exce])ting those which cnt<'r along 130 REFRACTION the meridian coinciding with tlie direction of th.o slit. Now if the eye he equally convex in all of its meridians^ vision will he equally clear at what- ever angle the stenopaic slit 1 e turned, because any meridian of the normal eye will focus tlie The Stenopaic Slit. rays ujoon the retina with equal correctness. If, however, the meridians of the eye are not equally convex, then it is manifest that when the slit is turned to coincide respectively with the two chief meridians the vision will be correspondingly most and least distinct. It must always be borne in mind, however, that the meridian whose condition is made manifest by the slit is the meridian at right angles to the direction of the slit. Con- versely, it must be remembered that, in estimating and correcting astigmatism with the stenopaic CORRECTION OF ASTIGMATISM 131 i^\'\[, the axis of the correcting cylinder should be at i-iglit angles to the defective meridian, i. e., parallel to the slit. ^lounting this disc, then, on the trial frame, we turn it around until the angle of the slit is found which gives the patient the best possible vision of the distant type. If this vision is 6/6, the refraction of the patient^s best meridian is normal, and if he has astigmatism at all it is a case of simple astigmatism. Now turn the slit at right angles to its best position. This gives the eye its worst vision. If at this worst angle the vision is still 6/6, the eye is normal in both meridians; no astigmatism ex- ists. If not, we find the spherical lens (either convex or concave) which enables the patient to read No. G type, and this is the measure of the astigmatism of the meridian at right angles to the slit. A cylindrical lens, of the same curva- ture and strength, with its axis at right angles to the defective meridian, i. e., with its axis par- allel to the slit, will correct the astigmatism. Simple Astigmatism. — If, with tlic slit at its best angle, vision is 6/6, then only one meridian of the eye is astigmatic (simple astigmatism). If, with the slit at its worst angle, a convex spherical lens is required to make vision 6/6, this means thatf the meridian at right angles to the slit is hypermetropic. The strength of the lens iMii ployed is tlie measure of the hypcrmetropia of the defective mcriflian and of the convex cvlin- 132 REFRACTION der which, with its axis at right angles to the defective meridian (parallel to the slit), will cor- rect it. - If, on the other hand, with the slit at its worst angle, a concave spherical lens is required to make vision normal, then the meridian at right angles to the slit is myopic. The strength of the concave lens employed is the measure of the myopia of the defective meridian, and of the concave cylinder which, with its axis at right angles to the defective meridian (parallel to the slit) will correct it. CoMPOLXD AstiCt.viatism. — If a lens was needed to make the vision 6/6, hoth with the slit at its best and at its worst angle, then both of the meridians of the eye are astigmatic (compound astigmatism). If the lenses requiredl to correct the best and worst meridian are of the same curva- ture (i. e., both plus or both minus) the astigma- tism is a compound hypermetropic or compound myopic astigmatism, as the case may be. In this case we ascertain, by means of the stenopaic slit, the degree of error in each of the two chief meridians, one of which will of course be greater tlian the other, for if tlie two chief meridians were equally defective the case would not be one of astigmatism at all, but of simple jiypermetropia or myopia. In other words, we as- certain the respective strengths of the lenses, con- vex or concave, which raise vision to normal with the slit at the ])ost and worst angles respectively. Now it is evident that if the worst ineridian CORRECTION OF ASTIGMATISM 133 be corrected to the extent of the difference be- tween it and the best meridian, tlicn both merid- ians will be equally defective to the extent of the best meridian, and we shall have reduced the case to one of simple hypermetropia or myopia, as the case may be, equal to the defect of the best merid- ian. Having found the respective degrees of error of the two chief meridians, therefore, the proper way to correct it would be to prescribe a cylin- drical lens, equal to the difference between the two meridians, with its axis at right angles to the worst meridian — this makes the defects of the two meridians equal, and renders the case one of simi)le hyperopia or myopia — and then prescribe a spherical lens equal in strength to the error of the best meridian. This corrects the remaining hyperopia or myopia, and renders the eye normal. Example. — The slit at 90° (vertical) gives the best possible vision, but this vision is not Q/(d, and requires a -|-1 D. spherical lens to make it normal. This means that the horizontal meridian of the eye is hyperopic 1 D. At 180°, the worst possible angle, it requires a +2.50 D. spherical lens to make vision 6/0. This meanS; that the vertical meridian is hyperopic 2.50 D. Here the difference between the two meridians is -["1.50 D. Hence a -f 1.50 D. cylinder, with its axis at right angles to the worst meridian, i. e., axis at 180°, will neutralize the astigmatism, makino- the two f'hief rneridiaps both hyperopic 1 D., and a spher^ 134 REFRACTION ical lens of +1.00 D. will then correct this re- maining hyperopia. With both meridians myopic, the result woukl work out precisely the same in concave lenses. AlixED Astigmatism. — This is the type of astig- matism in which the two chief meridians are both defective, but of different curvature, one being liyperopic and the other myopic. In this case we ascertain by means of the stenopaic slit the char- acter and degree of the defect in each meridian. In other words we find the respective strengths of the lenses, convex in one meridian and concave in the othei% which raise vision to 6/6 with the slit at its best and worst angle respectively. Now it is manifest that if either of these merid- ians be corrected to the extent of the sum of the errors of the two meridians, it will be over-cor- rected to the extent that the other is defective, and the case will tlien be reduced to one of simple hyperopia or nw opia ; as the case may be, equal to the defect of the uncorrected meridian. Having found, therefore, the respective degrees of error of the two chief meridians, the proper way to correct them is to prescribe a cylindrical lens, equal in strength to the sum of tlie two errors, and of appropriate correcting curvature, with its axis at right angles to the meridian requiring that curvature lens — this over-corrects the error of this meridian nnd renders it defective to the same de- gree and in the same direction as the opposite CORRECTION OB^ ASTIGMATISM 135 meridian, rendering the case one of simple hyper- opia or myopia — and then prescribe a spherical lens equal in strength and curvature to the error which now exists similarly in both meridians. This corrects the remaining hyperopia or myopia, 51 nd renders the eye normal. J n these cases it is customary to over-correct with the cylinder the meridian of least convexity, i. e., the one which requires the plus lens. How- ever, this will not be found practicable in all cases, as instances will be found wherein, for some reason or other, this form of correction does not give as good vision as the reverse. Example. — The slit at 90° (vertical) gives best possible vision, but this vision is not Q/Q, and re- quires a +1 D. spherical lens to make it normal. This means that the horizontal meridian is hyper- opic 1 D. At 180° (horizontal), the worst pos- sible angle, it requires a — 1.50 D. spherical lens to make vision Q/Q. This means that the ver- tical meridian is myopic 1.50 D. Here the sum of the two errors is 2.50 D., and the horizontal meridian is the meridian of least convexity. Hence a cylinder of -f 2.50 D., with its axis at right angles to the horizontal meridian, i. c., axis at 90°, will over-correct that meridian so that it becomes myopic 1.50 D., the same as the vertical meridian. The two meridians are now both myopic 1.50 D.. a case of simple myopia, and a spherical lens of — 1.50 will correct this remaining myopia. 136 REFRACTION Retinoscopy in Astigmatism. Straight and Oblique Edges. — In an astig- matic eye the image of the light which is made upon the retina is oval, with its greatest and least diameters corresponding to the least and great- est curvature of the cornea. This oval may have its edges vertical, horizontal, or oblique, accord- ing to the nature of the astigmatism. When the astigmatism is either directly with or directly against the rule, i. e., horizontal or vertical, the edges of the shadow are vertical and horizontal, and move horizontally or vertically across the pupil, giving the effect of a straight shadow as in hyperopia or myopia. When the astigmatism is oblique the edges of the shadow are oblique, and move obliquely across the pupillary field no matter how the mirror is rotated. Hence, while oblique shadows, when found, are diagnostic of astigmatism, straight sha- dows do not by any means exclude astigmatism. Testing Both Meridians. — The only safe method to pursue is to make a routine practice, in cases of vertical or horizontal shadows, of testing both meridians with the retinoscope. Example. — We find the shadow vertical, and moving against tlie mirror. We first test tlie vertical meridians, rotating the mirror liorizontal- ly, and find the strongest convex lens which causes the shadow to move with the mirror. Suppose this is found to be +2 D. We then test the horizontal meridian, rotating the mirror vertically, and find CORRECTION OF ASTIGMATISM 137 the strongest convex lens which makes the sliadow move with the mirror is -\-2 D. We know that the meridians are the same and the case is one of simple hypermetropia. Suppose^ however^ in testing the horizontal meridian we had found that -f-i I^- ^^'^^^ the strong- est convex lens making the shadow move witli the mirror. Then we have a compound hypermetropic astigmatism, 2 D. in the vertical meridian and 4 Illustrating the way the mirror must be held and rotated when the shadow is oblique, viz., perpendicu- lar to the edge of the shadow. D. in the horizontal, needing for its correction -|-2 1). cylinder, axis horizontal, and -\-2 D. spherical (i. c., tlie difference between the two- meridians). WJiere the shadow is oblique, we know at once that astigmatism exists, and proceed to eslini;iit' it as above, only in this case wc rotate the inin-or at right angles to the edge of the shadow and then parallel to it. and the axis of the correcting cylin- der is placed accordiiiLrly. Thus, if the edge of the shadow is tilted at 'iU^ tlie cylinder is pre- scribed with its a.xis at 110°, which is precisely at right angles thereto. 138 REFRACTION Astigmatic Band. — In cases of astigmatisiii in which one of the meridians has received its spher- ical correction the reflection of the retinoscope is usually seen as a band of light lying across the pujiil^ whose axis subtends the uncorrected merid- ian. Hence in refracting the horizontal meridian^ if, as soon as the movement of the shadow in this meridian is neutralized, a band of light is seen \\ing horizontally across the pupil, the observer knows that the eye is astigmatic, and that the ver- tical meridian is the astigmatic one, and should proceed to refract that meridian. In cases where the difference between the two meridians is very slight this band does not of course appear until the correction of the merid- ian first tested is very near completion, and it is then seen very faintly. For this reason, among others, it is advisable, as stated in a previous lec- ture, to reduce the area of the illuminating disc when one gets very near to the neutralizing point. Correction, — Having ascertained, by neutral- izing the movements of the shadow in the two chief meridians, the nature and degree of error in each, the correction is then made in the same man- ner as already des(]'il)(Ml under the stenopaic slit, namely by a combination of cylinder and sphere arranged as already explained. Betiiioscopij is a verv useful and rapid means of estimating and correcting astigmatism with experienced refractionists, who are usually able to do it by this method with great accuracy. But CORRECTION OF ASTIGMATISM 1^0 beginners will find it somewhat diiiicuit, and no one should ever rely on it without confirmation by some other metliod. Ophthalmoscopy in Astigmatism. Under ophthalmoscopy, direct method, the plan ite to lind the spherical lens, convex and concave, which affords a clear view of the principal me- ridians, testing one at a time. By the indirect method the disc is oval instead of circular, and increases or decreases in one di- rection more than another on withdrawing the ohjective. The spherical lens, convex and con- cave, which corrects this is the measure of the astigmatism in each meridian, which must then be corrected as already described. This method of estimating astigmatism, how- ever, is so diflicult and untrustworthy that it is of value only from a scientific or confirmatory point of view and i< not recommended for any other purpose. The stenopaic slit will generally be found, in ordinary cases, the most serviceable and prac- ticable method of working out astigmatism, and the author strongly commends it to hi? readers as superior to all others. All findings should. Imw- ever. be confiinu'd liv all tlie means at one's com- mand. CHAPTER Xn. PRACTICAL INSTRUCTIONS. General Procedure. Seat the patient in a good light, as nearly as possible six meters from the distance type, using the type-line marked No. 6 as a standard of dis- tance vision. ' • (N. B. It is not absolutely essential that this distance and type number be ohserved, as any dis- tance beyond six meters renders the rays parallel. The type-line used as a standard must correspond with the distance/and if less than 6 meters allow- ance must be made for the accommodation that is in force in the patient's eye. But No. 6 at 6 me- ters is the generally used standard.) If a good uniform natural light cannot be ob- tained, the test card should he illuminated from below with a good artificial light. Indeed, many operators prefer artificial light because of its greater uniformity and controlability. Separate Tests. — Tost each eye separately, shutting the other out of vision meanwhile by means of the opaque black disc found in the trial case, or by means of a strong plus lens. Some op- erators prefer the latter because it induces relaxa- tion of the eye under test. But, whatever is used, it is important that the untested eye be covered and not simply closed by the patient, because the act of cldsinir ihe eve tends to interfere with aeeonimoda- D-60. O E B T D— 60 C T D O E D-40 r E D O T c I>— 80. L N F C T D D-20 OPDLEFNC OETNFLDBP D-10. SOBSTKZ.CPF Test Type CliarL. In the above the "D" means distance and tlic number is feet. PRACTICAL INSTRUCTIONS 143 tion and innervation ui" ilic oilier eye. Jt is cus- tomaiy to begin with tlic right eye. Make a careful and systematic record of the findings in each test as proceeded with, designat- ing the right eye as E. V. and the left as L. V. 1. PiN-IIoLE Test. — Always begin with the pin-hole test. Instructing the patient to put on the trial spectacle frame, mount before the eye the black disc with the pin-hole in the center which is in every trial case. This has the same effect on the eye as a cutting out light from a camera, permitting rays to enter only along its central axis, and should therefore improve the vision of a healthy eye. If the pin-hole disc does not improve the vision, there is some physical trouble with the eye-ball, and an oculist must be consulted. If it improves the vision, the trouble complained of is an error of refraction and further tests should be proceeded with. 2. PiJiSMATic Test. — If your test case con- tains a chromatic lens, the prismatic test may next be employed with each eye separately. A white light is placed 6 meters from the pa- tient, and the chromatic lens mounted before his eye. If the patient sees a red rim around a central blue light, the eye is hypermetropic. If he sees a blue rim around a central red light, the eye is myopic. A screen is now placed before the light, with 144 REFRACTION a round opening 6 mm. in diameter immediately in front of the fiame^ and if the two colors, blue and red, are seen as bars at right angles to each other, tlie eye is astigmatic. 3. Distance Type. — The distance type should now be employed, each, eye being tested sep- arately. The patient is directed to look at the type card and tell how far down he can read. If at a distance of 20 feet lie reads type Xn. 20, Ills vision is expressed by the fraction 20/20. and is either normal or hypermetropic, Now mount a weak convex lens, say 0.50 or 0.75 D., before the eye, and if the vision is not blurred by it the eye is hypermetropic, and you may proceed to measure the hypermetropia by the methods before described. If a convex lens does injure the vision, the eye is probably normal and needs only rest and hy- gienic treatment. But no eye should be dismissed without other tests being made. If at a distance of 20 feet he cannot read type Xo. 20, l)ut can read, say ISTo. 40, his vision is ex- pressed l)y tlie fraction 20/40, and he is either my- opic or astigmatic — perhaps both. Wheel Test. — Xow instruct the patient to look at the fan-wheel which is at the top of the type card, and ask him if all of the spokes of the wheel look equally black to him. If they do, but he is si ill unable to read Xo. 20 lype at 20 feet, his error is simply myopia, and niav be measured and corrected as di'scrihed in a previous chapter. PRACTICAL INSTRUCTIONS 145 If the spokes of the wheel appear unequally black to him — one intensely black and another quite faint — he has an astigmatism, and must be dealt with as laid down elsewhere. Convex Lens for Relaxation. — If for any reason neither atropin nor any otlier drugs are available, a very satisfactory relaxation of accom- modation can be obtained by mounting a convex ]ens before the eye. Usually at 1.50 D. or 2.00 D. is about right, but the necessary strength varies with the patient. A sufficiently strong diopter should be used com- pletely to paralyze accommodation. When this method is used, the dioptrism of thei lens used must of course be added or subtracted from the result of the test, the same as in the case of atropin. Spectacles. It is highly important that the glasses them- selves be properly fitted to the patient's eyes, and the refractionist himself should attend to this fea- ture of the treatment. Tabulated instructions are usually found on the prescription blanks is- sued by optical firms for the proper measurement of the frames, and these should be fully and care- fully followed out in prescribing the glasses. From an optical standpoint the most important things to be observed are: 1. That the size of the lens is sufficient to cover the eve. Optician's prescription blanks usually 14(> REFRACTION designate this by a graded scale of sizes^, repre- sented by 0, 00, 000, etc. 2. That the center of the lens coincides ex- actly with the visual axis of the eye. This is in- sured by a proper measurement of the width be- tween the two pupil centers. For distance vision the lenses should then be made to stand perpen- dicularly; for near vision they should converge in accordance with the visual axes. 3. In myopia and astigmatism the lens should not be further than 13 mm. from the eye. Hy- permetropes, and especially presbyopes, may gen- erally suit their own comfort and convenience in this respect. 4. In astigmatism it is of course essential that the lenses always maintain the angle at which they are prescribed. To insure this, it is always advisable that astigmatic patients wear bow-spec- tacles. However, there are now in the market very improved makes of eye-glasses that provide for this necessity, and these may be worn in selected cases. Care must be taken that they do not be- come bent and out of shape, for this spoils the tilt of the cylinder axis and defeats the purpose of the lens. Latent Hyperopia. — It has already been seen that hypermetropes are obliged to use some of their accommodation for long distance vision. Hence there is always a certain amount of un- detected hyperopia, called latent hypermetropia which is made manifest only under atropin, when rRACTICAL INSTRUCTIONS 147 the ciliary muscle is completely relaxed, so that the results under atropin will be found to be about 1 D. more than without it. It, is advisable to provide for about one-half of this latent hyper- metropia in prescribing the glass. Hence if the tests have been made under atropin, about 0.50 should be deducted from the result in prescribing, and if made without atropin 0.50 should be add- ed, in prescribing the glasses. In hypermetrojyes, so long as N'o. 6 type can be read at 6 meters, the glasses need only be worn when near work is being done. If, however, even distant vision is defective, they should wear glasses to correct this. When hypermetropia is accom- panied by strabismus (squint) full correction (latent and manifest) should be worn constantly. Myopes whose myopia is of low degree may be given eye-glasses for use in distant vision, and be allowed to read and write without any glasses. In myopia of medium, degree they should wear their full correction constantly. To this general rule there are two exceptions: A. Where the myopia is of high degree the concave glasses diminish thq size of the retinal image so much that the patient brings the object close to his eye to make the image larger. In this case the purpose of the glasses defeats it- self, and it is wiser to gradually accustom the patient to his correction until his acuteness of vision is sufficiently improved to stand full cor- rection. 148 REFRACTION B, In cases of higli myojDia where the patient has got into the habit of converging in excess of his accommodation, full correction, while giving excellent distant vision, may cause him much pain when used for reading, and in this event he must be given full correction for distance and weaker glasses for reading, graduall}- increasing the strength of the latter until he can bear his full correction for both purposes. In these cases we subtract from the full cor- rection the lens whose focal length is the same as the distance at which he desires to read. Thus, suppose his full correction is — 9 D., and he wishes reading glasses for a distance of 33 cm. A 3 D. lens has a focal length of 33 cm. (the focal lengths of the lenses are marked upon the trial cases), and we therefore subtract 3 D. from — 9 D. and give him reading- glasses of — 6 D. Hygie>^ic Treatment. — Myopes should be carefully instructed in h3'gienic measures, even after correction. They should avoid long or strenuous convergence, and frequently rest the eye by looking into the distance. They should read and write in bold letters, with the paper at 33 cm. at least from the eye, and sedulously avoid the stooping posture when reading or writing, which induces congestion of the eyeball and ag- gravates the myopia. ^[alignant Progressive Myopia. — It should not be forgotten that myopia may be, and fre- qu(Mitly is. a progressive and malignant condition, PRACTICAL INSTRUCTIONS 149 and upon signs oi* such progression or malignancy a competent oculist should be consulted without delay. Astig malic jinHi'ids should wear their glasses constantly. If the astigmatism is associated with high degrees of hypermetropia or myopia, neces- Trial Case. sitating separate glasses for near and distant use, the full correction of the astigmatism should be put into both pairs of glasses. The astigmatism is due, as already seen, to the irregular shape of the cornea, hence it does not change \\ith accommodation or under any other conditions. 150 REFRACTION Diminishing Effect of Concave Lenses. — It should not be forgotten that convex glasses, by bringing forward the nodal point, enlarge the visual angle and so increase the size of the im- age, and concave glasses, by carrying back the nodal point, decrease the visual angle and so diminish the size of the image. This should be explained to patients, especially those wearing concave glasses, who are apt to complain that they do not see clearly with them, whereas the truth is they do not see as largely with them. Cardinal Points. Always have a good light, if possible behind the patient, in making the type tests. Always test each eye separately, by each method, excluding the other eye meanwhile by the opaque disc. Always begin with convex lenses. They relax the ciliary muscle. Never be satisfied with one kind of test. Con- firm it with other methods. Always relax the eye, if possible, in a patient under 30. Astigmatism in a young patient cannolt be properly estimated without relaxation. Look out for ciliary spasm. When the patient begins to get erratic in his replies — seeing first one thing and then another — stop testing and in- sist upon rest or use a convex lens. Have a system and follow it out. Nothing PRACTICAL INSTRUCTIONS 1.31 weakens a patient's conlidence and confuses the refractionist like aimless pottering. Having carefully attended to the correction of the refraction, pa}^ equally careful attention to the mechanical features of the glasses. They are of great importance, and often make just the dif- ference h( tween success and failure. CHAPTER XIII. STKABISMUS AND IMBALANCE. Strabismus. Strabismus is that condition of the eyes in which the yellow spots are not directed toward the same point in the object viewed. When the 'lines along which they are directed diverge from one another, the condition is called divergent strabismus ; when they converge toward each other, it is called convergent strabismus. In both cases there is donblc vision; in the latter variety there is direct double vision, i. e., the image seen by each eye appears to l)e on the same side as the eye perceiving it. In the former variety there is crossed double vision, i. e., the image seen by each eye appears to be on the opposite side from the eye which perceives it. This is easily diagnosed by mounting a colored glass before one eye and obsen^ing which image appears colored. Strabismus is real or apparent. Apparent strabismus is the effect of a disturb- ance in the relationship between the optic and visual axes, due to hypermetropia or myopia. The optic axis is a line drawn through the nodal point of the eye to the center of the cornea ; the visual axis is a line drawn from the object viewed through the nodal point to the yellow spot. The angle made, by these two lines is called the angle alpha, and in the normal eye is about 5°. 154 REFRACTION The Angle Alpha. — In myopia the nodal point is relatively further back than in the normal eye, and the angle alpha is thereby enlarged. This, by a confusion of judgment in the observer between the optic and visual axes, gives the pa- tient the appearance of having a divergent stra- bismus. In hypermetropia the nodal point is rela- tively further forward than normal, and the angle alpha is diminished, sometimes completely ob- literated, and occasionally the two axes cross, in which case the angle alpha is said to be negative. This gives the patient the appearance of having a convergent strabismus. It is of course highly important to be able to determine w^hether a given case of strabismus is real or apjDarent, and fortimately this is very eas- ily ascertained. We simply hold an object, such as a pencil, about a meter from the patient's eyes, directing him to keep looking at it, and gradually move it nearer to his eyes. If both visual axes continue to be directed toward the object the case is one of apparent strabismus. But if during the experiment one eye suddenly deviates, either in- ward or outward, then we have a case of real strabismus, which must then be diagnosed by fur- ther means. Beal strabismus is due to a defect in the func- tion of the recti muscles of the eye, and may be generally classified into Concomitant and Par- alytic strabismus . Properly speaking, these two varieties arc but different deirrees of the same STRABISMUS AND IMBALANCE 155 troubk', but for practical purposes we recognize as concomitant strabismus those cases in which the deviation of the squinting eye is constant, and its range of motion is practically equal to Illustrating convergent strabismus. that of the sound eye, while by paralytic strabis- mus we understand a condition in which the par- alysis of the recti is such as to seriously interfere with motion of the deviating eye. Diagnosis of Strabismus. — In some cases it is possible to determine roughly between concomi- Illustrating divergent strabismus. taut and paralytic strabismus, by simply having the patient follow with his eyes a small object moved in various directions, and observing whether or not the deviating eye follows the motions of the sound one. Bnt this is not at all a reliable loO REFRACTION test. The only trustworthy way of diagnosing the nature of the squint is as follows: First instructing the patient to look straight in front of him, make a mark on the lower eye- lid with a piece of crayon indicating the position of the pupillary center in each eye. N'ow cover one eye in such a manner as to ob- struct the patient's view, but so that the observer can watch the movements of the covered eye. In- struct the patient to look steadily with his uncov- ered eye at a small object held a short distance from him. (This is called "fixing" the object.) The covered eye will be seen to make a sudden movement inward or outward, and the extent of this movement should be indicated by another chalk mark on the eyelid. Now cover the other eye and repeat the experi- ment. If the deviating movements made by the two eyes, as indicated by the chalk marks, are equal, the strabismus is concomitant. If one eye made a greater deviation than the other, the stra- bismus is paralytic, and that eye which made the greater deviation is the sound e^-e. The deviation made by the squinting eye in this experiment is called the primary deviation, that made by the sound eye the secondary deviation, and the law is that in concomitant strabismus the primary and secondary deviations are equal ; while in paralytic strabismus the secondary deviation is greater than the primary. STRABISMUS AND IMBALANCE 157 Farahjtic strahismus is of course a matter for the neurologist to deal with, and need not be con- sidered in a work on optics. By far the greater proportion of cases of concomitant strabismus, however, are due to errors of refraction, high de- grees of hypermetropia or myopia, and are there- fore legitimate subjects for the optician. Treatment of Strabismus. — In recent cases, and in cases of what is called periodic strabis- mus, i. e., where the squint appears only under the strain of near vision or at times of great bodily fatigue, the proper correction of the re- fractive error and resting the eyes from near work is often sulficient to effect a cure. In more per- manent cases it is, frequently necessary to give the patient prolonged rest for his eyes by means of atropin, say three or four weeks. In divergent strabismus much good may some- times be accomplished by orthoptic exercises by means of prisms, for particulars of which, how- ever, a larger work must be consulted. Such treat- ment requires great patience and judgment, and should not be carried out by one who does not thoroughly understand the anatomy and physi- ology of the matter. For convergent strabismus no exercises of this kind are serviceable, as there is no known means of exciting the external recti to contraction. If none of these expedients succeed, the case becomes one for the ocular surgeon. 158 REFRACTION Muscular Imbalance. Of much more frequent occurrence than actual fctrabismus is the condition of latent deviation known as muscular imbalance^ or heterophoria, in which one of the extrinsic muscles of the eye has, from some cause or other, become less effi- cient than its antagonist, necessitating an ex- cessive innervation of the weaker muscle in or- der to prevent deviation, and giving rise to whai IS known as eye-strain^ with occasional lapses into periodic strabismus. Varieties of Heterophoria. — According to the direction in which the 63^6 tends to deviate, heterophoria has been divided into three general classes, dependent, of course, upon insufficiency of the muscle acting in the opposite direction, as follows : Variety. Tendency. Faulty Muscle. Exophoria Outward Internal Eectus Esophoria Inward External Eectus Hyperophoria Upward Obliques. Inasmuch, however, as this classification in- volves an inversion of the nomenclature of the condition and the muscle at fault, it is not a very convenient one, and the writer much prefers Gould's method of designating the trouble by the term "imbalance"' of the muscle in question. Of the above varieties of imbalance the last is so rare that it will be disregarded in the present work. STRABISMUS AND IMBALANCE 159 Causes of Imbalance. — The muscular insuffi- ciency may be due to a general lack of muscular or nervous tone, whose underlying cause is to be found in some constitutional disease, such as tu- berculosis, syphilis, anemia, neurasthenia, etc. Far more commonly, however, it is a result of long-continued error of refraction, involving an extraordinary disturbance of the normal relations between accommodation and convergence. In fact, every condition of ametropia inevitably and logically produces muscular imbalance, and it is simply a question of the degree in which it exists, ranging all the way from imperceptible hetero- plioria to actual strabismus. Rationale of Refractive Imbalance. — It has already been explained that, while accommodation and convergence are anatomically distinct and sep- arate functions, they are very intimately asso- ciated, and the exercise of one is a powerful stim- ulus to the performance of the other. Hence when, as in hypermetropia or myopia, the normal relation between the two functions is disturbed by tlie abnormal conditions of accommodation, the natural tendency of convergence is to follow suit, and it is only prevented from doing so by an excessive innervation of the muscle concerned in such prevention. Thus, .a hypermetrope, as we have seen, is obliged to use his accommodation, for objects at infinity, and wlicii he does so the natural ten- dencv of the internal recti is to contract and con- 160 REFRACTION \erge the eyes in proportion to the degree of ac- commodation exerted. But if this were done, con- vergence for distant vision would result, whereas the imperative desire of the brain is for single Illustrates the two dissimilar images seen through the Maddox rod. vision. Therefore an extraordinary innervation is applied to the external recti to prevent con- vergence. A myope, on the other hand, does not accom- modate for near objects, which nevertheless de- mand convergence in order to produce a single image. The normal stimulus to the internal recti is here lacking, and the external recti are nat- urally stimulated to contract in correspondence witli the negative accommodation. An excessive innervation is therefore necessary to bring the internal recti into adequate play. MusciFLAR Failure. — Tn both of these condi- STRABISMUS AND IMBALANCE 1(51 tions, if long continued and the eyes constantly used, th^ overworked muscle eventually becomes weakened, and there is a latent deviation toward the direction of its antagonist, which is only pre- vented by an increasing conscious effort which at Illustrates the fusion of images as seen by the nor- mal eye through the Maddox rod. times (especially under strain of bodily fatigue) fails, and temporary strabismus occurs. In hyper- metropic imbalance (by far the commonest tyi3e) it is the external recti that suffer and the ten- dency is to convergence; in myopic imbalance the internal recti are overworked, and the tendency is to divergence. 162 REFRACTION DlSTINCTlOX BeTWELIT IMBALANCE AND STRA- BISMUS. — Eventually, of course, imbalance, if not relieved, will end in strabismus, which is func- Illustrates the dissociated images as seen in di\'Terg- ent imbalance with the Maddox rod. tionally only an intenser degree of heterophoria. The practical difference between the two condi- tions is that in imbalance the brain is, by an effort, maintaining single vision at the expense of the muscle, whereas in strabismus the muscle is no longer able to maintain proper poise; single vision is then impossible, therefore the brain has ceased, to strive for it, but fixes the object with one eye or tlie other, disregarding tlio image on the unused eve. STRABISMUS AND IMBALANCE 163 Tests for Imbalance.— These are, in the gross, the same as those already given for strabismus, dependent upon the inability of the affected eye to accurately follow the convergent movements of the sound eye. Maddox Rod. — A much more delicate and re- liable test, however, is -that furnished by the ^laddox rod found in every trial case. This de- vice is an opaque disc into which is set a cylin- drical piece of glass, which is mounted before the suspected eye, the other eye being furnished with Illustrates the dissociated images as seen in con- vergent Imbalance with the Maddox rod. a colored piano lens, and the patient instructed to look at a small flame six meters away. The principle of the Maddox test is that the 164 RE FB ACTION image of the flame on the uncovered eye is that of a round colored flame, while on the covered eye it is drawn out into a long bar of white light, thus dissociating the retinal conceptions of the image and lessening the desire for single vision. Maddox Rod. If the muscles are in perfect balance the images on the two retinae will accord without effort and irrespective of the identity or difference of their form, and the round colored flame will be seen with the rod of white light running through it. If there is muscular imbalance, the dissocia- tion of the retinal images and the consequent weakening of the desire for single vision will induce the patient to give up his effort to pre- serve the single image; double vision will at once result, and the rod of white light A\ill l)e 'seen to one side of the colored flame — on the same side as the covered eye if the imbalance is convergent, on the opposite side if divergent. STKAniSMUS ANIJ IMIIAJ.ANCE 165 Use of the Maddox Rod. — It is important that the disc containing the Maddox rod be placed exactly in front of the patient's pupil, otherwise it will entirely fail of its effect, i It must also be remembered that as the rod is a cylinder it will draw out the flame into a ])ar precisely at right angles to the axis of the rod, and must therefore be placed before tlie eye at right angles to the direction in wliieh it is desired that the bar of light shall appear. In testing for convergent and diverii-ent imbalance^ (tlic oiilv Iwo varieties Showing how the Maddox rod draws out a bar of lig-ht at rig-ht angles to its axis. here considered), it is desirable to have the rod appear vertical, hence the rod should be placed horizontally. In testing for hyperophoria, or im- balance of the obliques, the reverse is desirable. This form of imbalance, however, is rare, and has no direct relation to refraction. 166 REFRACTION Prism Test. — In cases of bilateral muscular insufficiency, which, of course, can hardly be re- garded as imbalance, since both eyes are alike affected, the insufficiency is detected and measured by means of prisms, as described under Con- vergence. Eliminatiox of Refractive Errors. — As stated, every condition of ametropia is inherently attended by muscular imbalance, hence by the time the patient comes to the refract ionist a part of the muscular trouble has become permanent, due to anatomical changes in the muscle, and a part of it is still an integral factor in the ametropia. After testing the amount of imbalance in the un- aided eye, therefore, the operator should correct the error of refraction with appropriate lenses, and make another test, subtracting the result of the second test from tliat of the first to find the. net amount of ])ormanent imbalance which needs treatment. Treatment of Imbalance. — If the degree of heterophoria is slight it is usually sufficient to correct the error of refraction, and when this cause of the trouble is removed the muscle will right itself. In severer cases, however, a coursi^ of optic exercises must be carried out, with prisms base in or base out as the case may demand, and as indicated by what has already been said. These exercises should be nicely graduated, and care- fully supervised by the refractionist, and need to be persisted in witli great constancy and patience. STRABISMUS AM) IMBALANCE 1()7 111 cases of eoiivergeiit iiiibalaiice the results are usually veiT satisfactory; in divergent imbalance they are less encouraging as there is no known stimulus foi" the unilateral contraction of the ex- ternal rectus. CHAPTER XIV. ASTHENOPIA. Asthenopia is a broad term used to designate tliat group of symptoms which results from any form of eye strain due to functional causes, in- cluding that type of muscular and nervous ex- haustion which has already been dealt with at length in the chapter on Muscular Imbalance. In a general way these symptoms are the same, from whatever specific cause they arise, and in the last analysis they are essentially reflex in their character, dependent upon an excessive and un- v-^qual innervation, and mediated primarily through the ocular and facial nerves. Sympto^es of Asthenopia. — Asthenopia mani- fests itself by an inability to sustain a steady and prolonged convergence, and by more or less pain in the eye when this is attempted. It is an exceedingly common condition, so much so that the presence of pain in the eyes, with no other symptoms of inflammation, immediately suggests asthenopia. As a rule the pain is not very severe, although it may reach the point of agonizing neu- ralgia. Sometimes there is no actual pain at all, but after prolonged use of the eyes the vision be- comes indistinct, and the patient is obliged to niul the degree of decentration required is FITTING THE GLASSES 211 SO great as to increase the weight to an incon- venient extent. This method oilers a hirger range of usefulness in vertical than in lateral decentra- tion. Decentering Equivalents.^ — The following is a table of decentering equivalents, showing the amount of decentration in millimeters necessary to produce various prismatic angles with lenses of (litferent dioptric strength. Lens 1*^ 2° 3° 4° 5° 6° 8° 10° 1 D, 9.4 18.8 28.3 37.7 47.2 56.5 75.8 95.2 2 4.7 9.4 14.1 18.8 23.6 28.2 37.9 47.6 3 3.1 6.3 9.4 12.6 15.7 18.8 25.4 31.7 I.ens 1° 2° 3° 4° 5° 6° 8° 10° 4 2.3 4.7 7.1 9.4 11.8 14.1 18.9 23.8 5 1.9 3.8 5.7 7.5 9.4 11.3 15.2 19. 6 1.6 3.1 4.7 6.3 7.9 9.5 12.6 15.9 7 1.3 2.7 4. 5.4 6.7 8.1 10.8 13.5 8 1.2 2.3 3.5 4.7 5.9 7.1 9.5 11.9 9 1. 2.1 3.1 4.2 5.2 6.3 8.4 10.5 10 .9 1.9 2.8 3.8 4.7 5.6 7.6 9.5 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 13 .7 1.4 2.2 2.9 3.6 4.3 5.8 7.3 14 .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 ir, .6 1.2 1.8 2.4 3. 3.5 4.7 6. 17 .6 1.1 1.7 2.2 2.8 3.4 4.5 5.6 18 .5 1. 1.6 2.1 2.6 3.1 4.2 5.3 19 .5 1. 1.5 2. 2.5 3. 4. 5. 20 .5 .9 1.4 1.9 2.4 2.8 3.8 4.8 Below is a similar table, but with the degree of prismatic effect expressed in centrads or prism- dioptres: Lens 1 Cr. 2 Cr. 3 Cr. 4 Cr. 5 Cr. 6 Cr. S Cr. 10 cr. 1 D, 10. 20. 30. 40. 50. 60. 80. 100. 2 5. 10. 15. 20. 25. 30. 40. 50. 3 3.3 6.6 10. 13.3 16.6 20. 26.6 33.3 4 2.5 5. 7.5 10. 12.2 15. 20. 25. 212 EEFEACTION 5 o 4. 6. 8. 10. 12. 16. 20. 6 1.6 3.3 5. 6.6 8.3 10. 13.3 16.6 7 1.4 2.8 4.2 5.7 7.1 8.2 11.4 14.2 8 1.2 2.5 3.7 5. 6.2 7.5 10. 12.5 9 1.1 2.2 3.3 4.4 5.5 6.6 8.8 11.1 10 1. 2. 3. 4. 5. 6. 8. 10.9 11 .9 1.9 2.8 3.7 4.6 5.5 7.3 9. 12 .8 1.8 2.5 3.3 4.1 5. 6.6 8.3 13 .7 1.5 2.3 3. •3.8 4.6 6.1 7.6 14 .7 1.4 2.1 2.8 3.5 4.2 . 5.7 7.1 15 .6 1.3 2. 2.6 3.3 4. 5.3 6.6 16 .6 1.2 1.8 2.3 3.1 3.7 5. 6.2 17 .5 1.1 1.7 2.3 2.9 3.5 4.7 5.8 18 .5 1.1 1.6 2.2 2.7 3.3 4.4 5.5 19 .5 1.5 2.1 2.6 3.1 4.2 5.2 20 .5 1.5 2. 2.5 3. 4. 5. Decentering of Eeading Glasses. — All glasses that are prescribed for reading, sewing', and other forms of near work should be decen- tered, in connection with the inclination of their planes already referred to, to the extent demanded by the inclination of the visual axes when con- verged for the distance worked at. This, of course, is not for the purpose of producing a prismatic effect, but to prevent it, by permitting the visual axis to pass through the optical center of the lens. Glasses for constant use should be decentered to an extent half wav between the geometric center and the degree required for near work. Distance of Lens from Eye. — As a general proposition, tlie le^is should be placed as near to the eye as possible without touching the eyelashes. In estimating the refraction of the eye, the con- vergence or divergence of the rays as they enter or emerge from the eye are made the basis of cal- culation, hence the distance between the correcting FITTING THE GLASSES 213 k'lis and l.lie surface of the eve should be as near zero as possible. In cases of presbyopia, however, and other instances where the purpose of the glasses is to adjust the accommodation for some particular Morkiug-poiiit, tlie paiicjit may wear the glasses close to, or at a distance from, the eyes, according to the distance at which he is working. A convex lens gains in elfect the fur- ther away it is from the eye, hence the further away tlie patient holds his reading, or sewing, or v/hatnoi:, the nearer he requires the glasses to be to his eyes, and vice versa CHAPTER XVII. HYGIENE OF THE EYE. I'roiii what has bmi said on tlio subject of eye- strain it is apparent t!iat (lie hygiene of the eye, so far as its visual function is concerned, falls into two general divisions, namely, (1) that which pertains to the muscular elements, accommoda- tion and convergence, and (2) tliat which has to do with the retina, to which may be added (3) that which concerns the ordinary care of the eye itself. Cr.osE ArrLiCATiON. — Anyone who lias ever plaved the schoolboy trick of seeing how long he conld keep his arm stretched out in a horizontal position has had personal experience of the fa- tigue which comes from maintaining a muscle of the body in a continnons state of even partial con- traction for only a moderate lengtli of time. In the case of the experiment referred to the mind is of course concentrated upon the test itself, and the fatigue soon becomes so noticeable that the experimenter is obliged to give it up. In in- stances, however, where the continued contraction IS maintained for the purpose of carrying out some muscular work upon which the attention is fixed, the sense of fatigue does not manifest itself until a rest is taken or the muscle gives out and refuses to contract anv longer. This last is precisely what takes place in the ciliary and internal recti muscles of the eye when 216 REFRACTION • the vision is coucentrated for any considerable length of time upon a near object. These mus- cles are kept in a continuous state of quite pow- erful contraction — the slightest let-up would in- terfere with the vision — but the mind being con- centrated upon the book one is reading or the work one is doings the terrible exhausting effects do not make themselves felt until either the muscles are released or they give way and fall into little spasms, in which event the vision becomes blurred of course. The nearer the point for w^hich the eyes are used in this way, the greater the contrac- tion of the muscles and the more marked the fa- tigue. If this kind of abuse is persisted in (as it fre- quentlv is), the ciliary muscle — the muscle of accommodation — like any other overworked mus- cle of the body, becomes hypertrophied and the exercise of accommodation becomes to that extent a constant and fixed quantity. In this way a lat- ent hyperopia is developed, -which requires long and patient care, even after proper correction, to remedy. In addition, the continued and excessive de- mand of the muscles for blood during their pe- riod of activity induces congestive troubles in the eyeballs and lids. Frequent Eests. — For these reasons the vision should not be exercised continuously at close range for any great length of time. Persons whose oc- cupation obliges them to work at close range HYGIENE OF THE EYE 217 should make a practice of giving their eyes fre- quent short rests by removing them from the work and completely rehixing ihcir accommoda- tion and convergence for a few minutes, by look- ing into tlie far distance. Those wlio do excep- tionally close and exacting work for very long periods at a stretch should relieve their accom- modation as much as possible by the use of ap- propriate convex lenses when they work, and their convergence by properly adjusted prisms. Children's Eyes.— The mischievous results of continued maintenance of accommodation and con- vergence are particularly marked in young chil- dren, whose musculature is in a formative stage. The 'vast crop of' hyperopes, squints, and asthe- nopes that throng the offices of the refractionists at later ages have at least ninet^ per cent, of then- origin in the abuse of the eyes during school days. Fortunately we are now tardily waking up to tie dangers of the case, and taking steps to avoid them. In the first place, children should not be re- quired to decipher very small or close characters at all. Their books should all be printed in mod- erately large and very plain type, and held at a respectable distance from the eye. And in the second place, their tasks should be so arranged as to o-ive them the least possible amount of close work and the greatest possible alternation of work requiring no effort on the part of the eyes. All of the above mentioned evils of contmuea 218 REFRACTION application are of course intensified a hundred- fold when an}' error of refraction exists. Poor Illumination. — Near-work done under the handicap of poor illumination has precisely the same effect as that which is done under con- ditions of excessive exactingness. The inadequate intensity of the image made upon the retina hy the insufficient light requires that the image he held there for a greater length of time in order to make a proper impression upon the brain, and that faculty of the brain which represents acuity of vision is taxed to its utmost in recognizing the image. Thus the musculature and the central nervous system both suffer. The question of what constitutes adequate illumination will be discussed in a later paragraph. Retinal Hygiene. Excessive Illumination. — Too much light is almost, if not quite, as bad as too little. It must be remembered that the retina is, in effect, a sen- sitive web of nerve filaments,- upon which light acts as an irritant just as the teasing of a cutane- ous ner\'e-end causes sensations of pain, heat, etc., as the case may be. Now, if the stimulation of a cutaneous nerve-end be carried too far the pain, heat, etc., become injurious both to the nerve and to the individual. The same thing holds good in the case of the filaments of the optic nerve spread out upon the retina. A moderate degree of irritation by light HYGIENE OF THE EYE 219 IS necessary to the production of the sensation of vision; but if tlie stimulation be too vigorous the nerve-ends of tlie retina quickl}^ become inflamed or exhausted, and rciiiiilis or retinal asthenopia ensues. i -^ This is just what happens wlicn the eye is con- stantly receiving too much lights whether it come direct from the soui'ce of light, as by continually facing a bright window, looking into glaring fur- naces, etc., or is reflected from one's work, as in reading by a h^n-d brilliant light. These practices, therefore, should be avoided as much as possible, and if one's employment is such that they are un- avoidable, then he should wear smoked glasses to protect the retina. Guttering Material and Colors. — A very frequent cause of this tvpe of eye-strain is the excessive reflection of light by the object upon which one is engaged. The light may, in itself, be moderate enough, but the surface of the work is such that it reflects excessively. Printers and metal-workers experience this kind of trouble. The only effective prophylaxis is the wearing of tinted glasses, and frequent short periods of rest. Those who are obliged to concentrate their vision for long periods upon one or more bright colors suffer not only from general retinal ex- haustion, but from a specific exhaustion of those nerve fibers which respond to the color-vibrations in question. Their only means of relief lies in the wearing of glasses of such a color as will neu- 220 REFRACTION tralize the offending color into a more or less mixed and quiet tint. Failing this, they should frequently rest the fatigued color nerves by gaz- ing steadily for a few minutes at the color which is complementary to the one that distresses them. The Eyeball. Dust. — A very common source of trouble to the eyeball, especially in large cities in these days of busy traffic, is found in the clouds of dust that circulate in the air, and lodge upon the conjunc- tiva. Those who wear glasses are more fortunate in this respect than their fellows whose vision does not require them to do so, for the lenses serve to keep out a great amount of dust. The effect of this continued invasion of dust, in greater quantities than the tear-ducts and winkers can take care of, is frequently to set up a mild chronic conjunctivitis. In some cases it induces a still more chronic process of productive inflam- mation, by which a web of connective tissue i^'rows over the sclera, forming what is known as a pterygium. When this encroaches upon the cornea it gives rise to astigmatism, and has to be removed. The only effective safeguard against the inva- sion of dust consists, of course, in tlie wearing of goggles. It is hardly to be expected, however, that tlie ordinary man will submit to any such unes- thetic adornment, and most people will prefer to take their chances with ordinary precautions. Those who are out in the dust a great deal will HYGIENE 01-' THE EYE 221 find it bcneiieiai to wash their eyes each evening, Avheu they get lionie, with a copious instillation of boric acid sohition or other mild astringent. Cold. — The membrane of the eye is subject to the same ill results oi* sudden changes of tempera- ture as other membranes of the body having simi- lar exposure. As a rule it is quite able to adjust itself to moderate variations, such as one encoun- ters in daily life. But extreme variations, es- pecially when accompanied by high winds, will congest the conjunctiva and produce a troublesome conjunctivitis. When facing a more than ordi- narily severe cold or heat the eve should be kept closed for a little while until the conjunctiva be- comes gradually warmed or cooled, as the case may be. In cases of very extraordinary exposures it is, of conrse, necessary to wear goggles. Infection". — It cnnnot be too insistently boi-no in mind that the conjunctiva is a membrane of very rapid and ready absorptive capacity, and pre- sents a most facile surface for the entrance of in- fective matter of all kinds. The most criminal carelessness is displayed in this respect by the average person, and all sorts of infection are con- veyed to the eve, and by parents to their chil- dren's eyes, by ]'ub1)ing the e3Tlid, taking out for- eign objects, and otherwise manipulating the eye with fingers carrying infective matter. Scrupulous care should be exercised at all time.-: to preserve the eye from this type of injury. AAHienever it is necessary to manipuhito Iho eye or 222 REFRACTION the lids for an\- reasoii_, the hands should be cai'e- fully washed; if a haudkerchief or cloth is used it should be scrupulously cleau; any drops or wash that is instilled should be prepared in sterile water, or at least clean distilled water; and after any such manipulation the eye should be thor- oughly flushed with a solution of warm boric acid or other mild disinfectant. The necessity of precaution in this respect is es- pecially binding upon persons who are suffering with an infective trouble in any other part of the bod}^, particularly if their disease is accompanied by a discharge. But in this, as in all preventive measures, if the habit of carefulness be formed during times when there is no emergency, in the time of emergency its effectiveness will be exer- cised automatically. Posture. — The influence of posture upon the liealth and development of the eye is a most im- portant consideration. Nothing is more detri- mental to the eye and more destructive of good vision than the pernicious habit of poring over one's work with one's head bent down almost touching the knees Avhich is so common, especially among children^ in whom it is particularly injuri- ous. Such a posture induces general congestion of the eyeball, with an overproduction of its se- cretions, stretching the choroid, and producing choroiditis and progressive myopia by tlie elonga- tion of the eyeball. ]N"ot infrequently a progres- HYGIENE OF THE EYE 223 sive process has been started in this way tliat lias eventually destroyed the eyesiglit. Myopes are particiUarly ^ilty of this habit, and of course in tliose whose eyes are already myopic the mischievous effects of the habit are most pronounced. Every person, and especially myopes, therefore, should in reading or sewing or any other type of near-work form the habit of sitting up straight and holding the work at a comfortable distance fi'om the eyes. If their refraction will not permit of their doing this, then there is something wrong with their refraction and they should obtain such correction as will enable tliem to carry out the practice recommended. Lighting. ^ All of the plans and devices that ever have been or ever can be propounded for the best possible lighting of rooms resolve themselves into applica- tions of the basic principle that the light is for the purpose, not of calling attention to itself, but of revealing objects. The. whole problem of light- ing consists in attaining a light which shall ob- trude itself upon the eye to the least possible ex- tent, and at the same time disclose the objects to be viewed with the greatest possible clearness. The applicai icii of tliis ]n'inei))le necessarily implies the reflection of tlie light from tbe object to the observers eye. Its working out, therefore. involves tbe ennsidoration of two j^oucimI ff^iitures: 224 REFRACTION (1) the source and nature of the light, and (2) the course traversed by the rays from their origin to the object and thence to the observer. All questions concerning the light come either under the head uf its illuminating qualities or that of its propogation. ILI.UMI.^^ATIO^^ — la fulfillment of the require- ment already enunciated^ that the light shall at- tract as little attention as possible to itself, it is essential that it be as diffuse as practicable. This is the quality which the layman usually means when he calls a light '^soit," and it has its realiza- tion in the natural light of a clear day out of the immediate path of the sun's rays. Gould points out that the ideal method of attaining this con- dition with artificial light would be to have the source or sources of illumination concealed from view and arranged so as to exercise a uniform diffusion of the light proceeding from them throughout the space designed to be lighted. This effect is nowadays attained by the so called "in- direct" system of lighting, in which the light is reflected from ceilino- and walls : and this is the ideal system. Daylight, then (but not direct sunlight), is the ideal illumination, in spite of all foolish asser- tions to the contrary, and it should be admitted to the room in such a way as to render its distribu- tion as diffuse and uniform as possible. Daylight being inaccessible, or for any reason undesirable, the next best substitute is the indirect HYGIENE OL' THE EYE 225 liditins"' just referred to. After tliat the next best is a wliiie or sliglitly yellow artilieinl light, .sur- rounded hy a ^^pherieal globe of opacjuc glass for the purpose of dilTusion. The globe need not he a complete sphere, for it is rarely necessary to direct any of the light upwai-d. hut it sliould be of a spherical curvature, so as to obtain a uniform dif- fusion. Uniformity of distribution is best attained by a correspondingly uniform distribution of the sources of light throughout the room, in prefer- ence to grouping them in the center. This must be done with due regard to the matter of inten- sity, of which we shall treat directly, and of the purposes for which the room is to be used. Intensity. — The intensity, or what the layman calls the "strength,'' of the light is a matter whose adjustment depends largely upon the plirpose for which it is desired, and one in which the most paradoxical contradictions are perpetrated in act- ual practice. One does not require as much in- tensity of light for a dancing party or a reception as one does for reading or sewing, hut as a rule the dancing or reception room is lighted far more brilliantly :liau the reading or sewing room; in- deed, the former is usually lighted far too hril- lianlly for the peace and welfare of the eyes, wliil(> ihd latter is generally just the reverse. Too great intensity of light is injurious to the eyes chiefly by reason of its exhausting effects upon the retina, and also because it reveals more detail than is necessarv in the objects viewed 226 ftEFRACTION and thus induces other forms of eye-strain. Too little intensity, on the other hand, is harmful be- causd of the necessity which it imposes on the eye of close aiDplication and consequent muscular strain. For ordinary purposes of vision, such as dan- cing, dining, receiving, etc., the intensity should be just sufficient to afford a general view of ob- jects and people such as one desires on such occa- sions, and subdued enough to be restful to the eye — i. e., not to reveal unnecessary and nagging de- tail. For purposes of reading and sewing and sim- ilarly close work, the intensity should, of course, be greater — sufficient to reveal clearly all the nec- essary details of the work without undue applica- tion, but not enough to obscure these details b}'' excessive reflection or to tire the retina by over- stimulation. It is estimated that for the former require- ments an intensity of 32 candle power per 1,000 cub. ft. of space is about right, while for close work it should reach G4 candle power. Modes of Light. — A^iewcd from the standpoint of the foregoing qualifications, it will at once be seen that daylight is easily the most desirable mode of light. In addition to its other superiori- ties it possesses the distinctive advantage of fur- nishing a white light, which is not perfectly at- tainalile by any other method of lighting, and which is ilie most conducive to acuity of vision. Its chief disadvantages are its unreliability, its HYGIENE OP THE EYE 227 inadequate duration, and the difficulty under mod- ern crowded conditions of admitting it in sufli- cient quantities. These circumstances make it ab- solutely impossible to depend upon it for illumi- nating purposes in these days of competitive effort. Artificial modes of light possess degrees of de- sirability corresponding to the proximity with wliich they realize tlie advantages of daylight, and the extent: to wliich they avoid the disadvantages peculiar to artificial lighting. Chief among these disadvantages, common to all artificial methods, are tlie unsteady character of their light and the elfects produced by combustion upon the atmos- phere. Electric Light. — The mode of 'artificial light which comes nearest to fulfilling tliese conditions is undoubtedly the electric incandescent light. It possesses the following distinctive and positive ad- vantages; it gives an almost white light; it burns with comparative steadiness; it yields tlie greatest degree of intensity for the least bulk; its intensity is most easily measured and regulated. It lias, moreover, the following negative virtues: it does not vitiate the air; it does not affect temperature or humidity to any appreciable extent; it carries with it no danger of fire. Incidentally it may be added that it is the most economical light, for the reasons above set forth, both in tlie matter of diroct production and also of physical liealth. Qas. — Next to the eh-ctric incandescent lamp. 228 BEFBACTION illuminating gas with some form of mantle burner furnishes the most desirable artificial light. By means of these burners iruu-li of the objection to the color of tlie liaiuc and its unsteadiness is over- come, ami its intensity is enlumced. However, it still retains the disadvantages of vitiating the at- juosphere, affecting temperature and humidity, and affording constant danger of fire and explosion, to which may be added the ])ractical impossibility of ever obtaining a good grade of gas. Acetylene gas is an impr(/vement in every way u])on ordinary coal gas. Lamps. — In the absence of facilities for either electric or gas lighting, one must of course use oil lamps. They are very undesirable things at best, and all one can do is to lessen their objection- able features as much as possible by employing a high grade of kerosene — i. e., one with a high flasli point, a well-made air-tight lamp, a good circular wick, and a first-class chimney surrounded by an opaque white globe. An ordinary-sized room will require at least two such lamps of large size to adequately light it for reading or sewing. All the modes of artificial lighting above dis- cussed are, of course, subject to the requirements of diffusion and intensity previously set forth. Reflection. If the conditions ol' ditfusion. disti'ibution and intensity above set forth could in all cases be id('al1\- realized, then there would l»e no need to HYGIENE OF THE EYE 229 discuss the aspect of the subject wliicli coucerns rellectiou. For with ditt'used light, whetlier it be daylight or artificial light, uniformly distributed throughout the desired space, and of a net inten- sity proper to the required purpose, the light would be uniformly reflected, as to direction and inten- sity, from any point included in the illuminated room. Unfortunately, however, such ideal condi- tions are rarely, if ever, attained, and it is almost always necessary to make specific provision for the proper incidence and reflection of light, in the required degree of intensity, from some se- lected portion and aspect of the building or space. This is accomplished principally by adjustment of the postural relations of the sources of light to those of the objects and observers, either by arrangement of the windows or artificial liglits or by arrangement of the objects. Position' of Light. — This is determined ac- cording 1o the principles of diffusion, intensity, and reflection already enunciated, and with regard to tlu" illuminating purpose of the light. It is evident on nil of these grounds that tlie object to be viewed — the book to be read, for ex- ample — should not be held l)etween the person and the source of light, or that tlie source of light should he in front of the i)erson at all. For a light in fi'Diit of the o])server calls attenti(m to itself, exhausts the retina, is not reflected from the object and therefore does not illumine it, and in the case of sewing or writing casts a shadow 230 BEFRACTION toward the worker — in all of these respects vio- latiii*^ tlie canons of good illumination. It is ec[ually evident that the source of light ijnmediately behind the observer will not fulfil the required conditions, for in that case the ob- server's own body will be interposed between the source of light and the object, so that the light cannot reach the object to be reflected from it, and all of the room will ])e illuminated except the very portion -w'here illumination is desired — namely, where the object is — thus creating a very annoy- ing and tiresome contrast. The source of light directly to the side of the observer is as bad as, if not worse than, directly in front. In this position the rays of light strike the object at an extremely oblique angle and are reflected at an equally oblique angle to the other side of the observer; indeed many of them' pass laterally between himself and the object. Only those reach him from the object which are trans- mitted to it indirectly, like the cushioning of a billiard ball. In addition to these faults of re- flection, the light from the source of illumination is striking his eye directly at a lateral inclination, and irritating his retina in a peculiarly aggra- vating wa}'. The Correct Position. — The most satisfactory position for the source of light is midway between the rear and the left lateral ; sufficiently in the roar to nvoid any direct rays falling on the eye; sufficiently to the side to clear the observer's own JIYGIENE OF THE EYE 2:51 bod}-; aiKl suificieiitk high to clear his arm and shoukler. In this position the light falls upon the object at a slightly oblique angle, but not siillicicntly oblique to prevent it from reaching the observer's eye, especially if the object be appro- })riately tilted toward the source of light. The retina is now receiving no direct light, none in fact but what is properly reflected from the object, and no shadow is cast between the observer and the object. When the source of light is in a similar position over the right shoulder a shadow is thrown between the observer and the object un- less he moves the object to the left, in doing which he takes it out of the fixation field of the right, or dominant, eye. Some persons are sinistrocular, or ^'left-eyed,'' and for these persons the proper position for the source of light is over the right shoulder. Influence of Position on Intensity. — In ar- rangements of position such as we are contom- j dating; the source of light must not he too far from the oljject to illumine it in sufficient detail, noi- must it he so near as to unnecessarily exag- gei-ate detail and tire the retina. It should be suf- ficiently far removed to comfortahly reveal just the amount of detail required for the purpose in hand. Influence of Position on' DrFFUSiON. — The pame remarks apply to the diffusion of light. If the source of light he too near the ohject the light will not be properly diffused by the time it reaches 2L'2 REFRACTIOX the object; if, on the other hand^ it be too far removed, diffusion will be so great as to weaken the intensity and the reflective power. If the source of light be a window (daylight) and it be out of the direct rays of the sun, on ; can hardly be too near for either intensity or diffusion. GLOSSARY. Accoinniodation. — The power of changing the focus of the ej'e. by contracting tlie ciliary muscle. Ametropia. — An abnormal condition of refraction. Amplitude. — The amount of nervous energy neces- sary to perform a function. Angle Alpha. — The angle formed at the nodal point by tlie optic and visual axes. Ang-le Gamma. — The angle made by the visual axis and a line drawn from the object through the center of rotation. Anisometropia. — A difference of refraction in the two eyes. A.stlienopin. — Weakness of the visual apparatus. Astigmati.sm. — Inability to focus the rays at a point. Binoetilar. — Relating to both eyes used simultane- ously. Centering-. — Placing a lens before the eye so that the visual axis passes through the principal axis of the lens. Cliamber.s. — Divisions of the interior of the eyeball containing the liumors. Clioroid. — The vascular tunic of the eye-ball. Chromatic Test. — A refractive test made with a lens which separates up the ray into the primary colors. Ciliary. — The circular muscle of the eye which sur- rounds the pupil. Concave. — Diverging refracted rays. Convergence. — The directing of the visual axes of the two eyes toward a point nearer than infinity. Convex. — Converging refracted raj'S. Cornea. — The smaller spherical segment of the eye- ball. Cyeloplegric.^An agent for paralyzing the ciliary muscle. GLOSSARY 233 Cylinder. — A lens which is a segment of a cj'linder. Diopter. — The unit of refracting power, namely, that of focusing parallel rays at a distance of one meter. Dise (optic). — The raised circular spot where the optic nerve enters the retina. Einiiielropin. — A condition of normal refraction. Kye.straiu. — The nervous effects of abnormal re- fraction. Far Point. — The furthest point at which a com- pletely relaxed eye can clearly discern an object whose size corresponds to the visual angle. Finite Kays. — Rays which originate within six me- ters of the observer. Focal Distance. — The distance between the refract- ing surface and the focus. Focal I^engtli. — Focal distance as applied to a lens. Focal I*oint. — The point at which refracted rays are brought to a focus. Fovea Centrali.s. — The most sensitive point in the retina, situated in the center of the yellow spot. Fundus. — Tlie retina as seen through the ophthal- moscope. Heteroplioria. — A condition of unequal power in the ocular muscles. Humors. — The licjuid and semi-liquid contents of the eyeball. Hypernietropia. — A condition of refraction in which the rays are carried behind the retina to focus. Imbalance. — See Heteroplioria. Index of Kefraction. — The refracting power of a medium as compared with that of air, atmospheric refraction being ligured as 1. Infinite Rays. — Rays which originate more tiian six meters from the ol)sorver. liatent Hypermetropia. — Hypernietropia due to an- atomic ciianges in the ciliary. Macula laitea. — See Yellow Spot. Maddox Rod. — A device for testing tlie muscles of the eye. Manifest HypermcIa. — Hypermetropia due pure- ly to refiactive conditions and easily demonstrable by ordinary tests. Meridian. — A line of curvature of the cornea. Meridians, Chief. — Tiie meridians of greatest and least convexit\- icspectively of the eye. Myopia. — A condition of refraction in which paiallel rays are focused in front of the retina. Xear Point. — The nearest point at which the com- pletely accommodated eye can clearly perceive an ob- ject whose size corresponds to the visual angle. 234 REFRACTION INoKsitive C'onvtTseufe. — Tlie power of the external recti muscles to pull the eyeballs outward. Xodsil I*oiiit. — The center of the refracting system of the eye, through which all the rays pass as they cross. Oplitiiiiliiio.oicope. — A mirror for examining the fun- dus of the eye. Opiitlinlinoscopy. — The use of the ophthalmoscope. Optif Axis. — An imaginary line drawn through the nodal point to the inner side of the yellow spot. I'ri.siii. — A pyramidal shaped lens. I'reshyopia. — Hypermetropia due to old age. I'riiu-ipal Axix. — The imaginary line along which a ray travels which enters a medium perpendicularly to its surface. I'o.sitive Coiiverg-ence. — The power of the internal recti muscles to pull the eyeballs inward. Rang-e. — The maximum distance which a function can be exerted. Retincseopy. — The shadow test by means of a retino- scope. Strabl.simi.s. — Squint. A'i.sual Angle. — The angle formed by two lines drawn from the extreme boundaries of the object looked at through the nodal point. V'iNual Axis. — An imaginary line drawn from the yellow spot, through the nodal point, to the object. Yello^v Spot. — The most sensitive spot on the retina, situated in the centre of the retina, to the outer side of the disc. METRIC EQUIVALENTS. 1 meter = 3937 inches. Approximately 40 inches. 1 antimetre = .40 inch. Approximately i/^ inch. 1 millimetre = .04 inch. Approximately 1/25 inch. COMMOM.Y ISED DISTANCES. 6 meters = approximately 20 feet, n meters = api>roximately 16 feet. 4 metnrs = ;i pin'oximately 11 feet. 3 meters = api)i oximalely ]0 feet. 2 meters = approximately 7 feet. 1 meter = approximately 40 inches. INDEX Alicnal inn, Spluiiial 31 Ahiiuriual Acruiiiiiiutlalioii. . .174 — Com ( if^TiRe 175 Absolute' and JJiiiocular G7 — Negative Convergence. ... 74 AceoHiinodation Abnonnal. .174 — Anii)litude of 00 — and Convergence 63 — Observer's lOG — Range of 05 Achromatic Bi-Focals 200 Acuity' of Vision 42 Acute Conjunctivitis ISO Alpha Angle 42 Ametropia 4'J — Ophthalmoscope in 103 Anietropic Eye 49 Amplitude of Accommodation 00 — of Negative (.'onvergence. . 74 — of Positive CVmvergence. . . 74 Anatomic Construction of... 35 Angle Alpha 42, 154 — Gamma 42 — of Incidence 15 — of liefiection 15 — Visual 42 Anisometropia 50 Anterior Focus, Principal.. 32 Apparent Strabismus 153 Asthenopia 109 — Accommodative 171 — Symjitoms of 109 — Varieties of 1 70 Astigmatic Hand 138 Astigmatism 50 — C'omi)ound 132 — C'orrection 127 —Mixed 134 — Ophthalmosropy in 130 — OjihthalMiosctipe in 105 — Tfetinoscopy 95 — Retinoscopv in 130 — Simple 131 — Stenopaic Slit in 129 — Test Tvpe in 1 2S — Wheel Ti'st in 12S Atrophy. Optic IRS Axes and Points 3S — of the K.\-e 41 Axis Optic 41 — Princiiial 29, 41 —Visual 41 Bi-Focals 190, 204 • — Achromatic 206 — Cemented 2(t5 - — (jlround 2iM Binocular and Absolute .... 07 JJridges 201 Jiridge Measurements 202 Causes of Imbalance 159 Cemented Bi-Focals 205 C'enter, Optical 24 Chambers 37 Chief Meridians 53, 127 Children's E\es 217 Choked Disc" 187 Choroiditis 184 Chronic Conjunctivitis 181 Chromatic Test 113 Ciliary 37 Circles of Diffusion 33 Close Application 215 Cold in the Eye 221 Colors, Glittering 219 Commonly Used Distances. .234 Compound Astigmatism ....132 Computing the Correction.. 94 Concave Reflections of Light 24 Concentration of Light 92 Conjunctivitis 180 — Chronic 181 —Purulent 181 Convergence 03 • — Abnonnal 175 — Absolute Negative 74 — Amplitude of Negative. . . 74 — Amplitude of Positive. . . 74 -—Insufficiency of 75 — Measure of 72 — Range of 09 — Ratio of 74 Convex Ix^ns for Relaxation .1 45 Cornea. The 3(i Correction, Comi>uting the. . 94 — of .\stigmatism 127 — of ITvpermetropia 113 — of Myopia 121 Correct Position of Light.. 230 Decenfered Ij<>nses, Prescrib- ing 209 Decentoring of Lrns(s ....200 —of Reading GIass<'S 212 ■ — -Equivalents 211 - Afethods of 210 -Pnsm.Tfic Effect of 207 Detached Retina 187 Diagnosis of Strabismus ...15.'5 Dioptric System 48 236 INDEX Dioptrisni 48 Dittusion, Circles ot S'd Disc 38 — View of 110 Diseases of the Eye Connect- ed with Disturbances ot \'ision 179 Distance of Lens from Eye. 212 — Principal 30 — Principal Posterior 32 — Tj'pe 144 Distances Connnonlv rsed..234 Distant Tvpe Test." 114 Dust in Eyeball 220 Edges, Oblique 95 ■ — of Shadow 94 Electric Light 227 Emnietropia 49, 88 Emmetropic Point of Revers- al 84 Errors, Estimation of Re- fractive 100 — of Refraction .50 — Possible 102 Examination 79 — of Refractive Errors 160 Excessive Illumination 218 External Rectus 45 Eye, The 35 — Anatomic Construction of 35 — at Rest 63 — ^Axes of 41 —Cold in 221 — Diseases of 170 — Distance of Lens from ..212 — Hygiene of 215 — Infection in 221 — Glasses 203 — Muscles of 44 — Refraction of 47 — Size of 204 — Strain, Reflex Effects of.. 172 Eveball 3.), 220 — Dust in 220 Eves, Children's 217 Far Point 04, 73 Fitting Glasses 191 F'ocal Ijcngth 25 — Length of Mirror 93 — Point, Negative 31 — Point, Positive 31 — Point, Principal 30 Focus, Principal 25 — Visual 20 Formation of Images 32 Fundus. Observation of ....102 Gamma Angle 42 Gas Light 227 Glasses, Decentering of Read- ing 212 —Fitting 191 — Periscopic 196 — Prescribing 198 Glaucoma 184 Glittering Material or Colers.219 Glossary 232 Gradual Relaxation 116 Ground Bi-Focals 204 Half Lenses 206 Heterophoria, Varieties of.. 158 Humors 37 Hvgiene of the Eve 215 —Retinal ' 218 II\-]iermetropia ....49, 51, 88 • — Correction of . .113 — Ophthalmoscope 103 Hypermetropic Point of Re- versal 85 Hvpermetropia, Retinoscopy in 117 Hypei-opia, Latent 146 — Measure of 116 H^peropic Eye 49 Illumination 224 — Excessive 218 —Poor 218 Images, Formation of 32 Image Reflected 22 —Visual 26 Imbalance 153 ■ — Causes of 159 — Muscular 158 —Tests for 163 — Treatment of 166 Incidence, Angle of 15 Index of Refraction 48 Indirect Ophthalmoscopy. . . .106 Infection in Eye 221 Inferior Oblique 45 — Rectus 44 Influence of Position on Dif- fusion of Light 231 — of Position on Intensitv of Light ;...231 IiisuHiciency of Convergence. 75 Intensity of Light ... 223 Internal Rectus 44 Intei-pretation of Shadow. . . 88 Interstitial Keratitis 183 Intrinsic Muscles 44 Iris 30 Iritis 183 Keratitis Interstitial 183 Lamps 228 Latent Hyiieropia 146 Laws of Reflection 16 Lens 36 — Focal Ijcngth of 58 — Focal Point of 57 — Measurement of Accommo- dation 66 Lenses 57 • — Bi-Concave 60 INDEX 237 — Bi-C'onvpx 00 — (.'oncav(>-( 'onvpx 60 — Convi'if^iiif;' Coiifavo Con- vex 60 —Cylindrical 16, 59 — Decenteiing of 206 — I)iiiiinislunf4' Ett'oct of Con- cave 150 - — Diverj^inj;' Concavo-Cnnvcx. 60 — Crindinj;- 60 — foi" Myopia 122 — Piano-Concave 60 — Piano-Convex 60 — Prescrihiiif^' Decentered ...209 —Spherical 58 Lifiht 9 — Al)sorption of 10 —Color of 13 — Concentration of 92 —Concave ReHection of 24 — Correct I'osition of 230 • — Divergence of Kays of . . . . 14 — Dynamics of 11 —Effects of on Matter 12 —Electric 227 - — Finite Ray.s of 15 — Force of 12 — for Retinoscope 80 —Gas 227 — Generation of 9 — Geometries of 14 — -Infinite Hays of 14 — InHucnce of I'osition on Diffiisi(m of 231 — Influence of Position on Intensity of 231 — Intensity of 13, 223 — Linear Propagation of.... 14 —Modes of 226 — Nature of 9 — Oscillatory Velocity of.... 12 —Position of 92. 229 — Kavs 14 —Reflection of 10, 22 —Refraction of 28 — Sources of 9 — Transmissir)n of 9 — Velocity of U — \'il)rations 13 Lighting 223 •Maddox I'upil Localizer 192 — i{..d I(i3 — Use of 165 Malignant I'rogressive My- opia 148 Measure of Convergence ... 72 — of nyp(roi)ia 116 Meridians, Testing 136 Mitiiod of Estimation !)5 Nfrtiiods of Dccentering. . . .21(1 .Met lie .\ngle 73 — E.piivalents 234 Minus Lenses 53 Mirror. Focal Length of . . . 93 — Movements of 93 Mixed Astigmatism 134 Modes of Light 22(1 Movements of Mirror 93 Muscles of the E\e 44 Muscular Lnhalancc 158 — Mechanism 45 Myopia .4!), 52, 8!t — Correction of 121 — Distant Type Test for... 122 -Lenses for 122 — .Malignant Progressive ...148 — (Ophthalmoscope in 104 — Retinoscopy in 123 Myopic Point of Reversal.. 86 Near Point 65, 74 Negative Focal Point 31 Neuritis, Optic 187 Nodal Point 39 Normal .\mplitude of .\ccm- modation 67 Observe'r's .\ccommodation . . .106 — Refraction 105 Oblique Edges 95 Observation of Fundus 102 Opacity 10 Operator, Position of KM Ophthalmoscope 97 — in .\metropia 103 ■ — in Astigmatism 105 • — in Hypermetropia ......103 — in Myopia 104 Ophthalmoscopy 97 —Direct l