THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES TRANSLATED FROM THE DANISH BY ELISABETH AAGESEN CENTRilTRYKKERICT- NYKOBINO F. 1 MicUl Loutl-fliU yix4/2AA^ /h.A SKIASCOPY AND ITS Practical Application to the Study of Refraction BY Edward Jackson, a. m., m. d., PROFESSOR OF DISEASES OF THE EYE IN THE PHILADELPHIA POLYCLINIC AND COLLEGE FOR GRADUATES IN JIEDICINE ; SURGEON TO WILLS EYE HOSPITAL; CHAIRMAN OF THE SECTION ON OPHTHALMOLOGY OF THE AMERICAN MEDICAL ASSOCIATION ; MEMBER OF THE AMERICAN OPIITHALMOLOGICAL SOCIETY ; ETC., ETC. WITH TWENTY-SIX ILLUSTRATIONS PHILADELPHIA: THE EDWARDS & DOCKER CO. 1895 COPYRIGHT, i8o5 BY EDWARD JACKSON CONTENTS PREFACE CHAPTER I.— History, Name, Difi--icui,ties and Study History . . Name . . . . ' . Difficulties How to Stud}' the Test ...... CHAPTER II.— General Opticai. Principles. The reversal of movement Real Movement of light on retina, Plane Mirror Real Movement of light on retina. Concave Mirror Apparent ISIovement of light in pupil Rapidity of Movement of light on the retina INIagnification of the retina Form of the light area .... Brilliancy of light in the pupil Finding the point of reversal . CHAPTER III— Conditions oe Accuracy. The Source of light ..... Focusing of light on the retina I'osition of greatest accuracy . Irregularities of the INIedia and Surfaces Distance of Surgeon from Patient . CHAPTER IV.— Regular .\stigmatism. Two points of reversal ....... The Band-like appearance Changes in the Light area at different distances Direction of movement of the band CHAPTER V. — Aberration and Irregular Astigmati.sm Appearance of Irregular .Astigmatism .... Synmietrical Aberration ...... The Visual Zone ........ Appearance of Positive .Vljerration ..... 7 lO II 13 19 23 24 26 28 30 32 33 34 36 3S 40 41 43 46 47 53 55 56 5S 59 60 Appearances of Negative Aberration ..... 63 Appearance of Conical Cornea 65 Scissors-like Movement . 68 CHAPTER VI.— Practical Application with Plane Mirror. Position and Arrangement of Light . . , . .71 Hyperopia ........... 72 Myopia 74 Emmetropia 76 Regular Astigmatism 77 Aberration and Irregular Astigmatism 84 Measurement of Accommodation 86 CHAPTER VII. — Practical Application with Concave Mirror. The Source of Light 89 Hyperopia .......... 90 Myopia 91 Emmetropia ... ...... 93 Regular Astigmatism 93 Aberration and Irregular Astigmatism 99 Measurement of Accommodation 99 CHAPTER VIII —General Considerations. Apparatus 100 Mydriatics 106 Relative advantages of plane and concave mirrors . . 107 INDEX Ill PREFACE THIS little book was written to bring about the more general adoption of Skiascopy as an essential part of the examination for ametropia. It is not supposed that any ophthalmologist is quite ignorant of the test ; but many do not know its full practical value, or how best to apply it. The demonstrations and descriptions here given assume a general knowledge of the eye and of physiological optics. And the writer, having observed that students of this subject do not generally think in the terms of algebraic formulas, but more readily grasp the graphic or geometric presentation of a fact, has governed himself accordingly. The claims of this subject to careful consideration are : First. — Skiascopy, is an objective test, independent of the patient's intelligence or visual acuteness, and more largely than any other, independent of the patient's coopera- tion. Second. — It is by far the most accurate objective test. The limits of its accuracy depend on details of its execution, and the skill and patience of the observer ; but, it does not require any rare natural qualifications, to carry it, for many eyes, to the extreme limits of accuracy for subjective tests. Third. — It requires but little more time than the use of the refraction ophthalmoscope or the ophthalmometer, which are able to give very inferior information. It saves time in making a complete diagnosis. Fourth. — It requires no costly, complex or cumber- some apparatus. Fifth. — It lays before the surgeon the refraction in each particular part of the pupil as it is revealed by no other test, and opens up the principal avenue for farther advance in the scientific study of the refraction of the eye. Of the use of the data obtained by means of skias- copy, it is not the purpose of the present monograph to speak. These data include the refraction of the visual zone, corresponding to the refraction of the eye obtained by other methods ; an accurate knowledge as to the loca- tion and limits of that zone ; and the refraction outside of it ; the latter having in some cases important bearings on the practical adjustment and use of lenses. Denver, Col., March, 1895. CHAPTER I. HISTORY, NAME, DIFFICULTIES, AND METHOD OF STUDY- ING THE TEST. History. — From the earliest use of the ophthalmoscope, by the direct method, it has been recognized that the see- ing of an erect image of the fundus at some little distance from the eye indicated hyperopia, and the seeing of an in- verted image indicated myopia. So long ago as 1862, Bowman {The Royal London Opthalmic Hospital Reports, Vol. II, p. 157), called attention to the rotation of the mir- ror as a means of bringing out appearances characteristic of irregular astigmatism and conical cornea, and Bonders, in his work on Accommodation and Refraction of the Eye, pub- lished in 1864, included (p. 490) the following note: *' My friend Bowman recently informs me that ' he has been sometimes led to the discovery of regular astig- matism of the cornea, and the direction of the chief meridi- ans, by using the mirror of the ophthalmoscope much in the same way as for slight degrees of conical cornea. The observation is more easy if the optic disc is in the line ot sight and the pupil large. The mirror is to be held at two feet distance, and its inclination rapidly varied, so as to throw the light on the eye at small angles to the perpen- dicular, and from opposite sides in succession, in successive meridians. The area of the pupil then exhibits a some- what linear shadow in some meridians rather than in others.' " (7) 8 SKIASCOPY. The use of the ophthalmoscope above referred to, for the detection of irregular astigmatism, became widely pop- ular. It was generally adopted as the most satisfactory test for this kind of defect. But the observation that the same method was capable of revealing regular astigmatism and the direction of its principal meridians, does not seem to have attracted the same attention. In 1872, Couper, in his paper before the Fourth Inter- national Ophthalmol ogical Congress (see Trans, page 109), alluded to Bowman's observations, and said : " The greater dispersion in one meridian than in the opposite, gives rise to the linear shadows. Only the fact of astigmatism is thus established." He then went on to describe a method of using the ophthalmoscope as an optometer in astigma- tism, which is rather a modification of the ordinary use ot the ophthalmoscope than a variety of skiascopy, since it depends on the recognition or non-recognition of the retinal vessels in different meridians, when the ophthalmoscope mirror is held at a considerable distance from the eye, and takes no account of the movement of a light area on the retina. It had already been pointed out (Bonder's Accom. and Eef. ojthe Eye, page 106), that the distance of the in- verted image before the eye indicated the degree of myopia. In 1873, Cuignet, of Lille, published (Rec. d'Ophtal- mol, 1873, pp. 14 and 316) an account of the test, as he had used it, as one capable of revealing not only the pres- ence of hyperopia or myopia as well as astigmatism, but also as giving a practical method of measuring the amount of these errors of refraction. He seems not to have appre- ciated fully the optical principles involved in the test, and his account of it attracted no attention. However, in 1878, his pupil, Mengin, introduced the practice of the method at Galezowski's clinic in Paris. There it was taken up, HISTORY. 9 and Parent demonstrated its true optical basis and urged its advantages in a series of articles published in the Recucil d' Ophtahnologie in 1 880-81, pp. 65 and 229. Lytton Forbes, in the Royal London Ophthalmic Hos- pital Reports for 1880, (p. 62), published a paper on the test, giving a minute account of the various forms assumed by the light and shadow in the pupil, but without full expla- nation of their optical significance. In 1881, A. Stanford Morton included a full description of the test in his little work on the Refraction of the Eye. In 1882, Charnley gave the fullest demonstration of its optical basis in the Royal London OphtJialmic Hospital Reports^ X, 3, p. 344. And Juler called attention to it in the Ophthalmic Review Vol. I, p. 327. The method described and advocated by Parent and those who followed him, had been that with the concave mirror. Cuignet had used the plane mirror, and, in 1882, Chibret pointed out {Annales d^ Oculistiqiie^ Vol. xxxviii, p. 238) the advantages of the plane mirror in determining the presence and degree of myopia in the examination of large numbers of recruits. In 1883, Story {Ophthalmic Review^ Vol. II, page 228) advocated the use of the plane mirror,, but in the same manner as the concave, except that the ob- server should place himself at a distance of four metres, from the patient, a distance which renders the test of little value for a considerable proportion of cases. In 1885, was published {American Journal of the Medical Sciences^ April, 1885) the writer's account of the test with the plane mirror, as applicable to all varieties of ametropia, the determination being made by measuring the variable distance of the surgeon from the patient. Since that time the test has been widely recognized, but even yet is far from being universally adopted and depended upon as it deserves to be. The literature of the subject has since grown 1 10 SKIASCOPY. quite extensive. But it must be noted that a considerable proportion of the accounts of the test bear evidence that their author's acquaintance with it has been theoretical rather than practical, and the mass of them contribute nothing to the common fund of professional knowledge. The writer's contributions as to the retinal illumination (Ophthalmic Review^ Feb., 1890) and the relative positions of the source of light and the observer {Archives of Ophthal- mology, July, 1893), with the suggestions as to the special pieces of apparatus to facilitate the test, to be mentioned in Chapter VIII, complete the evolution of this method of diagnosis as now practiced, and here described. Name of the test. — Neither Bowman nor the others, who early employed the test for the detection of irregular astigmatism and conical cornea, proposed for it any special name. Cuignet, who brought it forward as a distinct method for the diagnosis of the refraction of the eye, seems to have thought at first that the play of light and shade in the pupil depended entirely on the curvature of the cornea, and described it under the name keratoscopie. Considering the real causes of the movement of light and shade in the pupil and the purposes for which it is employed, this name seems especially inappropriate. Parent, realizing this inappropriateness, proposed retino- scopie, in allusion to the fact that it was the movement of light and shade on the pigment layer of the retina that commonly gave rise to the phenomena studied. Yet this name was obviously open to criticism, in that the condition of the retina itself was not at all the matter in considera- tion, and that the same play of light and shade could be watched on the head of the optic nerve, or, where the reti- nal pigment was wanting, upon the choroid or sclera. HISTORY. 11 Cliibret, to bring out the point that it was the movement of a shadow that was the subject of investigation, proposed the name of fantoscopie retinienne, and, Mr. Priestley Smith, probably anglicizing this term and dropping the allusion to the retina, called it the shadow-test. This name though a compound word, where a simple one should do, became extremely popular, and its appropriateness led Chibret to call to his aid, the linguistic skill of M. Egger, who ren- dered it in the term skiascopia, which in its French form skiascopie, or its English form, skiascopy, has been most widely accepted as the proper term to designate the test. Umbrascopy, proposed by Hartridge, is indefensible on linguistic grounds, and the same is true of papilloscopie proposed by Landolt, and for which he afterwards offered the equivalent, koroscopie. Dioptroscopie was advocated by Galezowski {Atlas d' Ophthalmoscopie) and is appropriate, though equally applicable to other methods of measuring refraction. Retinophotoscopie and retinoskiascopie have have also been recently suggested by Parent, but there seems to be no suf- ficient reason for retaining in the name any allusion to the retina. Fundus-reflex-test suggested by Oliver is also unnec- essarily long for a name. The suggestion has sometimes been made to apply one of these names to one form, and another name to another form of the test. But such a use of them is not warranted by their original suggestion or by custom, nor is there any sufficient reason for the employment of separate names to differentiate various forms of the test. In all its different forms, the test is essentially the same ; the difference being merely as to the apparatus and mechanical detail. Difficulties of the Test — That skiascopy, though a val- uable method of examination, is one difficult to master, 12 SKIASCOPY. becomes more and more evident as one continues to work with it. The theoretical basis is perfectly simple ; the fundamental phenomena readily observed ; and, with a few days practice, the merest tyro may be able by it to estimate the refraction in favorable eyes with an accuracy not to be attained by any other objective method. But long after the stage of such acquirement has been passed, the surgeon will again and again encounter cases that still prove diffi- cult and puzzling. Nothing but a thorough understanding of the optical principles involved, and patient study of the eyes which prove most puzzling, under carefully arranged favorable conditions, will enable one to master the test. The importance of careful arrangement of the relative positions of the light and of the observer, and the adapta- tions of the mirror have not heretofore been sufficiently insisted upon. What these adaptations and arrangements are will appear under their proper headings in chapters III and IV. It is here only necessary to emphasize their importance. For instance : All descriptions of the shadow- test allude to the characteristic band-like appearance of the light in astigmatism. Now, as a matter of fact, even in the highest degrees of astigmatism, such an appearance cannot be perceived, except with certain lenses, or at cer- tain distances in front of the eye ; and it is a distinctive and exact indication only when the light, the mirror, and the patient's and the observer's eyes are brought into a certain relation. It would be as rational to attempt to measure refraction with an ophthalmoscope devoid of any lens series, or to test the acuteness of vision in a darkened room, as to expect definite and satisfactory results from skiascopy, applied without careful attention to details that have usually not been referred to in descriptions of the test. DIFFICULTIES. 13 The fact that this test shows, as does no other, the actual refraction of the eye for each particular portion of the pupil, increases enormously the wealth of phenomena it offers for study, adding to its scientific and practical value, but also making it more difficult by rendering it necessary to discriminate between the particular portions of the movement of light and shade which are of practical im- portance, and others which are not. How to Study the Test. — The study of skiascopy is something quite different from its practical application. To start from a few bare rules as to the placing of glasses, and the movements of the mirror, and the light in the pupil, and attempt to learn the test by using it will never give a mastery of it. It is better to make a careful study of it before attempting to employ it as a method of ascertaining the refraction. Such a study is chiefly a use of the test, but from a standpoint entirely different from that of its application in practice. To study the test, one should as far as possible, start with known conditions of refraction, with lenses of known strength, with the eye at a known distance, and should observe the character of the movements of light and shadow in the pupil, which belong to these known condi- tions. He should work from known refraction to the pupillary appearances that belong to it. While in using the test for the measurement of ametropia, he has to deduce from observed pupillary appearances the state of refraction causing them. The student may, from time to time, test his progress towards proficiency by attempts to measure refraction by skiascopy, but familiarity with the appearances indicative of known conditions of refraction is chiefly to be sought. The appearances upon which the attention is fixed in .14 SKIASCOPY. skiascopy are those of the red reflex in the pupil. The first step is to learn just what this appearance is and some of the variations of which it is capable. Let the beginner with his eye at the sight-hole of the skiascopic mirror throw into the observed eye, from a distance of 20 or 30 inches, the light from a lamp flame as in the ordinary oph- thalmoscopic examination. Looking into the observed eye with the light properly directed, he will see the brilliant point of light, the reflection from the surface of the cornea of the lamp flame he is using ; and he may also see reflections of his own face or of other objects from the surface of the cornea. These are to be disregarded. The real object of study, the phenomena upon which attention is to be fixed, is the general red glow perceived within the pupil, the fundus reflex. If the mirror be rotated about an axis lying in the plane of the mirror, the area of light thrown by it upon the face will move in the direction towards which the mir- ror is turned. As the test becomes familiar, the direction of this movement will be known without any conscious effort to discover it. With the concave mirror at a greater or lesser distance than its focus or with the plane mirror at all distances, except at the point of reversal which it is the object of the test to determine, the rotation of the mirror also causes a movement of the red reflex in the pupil. As the reflex disappears from the pupil, it is followed by an area of shadow, and, as it returns to the pupil, the shadow passes out before it. The movement of the light area really goes on when no shadow is visible in the pupil, but only when light and shade are both seen can the movement be recognized. We know the movement of light in the pupil by the movement of the boundary between light and shade. STUDY OF THE TEST. 15 Having learned what it is that he has to watch in the pupil, the student should make himself familiar with the various appearances of the fundus reflex by viewing it from different distances, with different lenses before the eye, with different mirrors, and later, if he chooses, in a number of different eyes ; and all this without concern- ing himself as to the state of their refraction, or the especial significance of what he does. That is, he should learn to some extent what are the variations in the pupillary reflex, a few of which are illustrated on the following pages, before attempting to appreciate their significance. Without a good understanding too of the simple optical principles underlying the test, it must remain a blind routine and rule of thumb work, and can never be of the highest utility. To aid in such an understanding of them, one may take a strong (15 D. to 20 D.) convex lens and a piece of card-board with a dot on it. The lens can repre- sent the dioptric media of the eye, the card-board the retina, and the dot the light area upon the retina. The card-board should be held back of the lens a little farther than its focal distance, and the dot looked at through the lens from various distances. Nearer the lens an erect image of the dot (blurred of course), and, farther away, an inverted image will be seen, and between the two the phenomena of rever- sal. The movement of light on the retina may be imitated by a slight movement of the card in different directions. The apparent enlargement of the dot, as the point of reversal is approached, and the diminution of its apparent size as the point of reversal is departed from, its diffusion and indistinctness near the point of reversal, and its con- centration and greater definiteness away from the point of reversal, are to be observed. Such a combination of dot and lens will also beautifully exhibit the phenomena of aberra- 16 SKIASCOPY. tion [See Chap. V] with its central and peripheral areas of differing movement, the one an erect and the other an inverted image. The difficulty of keeping the dot in view when the point of reversal is approached, will illustrate how small a portion of the retina is visible from the point of reversal when the test is applied to the eye. By holding in combination with the spherical lens a cylindrical lens of 5 D., the distortions of the fundus reflex produced by astigmatism, and the band-like appearances it causes at certain distances, should also be studied. This is not all to be done at a single trial, but the lens and card should be kept at hand where they can be used to- parallel and elucidate the different conditions as they arise in studying the pupillary reflex. The study of the appearances in the eye may thus be carried on : Take an eye, the refraction of which is known, and from a distance that will give an erect movement, throw the light into the eye, and, by the rotation of the mirror, produce and study the erect movement. Then with a lens which it is known will give an inverted movement,, the inverted movement is to be similarly studied. Finally the lens, or position of the observer is to be so varied as to bring the point of reversal to the eye, and the appearance of the pupil from this point is also to be studied. In these studies, and, indeed, throughout the whole course, the student will find it easier to master and understand first the appearances with the plane mirror. If it is possible to get an eye free from astigmatism or aberration of any notable degree, these earlier studies of the appearances will be much simplified. After the above have become familiar, the phenomena of astigmatism may be studied by placing before the same eye, a cylindrical lens of known strength. The point of reversal with such STUDY OF THE TEST. 17 a lens will give an observer the appearances presented by the pupil at this distance and at the other distances the other appearances presented in astigmatism can be ob- tained. For example, suppose the eye at the student's disposal is hyperopic i D. Let him first place before it the convex 2 D lens. This will bring the point of reversal one metre from the eye. With the plane mirror, let him first study the erect movement at one-half metre ; then study the inverted movement at a distance of two metres ; then observe the eye from the point of reversal at one metre, and then vary his distance so as to study it from intermediate points. When he takes iip the study of astigmatism, he should place before such an eye, a convex cylindrical lens of 2D in addition to the spherical. Then from the distance of one-third of a metre he will be able to observe the band of light at right angles to the axis of the lens, from a distance of one metre the band of light running in the direction of the axis of the lens, and from other distances the other appearances indicative of astigmatism. Familiarity with the many appearances due to aberra- tion and irregular astigmatism is only to be obtained by study of eyes presenting those defects. But, as the great majority of eyes present them in notable degree, material for such a study is not difficult to obtain. Careful observa- tions of the corresponding appearances, with the lens and card-board already referred to, will enable the beginner promptly to recognize the appearances of aberration. And, when once he has found an eye that presents them, let him observe them with different lenses, and [with the plane mirror] from varying distances. A considerable part of the study of skiascopy and espe- 2 18 SKIASCOPY. cially of the appearances of positive aberration can be carried on with the aid of an^artificial, schematic, or model eye. That of Frost is one of the best, although any, even the rudest, will answer. In the studies on the human eye, it is better to study one eye long and repeatedly, or at most to confine the earlier observations to a few eyes than to attempt to employ a large number. Each additional eye will introduce variations in the appearances presented, which will at first be onK\,puzzling and retard, rather than assist, the mastery of the test. CHAPTER II. GENERAL OPTICAL PRINCIPLES. MYOPIA, EMMETROPIA, HYPEROPIA. Skiascopy is a method of measuring myopia, either the myopia originally present in the eye or that produced by a lens of known strength for the purpose of measurement. In myopia, we have the retina situated back of the princi- pal focus of the dioptric media, so that the rays of a certain divergence, that is coming from a point a certain finite dis- tance in front of the eye, are brought to a focus upon the- retina. Conversely, the rays coming from a point on the retina and passing out through the crystalline lens and cornea, are brought to a focus at the same distance in front of the eye. The point for which the eye is focused, and the point on the retina, on which the focused rays are: received, have the relation of conjugate foci to the refract- ive surfaces of the eye. The Reversal of Movement. — The amount of m3'opia is known when we know the distance of the point in front of the eye, which has this relation of a focus conjugate to the retina. Skiascopy furnishes a method of determining the position of this point. Closer to the eye, than this point for which it is focused, the observer may see an erect image of the fundus. Farther from the eye than this point, he can perceive an inverted image. Skiascopy is a means of determining when the image seen is erect and when it is inverted, or when it passes from the erect to the inverted^ (19) 20 GENERAL OPTICAL PRINCIPLES. This may be understood from a study of figure i. Let M represent a myopic eye, A and B being two points of the retina from which rays emerge to reach the ob- server's eye ; and C and D the points at which these rays coming from the retina are focused, the rays coming from A being focused at C and those from B at D. The apparent position of a point is determined by the direction of a ray coming from that point and passing through the nodal point of the observer's eye. Suppose the observer's eye is placed at N, closer than the point for which the observed eye is focused. The apparent position of the point A is determined b}' a ray which passes through the upper part of the pupil and is turned down. It appears Fig. I. in the direction of a. The apparent position of the point B will be located by the ray coming through the lower part of the pupil and turned up. It will be seen in the direction of b. Thus, from this position N, the point A, which is really above appears above, and the point B, which is really below appears below. The observer sees an erect image. When, however, the observer places his eye at N', at a greater distance than that for which the eye is focused, the ray which reaches his nodal point from A, will be one that comes through the lower part of the pupil and is turned up ; so that A will appear to be located in the direction of a' in the lower part of the pupil. From this REVERSAL OF MOVEMENT. 21 position he will judge the location of B by the ray which comes through the upper part of the pupil and is turned down, so that B will appear to be located in the direction of b' in the upper part of the pupil. That is, the point A, which is really above, will appear to be below, and the point B, which is really below will appear to be above. The image observed is inverted. The Point of Reversal. — It is evident that this change in the relation of the rays that brings about the change in the apparent position of A and B occurs at the distance of the points C and D, at which, the rays coming from the retina are focused. Here it is that these rays intersect and take their new relation which gives the reversal of the apparent position of the points of the retina from which they come. It is, therefore, convenient in connection with skiascopy to designate this point as the point of reversal. This name indicates the significance of this point with reference to this test. Of course, it is really the same point as the far point of the myopic eye — the point for which the eye is focused — the conjugate focus of the retina — these latter names indicating the relations of the same point in other matters. It is only when the rays leave the eye, convergent only when the eye is myopic, that they ever come to a focus in front of it. If the eye be emmetropic or hyperopic, the rays emerging parallel or divergent remain so at all dis- tances. Hence, in emmetropia and hyperopia, there can be no point of reversal. From whatever distance the eye is viewed, the image perceived is erect. In myopia, the distance of the point of reversal from the eye depends on the degree of convergence of the rays as they leave the cornea — depends on the amount of myo- 22 • GENERAL OPTICAL PRINCIPLES. pia. The distance of the point of reversal from the eye being the distance from the eye to its far point is the focal distance of the lens required to correct the myopia. So that to ascertain the amount of myopia, we have only to determine the point of re\'ersal and then measure its dis- tance from the eye. Skiascopy determines the position of the point of re- versal by observation of the direction of the movement of light and shade in the pupil. Other kinds of ophthalmo- scopic examinations attempt the recognition of the details •of the fundus image. But, as the point of reversal is approached, the details of the fundus image become indis- tinct and fade away entirely, so that the location of the point of reversal cannot be accurately determined by such an examination. On the other hand, when this point has been so closely approached that the fundus details are quite indistinguishable, it still remains easy to recognize the direction of the movement of light. and shade in the pupil ; and, from it, to deduce the erect or reversed character of the image. Skiascopy, therefore, determines the point of reversal, and measures the degree of myopia with much greater exactness than the fundus-image tests. In skiascopy, we watch the apparent movement of light and shade in the pupil, due to the real movement of an area of light upon the retina. This area of light is secured by reflecting into the eye the light from a lamp with a skiascopic mirror. This is done in a darkened room, in order that the retina outside of this light area maybe dark, furnishing a contrast for the movements to bfe watched. The movement of the light area upon the darkened retina is secured b)' varying the inclination of the mirror — rotating it about some axis passing through the sight hole. The movement produced by a certain change in the position of the mirror depends on whether it is plane or concave. MOVEMENT OF LIGHT ON THE RETINA. 23 Real Movement of the Light on the Retina. The Source of Light. — The lamp flame, or similar source of light used for the test, may be called the original source of light, in contra-distinction to the reflection of it from the mirror, which being more immediately related to the move- ment of the light on the retina, we shall call the immediate source of light. The Plane Mirror. — With the plane mirror the imme- diate source of light is behind the mirror as far as the original source of light is in front of it. The rays reflected from the mirror enter the eye under observation as though they had started from this immediate source. As the mir- ror is rotated, the apparent position of the immediate source of light changes ; for this immediate source is sit- uated upon a line drawn through the original source per- pendicular to the surface of the mirror, and necessarily changes with that perpendicular as the inclination of the mirror changes. With the change of position of the immediate source ■of light, the rays coming from it and falling upon the eye, are made to fall upon a new part of retina, and thus the inclination of the mirror causes the change in the part of the retina that is lit up by the light reflected into the eye. IM 'a 1' \ B A Fig. 2. What these changes are can be better understood by a study of figure 2. L represents the position of the lamp iiame, the original source of light. When the mirror is 24 GENERAL OPTICAL PRINCIPLES. held in the position A A, the immediate source of light is situated at 1, and light entering the eye from that direction falls upon the retina toward a. When, however, the posi- tion of the mirror is changed to BB, the immediate source of light is changed to 1', from which, light falls upon the retina towarb b. As the mirror is rotated from A A to BB, the position of the immediate source of light moves from 1 to 1', and, as a consequence, the area of light upon the retina moves from a to b. The light on the retina then, moves in the direction that the mirror is made to face. It is said to move with the mirror. Only a portion of the light reflected by the mirror enters the eye, the remainder falls upon the face and makes a light area on the face. One may readily demonstrate by trial that this area of light cast by the mirror on the face also moves with the mirror under all circumstances. The rays of light coming from 1 and 1' intersect at the nodal point of the eye ; and passing directly on do not again change their relative position. Whatever the distance of the retina from this nodal point, the movement of the light upon it will be in the same direction, so that whether the retina be at H. as in hyperopia, at E. as in emmetropia, or at M. as in myopia, the real movement of light upon it from a certain movement of the mirror is alwa}'s in the same direction. Therefore, with the plane mirror, the real movement of the area of light on the retina is with the mirror — ivith the area of light on the face, in all states of refraction. This is true for all distances of the light from the mirror, or of the light and mirror from the tested eye. The Concave Mirror. — With the concave mirror as used in skiascopy, the immediate . source of light is a real focus of the mirror, conjugate to the position of the light, MOVEMENT OF LIGHT ON THE RETINA. 25 and is usually situated between the mirror and the eye to be tested. The position of this immediate source varies with the position of the mirror, moving in the direction that the mirror is made to face and causing an opposite movement in the area of light that falls from it upon the retina. B A ^-^ Fig. 3. In figure 3 L again represents the original source of light. When the mirror is in the position AA, the light falling upon it from L is focused at 1., and the little inverted image of the lamp flame there formed is the immediate source of light. From it the rays diverge, some to fall upon the face, and those entering the eye to fall upon the retina toward a. When the mirror is turned to occupy the position BB, the light falling upon it is focused at 1', which becomes the new position of the immediate source of light, and from which the ra}-s entering the eye fall upon the retina toward b. As the mirror is rotated from AA to BB, the immediate source of light moves from 1 to 1' and the light upon the retina from a to b. This will be the direction of its movement in all states of refraction whether the retina be situated at H. as in hyperopia, at E. as in emmetropia, or at M. as in myopia. The portion of the light which falls upon the face, however, and forms the facial area, as can be readily demonstrated by trial, moves in the direction that the mirror is made to face. 26 GENERAL OPTICAL PRINCIPLES. We have then, with the concave mirror, the real movement of the area of light on the retina is against the mirror, and against the light on the face, in all states of refraction. The above is the movement that occurs with the con- cave mirror used as in skiascopy, so far from the original source of light and from the eye to be tested, that the con- jugate focus of the original source of light falls in front of the eye. If, however, the original source of light be brought so close to the mirror that the rays from it are not rendered convergent, but continue to diverge after reflec- tion, the immediate source of light will be a magnified image of the lamp flame, situated behind the mirror as in the case of the plane mirror ; and the movement of the retinal light area will be precisely the same as with the plane mirror. Again, if the rays reflected by the mirror are rendered convergent, but the eye to be tested is brought so near that they cannot come to a focus in front of its nodal point, the light will pass in as though from an imme- diate source back of the mirror, and the movement of the area of light on the retina will again be like that with the plane mirror. If the light reflected upon the eye be con- vergent so as to be focused just at its nodal point, no move- ment of light on the retina such as we have been consider- ing will occur, but whatever direction the mirror is turned, so long as the light enters the eye, the retinal light area remains stationary. It is to be borne clearly in mind that the movement so far spoken of is the real movement of the area of light upon the retina as it would appear from within the eye itself, or when viewed from behind the retina with the sclera and choroid cut away. The Apparent Movement of the Light in the Pupil. — What we observe in skiascopy, however, is the apparent APPARENT MOVEMENT IN THE PUPIL. 27 movement of the light in the pupil as viewed from the position of the observer some distance in front of the eye. When an erect image of the retina is viewed, this apparent movement of the light will be in the same direction as the real movement. When an inverted image is viewed, the apparent movement will be in the direction opposite to that of the real movement. The observer can always watch the movement of the light area on the face, and know that with the plane mir- ror the light area on the retina always has a real movement in the same direction, and with the concave mirror it always has a real movement in the opposite direction ; and he has only to compare the apparent movement of the light which lie watches in the pupil with the known direction of the real movement on the retina, to determine whether he sees an erect or an inverted image, When the apparent and real movements are in the same direction, he knows (page 19) he is looking at the eye from a distance shorter than that for which it is focused. When the apparent and real movements are in opposite directions, he knows that he is looking at the eye from a distance greater than that for which it is focused. The direction of the apparent movement of the light then, will be with the light on the face in hyperopia and in emmetropia at all distances, and in myopia when the eye is viewed from a point nearer than its point of reversal, and the apparent movement in the pupil will be the opposite of the real movement only in cases of myopia when the eye is viewed from somewhere beyond its point of reversal. With the plane mirror, the apparent movement iv'dl be with the light on the J ace in hyperopia, emmetropia, and myopia with the point of reversal behind the observer; and against the light on the face in myopia viewed from beyond the iioint of reversal. 28 GENERAL OPTICAL PRINCIPLES. With the concave mirror the apparent movement is against the light on the face in hyperopia, emmetropia, and myopia with the point of reversal behind the observer ; and is with the light on the face only in myopia viewed from beyond the point of reversal. This statement made to conform to the practice customary in the use of the concave mirror where the observer keeps a constant distance of i metre from the e}'e [corresponding to i D. of myopia] would be : the light moves against the light on the face and against the mirror in hyperopia, emmetropia and myopia of less than 1 D., and only moves with the light on the face in myopia of more than 1 D. These statements are made with reference to the apparent movement of the light before the state of refrac- tion has been modified by any glass placed before the eye for that purpose. But they hold equally as to hyperopia, emmetropia, or myoj^ia remaining imcorrected, or produced by a lens placed before the eye. For instance : — In myopia the movement remains against the light on the face with the plane mirror, or with the light on the face with the concave mirror, so long as the concave lens employed is not strong enough to bring the point of reversal to the dis- tance of the observer's eye. In hyperopia or emmetropia, where the movement is watched through a convex lens, the movement remains with the light on the face for the plane mirror, and against the light on the face for the con- cave mirror, until a convex lens is used strong enough to over-correct the hyperopia and cause enough myopia to bring the point of reversal nearer to the eye than the posi- tion of the observer. Rapidity of Movement of the Light on the Retina. — The rapidity with which the light and shadow appear to move across the pupil depends first, on the rapidity of the real movement of the light area upon the retina ; and, RAPIDITY OF MOVEMENT. 29 second, upon the magnification of the retina. The rapidity of the real movement on the retina depends : On the rate of movement of the mirror in the observ- er's hand. On the distance of the mirror from the observed eye. On the distance of the original source of light from the mirror. And upon the distance of the retina from the nodal point of the eye. The rate of movement of the mirror and the distance of the light from the mirror determine the rapidity of the movement of the immediate source of light ; this being greater as the mirror is moved more quickly, or as the ori- ginal source of light is more distant from the mirror. The excursion which the immediate source of light can make is limited by the width of the mirror, and the extent of movement of the light area on the retina produced by the movement of the immediate source of light entirely across the mirror depends on the relative distance of the mirror and the retina from the nodal point of the eye. The wider the mirror, or, the nearer it is to the nodal point of the eye, or the farther the retina is from that nodal point, the greater the extent of movement produced in the retinal area of light by a given movement of the mirror. On account of the relative distances of the retina from the nodal point, the extent of the movement of the light on the retina is, other things being equal, least in the highest hyperopia and greatest in the highest myopia. The rapidity of the real movement of the light on the retina then, is increased : By moving the mirror faster. By carrN'ing the original source of light farther from the mirror. 30 GENERAL OPTICAL PRINCIPLES. By bringing the mirror closer to the eye. By elongation of the antero-posterior axis of the eye- ball. The real movement of the light upon the retina is made slower : By moving the mirror more slowly. By bringing the original source of light closer to the mirror. By carrying the mirror farther from the eye. By shortening of the antero-posterior axis of the eye- ball. In using the test, the distance of the light from the mirror is practically constant, and the ordinary variations in the antero-posterior axis of the eye-ball are so slight as to have no appreciable influence. So that the rapidity of the real movement of light on the retina depends princi- pally on the rapidity of the movement of the mirror and the distance of the mirror from the eye. Magnification of the Retina. — In practice the rapidity of the apparent movement of the light in the pupil depends far more on the extent to which the retina and the real move- ment of light upon it are magnified, than upon the actual rate of that real movement. The retina as viewed through the pupil from different distances, is seen under different degrees of magnification. When the observer's eye is placed at the point of reversal, the rays from a single point of the retina, passing through all parts of the pupil, con- verge to the observer's nodal point, so that the one point of the retina appears to occupy the whole of the pupil, and the retina is seen indefinitely magnified. As the observer's eye departs from the point of reversal, it receives the rays from an increasing area of the retina, more and more of the retinal image occupies the same space of the pupil and MAGNIFICATION OF THE RETINA. 31 the retina is seen less magnified. This is ilhistrated in figure 4, which represents an eye with its point of reversal at A. If the observer's eye be placed at A it receives rays only from the point a, and this point appears to occupy the whole pupil. If, however, the observer's eye be placed at B, from which rays would be focused at b behind the retina, and, at which, rays from b would be focused, the observer will be able to see in the space of the pupil all of the retina, m n, included between the broken lines passing from B to b — all of the retina, which would receive a circle of diffusion if the rays were coming from the point B, Or, again, if the observer's Fig. 4. eye be placed at C, from which rays will be focused at c in front of the retina, and, at which, rays coming from c would be focused, he will be able to perceive the portion of the retina, m n included, between the dotted lines, passing through c and continued on to the retina — the area upon which would be formed a circle of diffusion by rays coming from the point C. It follows then, that the closer the observer's eye to the point of reversal, the more is the real movement of light upon the retina magnified, and, therefore, the swifter does it appear. The farther the observer's e}e is removed from the point of reversal, the less is that real movement of the light on the retina magnified ; and the slower is the apparent movement as watched in the pupil. 32 GENERAL OPTICAL PRINCIPLES. And, as this source of variation overcomes all other sources of variation in the rate of the apparent movement of the light, [except the rate of movement of the mirror, which is to a considerable extent under the control of the observer] the raj^idlty of the apparent tnovement of light and shade in the pupil increases as the point of reversal is approached and diminishes as that point is departed from, and constitutes a measure of the degree of ametropia remaining uncor- rected. Form of the Light Area. — The real form of the light area on the retina, except under certain conditions in astigmatic eyes, will be circular. If the light be perfectly focused on the retina it is circular, because that is the form of the source of light employed (see Chapter III). If the light be not perfectly focused on the retina, the circular pupil gives its form to the resulting area of diffusion. Fig. 5. Fig. 6. The apparent form of the light area as seen in the pupil of astigmatic eyes will be discussed in Chapters IV and V. But in eyes free from astigmatism this form varies with departure of the observer's e}'e from the point of reversal. If the magnification of the retina is so slight that all of it occupied by the light area is visible in the pupil at one time, that area appears circular as represented in figure 5. But when the point of reversal is approached so that the magnification of the retina prevents all of the retinal light FORM OF THE LIGHT AREA. 33 area from being seen at one time, only a portion of its ont- line is visible as an arc of the greatly enlarged circle, as shown in figure 6 ; and the nearer to the point of reversal that the observer comes, the nearer does the boundary be- tween light and shade approach to a straight line. It must be borne in mind, however, thai this is still part ot the boundary of a circle, and hence that different parts will run in all the different directions ; in contradistinction to the band-like appearance of astigmatism, the direction of which always conforms to one or the other of the principal meri- dians (see pp. 47, 55). Brilliancy of the Light in the Pupil. — This depends on the illumination of the retinal light area and the extent to which that area is magnified. The illumination of the light area on the retina depends on the brightness of the original source of light and the accuracy with which the light coming from it is focused on the retina. The brighter the source of light and the more accurately it is focused, the brighter the illu- mination of the retina. The dimmer the light and the larger the circle of diffusion over which it is dispersed, the more feeble the retinal illumination. As the immediate source of light is usually near the mirror (in front for the concave, behind for the plane)) when the mirror and the observer's eye approach the point of reversal, or the point of reversal is brought to them by a change of lenses, the light being more nearly focused on the retina, the retinal illumination becomes brighter. But, as the point of reversal is approached, the appar- ent brightness of the light area in the pupil is diminished by the increasing magnification of the retina, which causes the light from a smaller part of the retinal area to occupy the whole space of the pupil. Hence the brightest light 34 GENERAL OPTICAL PRINCIPLES. reflex is never obtained at the point of reversal, but usually in practice at one or two dioptres from the point of reversal, its exact position being dependent on the arrangement of the source of light. Finding the Point of Reversal — The point of reversal is to be recognized only when the observer's eye is in its immediate neighborhood. This may be effected either by varying the distance of the observer's eye from the observed eye until it comes to the position of the point of reversal, or by varying the position of the point of reversal by changes in the lenses placed before the observed eye until the point of reversal comes to the chosen position of the observer's eye. For reasons to be stated in Chapter VI, the former method is the better when using the plane mirror, and the latter is to be resorted to when the conca\'e mirror is employed. In any case, the trial movement across the pupil shows by the direction of the movement whether a point of reversal exists between the observer and the obser\-ed eye, and the rapidity of movement shows approx- imately [when the observer has learned to appreciate its significance] the extent of the interv^al between the position of the observer and the point of reversal. If the movement be slow, the inter\'al may amount to several dioptres. If it be rapid, the interval is less. Upon these data of the direction and rapidity of the movement, the surgeon bases the next step of the test, the selection and placing of the lenses before the eye. This being accomplished, the test is repeated, the movement with the lens noted both as to its direction and rapidity, and the distance of the observer from the patient, or the strength of the lens before the observed eye, varied in accordance therewith. This process is continued until the obser\'er's eye reaches the point of reversal, or the point of POINT OF REVERSAL. 35 reversal is brought by the lens to the observer's eye. But the test should not be regarded as completed until the movement has been repeatedh' viewed both from within and beyond the point of reversal, as well as from that point. Only by this precaution of observing from a slightly greater and a slightly less distance, or with a slightly stronger or slightly weaker lens than that which brings the point of reversal to the surgeon's eye, can the certainty of a correct result be assured. CHAPTER III. CONDITIONS OF ACCURACY. Since in skiascopy one has to observe the movement of an area of light across the shaded retina, the size, bright- ness and sharpness of the contrast between the margin of this light area, and the shadow immediately adjoining it are very important factors in determining the definiteness and accuracy of the test. For reasons to be presently dis- cussed, the contrast between light and shadow as seen in the pupil necessarily diminishes as the point of reversal is approached. It is, therefore, important to have the contrast between light and shadow upon the retina as sharp as pos- sible. Darkening the Room. — To secure this contrast, the retina outside of the proper light area should be in absolute darkness. This requires a complete darkening of the room in which skiascopy is practiced, including the shading of the source light, except in the direction in which it is used. The difference in the ease of the test as applied in a completely darkened room, as contrasted with its use in a partially darkened room, can only be appreciated by one accustomed to applying it under the former condition. The Source of Light. — To secure the brilliant illumi- nation of the light area, in contrast with the complete shadow around it, the source of light must be as bright as possible. On account of the difficulty about the sight hole to be referred to later, the arc electric light cannot be (36) THE SOURCE OF LIGHT. 37 ■employed, except to illuminate a piece of ground glass as suggested by Derby. The incandescent electric light is not available on account of its form, so that recourse must be had to one of the various illuminating flames. Of these the paraffin candle is the most brilliant. Next come the heavy mineral oils, and gas flames reinforced with the richer hydro-carbons, or used on the Welsbach mantle, and after this the ordinary illuminating gas. But a good flame of the latter furnishes a satisfactory illumination. It is more important whatever flame is used that the brightest part of it should be employed. With all flames there is at the margin a comparatively gradual shading from light to darkness, which interferes with the sharpness of the boundary of the light area on the retina. To secure that sharp boundary as well as to prevent the diffused illu- mination of the room, and to limit the size of the source of light, the flame should be entirely covered by an opaque shade with an aperture of the proper size placed opposite the most brilliant part of the flame. This gives, under proper conditions of focusing, a perfectly sharp margin to the light area on the retina. The size of this opening in the opaque screen deter- mining the size of the original source of light is governed by various conflicting requirements. Enough light must be available to give a distinct area of light upon the face, as well as to give sufficient illumination within the pupil. The source of light must be considerably larger than the sight hole in the mirror. As the mirror is rotated, the immediate source of light appears to move across it, and if this source were not larger than the sight hole, it would, at times, entirely disappear within that opening. At such times, the light area would disappear from the retina and from the pupil, causing delay and uncertainty in the test. 38 CONDITIONS OP ACCURACY. If, however, the immediate soiirce of light is sufficiently larger than the sight hole, no such disappearance of the light occurs. On the other hand, the ease of distinguishing special forms of the light area, or the different movements in the different parts of the pupil is proportioned to the smallness of the source of light used. The characteristic band-like appearance of astigmatism is developed in proportion as the light area upon the retina approaches the limit of a mathematical point. The size of the opening through which light is ob- tained, is then a compromise between the requirements of light and the size of the sight hole on the one hand, and need to have the retinal light area as small as possible on the other. In practice the writer prefers an aperture five millimetres in diameter, but the beginner may find one of double that diameter more satisfactory. Focusing of the Light on the Retina. — When the rays coming from the immediate source of light are accu- rately focused upon the retina, the area of retinal illumina- tion will be the smallest and brightest, and will have the most definite edge. This accurate focusing is secured only when the immediate source of light is situated at the point of reversal. In searching for the point of reversal, it is, therefore, advantageous to keep the immediate source of light as close to the mirror as possible. With the plane )nirror the immediate source of light being a reflection of the original source as far behind the mirror as the immediate source is in front of it, the closer the original source of light can be brought to the mirror, the closer will its reflection be to the observer's eye, and to the point of reversal at the critical moment when the observer's eye reaches that point. The original source of FOCUSING OF THE LIGHT ON THE RETINA. 39 light then should be kept as close to the mirror as possible being moveable to follow the movements of the observer's eye and the mirror when the distance of these from the eye under observation is varied. When the observer withdraws to the distance of two metres or more from the patient, it may not be practicable to keep the light very close to the mirror, but at such a distance, the separation of the source of light from the mirror becomes of small importance. For, if the original and immediate sources of light were at the mirror, the rays from the latter would have a divergence of one-half D. when they reached the eye ; and, if the original source of light were a metre in front of the mirror, so that the imme- diate source would be one metre behind the mirror, that is three metres from the eye, the rays from it would reach the eye one-third D. divergent, and the difference between the one-half and the one-third D. is so trifling as to be of no practical importance. On the other hand, when the surgeon approaches close to the patient's face, the slight distance that must necessa- rily remain between the original source of light and the mirror becomes a source of imperfect focusing of the light on the retina ; and, therefore, of inexactness in the deter- mination of the point of reversal. Suppose the mirror to be at five inches from the eye and the original source of light three inches from it, this will make the immediate source of light eight inches from the eye, and the ra}'s from it will reach the pupil 5 D. divergent when the surgeon is seeking the point of reversal corresponding to 8 D. of myo- pia. This difference of 3 D. interferes greatly with the delicacy of the test. With the concave mirror, the immediate source of light being a real image of the original source in front of the 40 CONDITIONS OF ACCURACY. mirror, cannot be brought closer to the mirror than its principal focal distance. It is brought closest by carrying the original source of light as far away from the mirror as possible. The original source of light then, for the con- cave mirror, should be behind the patient as far as possi- ble. Position of Greatest Accuracy. — With the plane mir- ror the immediate source of light is necessarily behind the mirror. It will, therefore, be exactly at the point of rever- sal when the mirror and the observer's eye are slightly within the point of reversal. Hence the conditions of accuracy are better complied with for the observation that is made from within the point of reversal, where the light still moves in the pupil with the light on the face, than for the obser\'ation that is made from beyond the point of reversal, where movement is inverted. The point of rever- sal is then, with the plane mirror, most closely approxima- ted from the side toward the observed eye ; and in practice the greatest accurrcy is attained by considering the point of reversal as located at the greatest distance from the eye at which erect movement can be seen in the visual zone of the pupil. With the concave mirror the immediate source of light being necessarily in front of the mirror, can be brought accurately to the point of reversal only when that point of reversal is the focal distance of the mirror in front of the observ^er's eye. It is approached more accurately by the lens which still leaves it in front of the observer's eye, than by the lens which removes it back of the observer's eye. Hence, with the concave mirror, the strongest con- cave lens, or the weakest convex, which allows the move- ment of light in the pupil with the light on the face, is the lens which brings the point of reversal most accurately to the distance chosen. POSITION OF GREATEST ACCURACY. 41 In regular astigmatism, as will be indicated in the ■chapter (IV) upon that subject, the arrangement of the light must be modified, when it is desired to develop the band-like appearance characteristic of that condition. For the measurement of refraction in either of the principal meridians, the adjustment of the light should be precisely the same as for simple hyperopia or myopia. But the band- like appearance cannot certainly be recognized unless the necessary conditions as to the position of the observ'er and source of light are carefully observed. When the proper precautions are taken one can get a characteristic band of light with even one-half dioptre of astigmatism, and by that band can fix the direction of the principal meridians with great accuracy. In the higher degrees of regular astigmatism there is •considerable difficult}^ in measuring the refraction of the principal meridians with accuracy. It is, therefore best, before regarding the skiascopic test as completed to place before the eye such a cylindrical lens as appears to be required to correct the astigmatism, repeat the test, and so ascertain whether the astigmatism has been accurately cor- rected. Fuller references to this matter will be found in the Chapters VI and VII. Irregularities of the Media and Surfaces. — These in- terfere with skiascopy not only by changes in the apparent movement of the light as watched in the pupil, but also by preventing the perfect focusing of the light which falls upon the retina, and in this way, they limit to some extent, the accuracy of the test, since they are present in some degree in nearly all eyes. In the case of positive aberration (see Chapter V), the interference with focusing is of the same kind as the defect in the refraction of a strong convex spherical lens. If one 42 CONDITIONS OP ACCURACY, takes such a lens and intersects the narrowing pencil of rays that have passed through the lens with a piece of card board, he will find that the strong refraction at the margin of the lens causes a ring of condensation at the periphery of the circle of diffusion. This ring is exhibited from close behind the lens back to its principal focus, beyond which, we have the condensation at the centre of the light area and a gradual fading away of light around it. Hence, the circle of diffusion in front of the principal focus pre- sents a brilliantly illuminated edge in sharp contrast with the shadow around it, while at the principal focus and be- hind it, the light area has a sharply defined edge, but fades gradually into the shadow around it. Therefore, in making the test, the influence of posi- tive aberration upon the distribution of light in the light area is to be utilized by having the light focused not exactly on the retina, but slightly back of it. This may be brought about by having the immediate source of light closer to the eye than the obseryer's eye or the point of reversal ; conditions that are secured in the use of the concave mir- ror. Hence, for positive aberration of a certain distribu- tion in the pupil, a sharper and more definitely bounded lieht area is to be obtained bv the use of the concave mir- ror than can be had with the plane mirror. With negative aberration, where the refraction is weaker near the peripher}^ of the pupil, the condensation ring of light is less pronounced and is found back of the principal focus for the central visual area. For this form of aberration the plane mirror enabling the observer by pushing the source of light from the mirror to get the light focused in front of the retina has some advantage over the concave mirror. The interference with the focusing of the light on the IRREGULARITIES OF THE MEDIA AND SURFACES. 43 retina due to irregular astigmatism cannot be overcome in any way, and it impairs the value of the test and makes it more difficult to apply in eyes presenting marked defects of this kind. Distance of the Surgeon from the Patient It will always be impossible to determine the point of reversal with perfect exactness. The best that can be done is to make out that it lies, within narrow limits of possible error, at about a certain distance. It may be an inch or two nearer, it may be an inch or two farther off. If the distance be a short one, if the lens used is such that the point of reversal is brought close to the observed eye, the possible inaccuracy of distance will cause an appreciable error in estimating the refraction measured in dioptres. For instance : at eight inches from the eye, two inches additional, making ten inches, will mean a whole dioptre of refraction, and two inches less, making the dis- tance six inches, will mean a difference of a dioptre and a half. On the other hand, at eighty inches, a foot either way will correspond to less than one-quarter of a dioptre of inexactness. Hence, for accurate work it is best to make the determination of the point of reversal at the greatest distance at which it can be certainly made in the visual zone. The importance of this has been especially dwelt upon by Randall (Trans. Section on OpJdhalmoL Am Med. Assoc, 1894, p. 63). What this distance may be will var}- in different eyes. In general, it is limited by the size of the area in which the movement of light and shade is to be watched. The pupil fully dilated may be eight or ten millimetres in diameter, and movement across the whole width of such a pupil could be readily watched at a distance of 4 to 6 metres. But the diameter of the visual zone of the pupil, 44 CONDITIONS OF ACCURACY. the only area in which the movement is of practical importance, is commonly much less than this, say from 4 to 6 millimetres, and the movement of light across it can only be satisfactorily studied within the distance of two or three metres. Beyond one metre, however, the necessary inaccuracies of distance become usually of slight practical importance. In cases of aberration invading the central portions of the pupil and still more in cases of irregular astigmatism, the visual zone is considerably less in area than in the ordinary normal eye. In these cases, the test must be applied from a still shorter distance, often one-half or one-third of a metre, or even less. With the plane mirror it is easy to adopt any distance that suits the particular case. With the concave mirror any considerable variation in the distance requires a corre- sponding variation in the focus of the mirror used. A mir- ror of shorter focus being employed when the distance between the observer and patient must be short ; and of longer focus if a greater distance is to be maintained. The reason for this is that if the concave mirror be brought too close to the observed eye it gives an immediate source of light relatively too large, while if it be removed too far from the patient's eye, the diffusion is so rapid that it gives an illumination that is too feeble. These changes are much more rapid with the concave than with the plane mirror ; as one may readily demonstrate by holding both mirrors in his hand in the darkened room and reflect- ing areas of light upon a wall from various distances. I have elsewhere {Journal of the Am. Med. Assoc, Sept. 4, 1886) demonstrated the relations of the one to the other. In general the distance at which a concave mirror can be DISTANCE OF THE SURGEON FROM THE PATIENT 45 used to best advantage is a little over four times its focal distance. For the majority of cases then, a distance of from 5/2 to 2 metres is convenient for the plane mirror ; and one metre or a little less for the concave mirror, which shonld have a focal distance of from 20 to 25 centimetres. When it is desired to make the shadow test as accurate as possible, it is well to complete the test by placing before each eye lenses representing its supposed correction, with such addition to the convex or diminution of the concave spherical as shall bring the point of reversal to the great- est distance at which it can be satisfactorily studied in the particular eyes in question ; and then to test the movement of light and shade at that distance, looking especially for uncorrected astigmatism, and comparing the one eye with the other for any evidence of remaining inequality of refraction. CHAPTER IV. REGULAR ASTIGMATISM. The essential fact of regular astigmatism is that in two different directions, at right angles to each other [the prin- cipal meridians], the curvature of the dioptric surfaces dif- fer, so that they exert unequal refractive power ; and that in all other directions, or meridians, the refractive power bears such a relation to the refractive power of these prin- cipal meridians, that it is only necessary to consider what happens in their direction. Two Points of Reversal, — In such an eye, the rays coming from the same point of the retina, and passing out through surfaces that refract imequally in different merid- ians, must leave the eye with different degrees of divergence or convergence in the directions of these different meridians. If the ra)'s are convergent, or rendered so by passing through a convex spherical lens, they will be more convergent in one principal meridian than the other, and the point of reversal for one principal meridian will be at a different distance from the eye, from the point of reversal for the other principal meridian. The position of the point of reversal, giving the amount of myopia (either original or produced) in the principal meridian to which it belongs, the difference between the amounts of myopia in the two principal meridians will be the astigmatism. The general plan of measuring astigmatism by skiascopy, therefore, is to ascertain the point of reversal and measure the degree (46) THE BAND-LIKE APPEARANCE. 47 of myopia for each principal meridian, and by subtracting the one from the other, to find the amount of regular astigmatism. The Band-like Appearance. — This difference in the position of the points of reversal for the different meridians, gives rise to certain phenomena of great practical import- ance in skiascopy. It is true of the astigmatic as of the non-astigmatic eye, that, as the point of reversal is ap- proached, the image of the retina seen through the pupil becomes magnified (see Chapter II). And, it necessarily follows that when the observer's eye is nearer to the point of reversal for one meridian than it is to the point of rever- sal for the other meridian, that the retinal image is more magnified in the direction of the principal meridian, to which the nearer point of reversal belongs. When the observer's eye is placed at the point of re- versal for one meridian, the retinal image becomes indefi- nitely magnified in the direction of that meridian, while comparatively little magnified in the direction at right angles to it. Each point of the retina then appears in the pupil as a line running in the direction of that principal meridian , and the retinal light area, which consists of a number of these points, takes the form of an elongated band of light running in the direction of the principal meridian, which has its point of reversal at the observer's eye. This is the band-like appearance of the light in the pupil, char- acteristic of astigmatism bounded by the " linear shadow " of Bowman. Figure 7 represents this appearance when the eye is placed at the point of reversal for one principal meridian, represented about twenty degrees from the vertical ; and figure 8 represents the appearance presented at the point of reversal for the other principal meridian, twenty degrees from the horizontal. Its direction is always 48 REGULAR ASTIGMATISM. that of the principal meridian, at whose point of reversal it is seen, and it is more pronounced, in proportion to the degree of astigmatism, the nearness of approach to the point of reversal, and the perfection of the focusing of the light upon the retina in the direction perpendicular to this prin- cipal meridian, that is, in the other principal meridian. Fig. Fig. 8. In estimating astigmatism by skiascopy, two distinct things are to be done, which require different arrangements of the source of light. The first is to determine accurately the direction of the principal meridians by bringing out most distinctly this band-like appearance in the pupil, in- dicating the direction of one of these principal meridians ; the other being always, for regular astigmatism, at right angles thereto. The second thing to be done is to measure accurately the refraction in each of these principal merid- ians, testing them, of course, one at a time. The test proceeds at first as for myopia or hyperopia in a non-astigmatic eye, until a point of reversal is found. Then it is discovered that this point of reversal is only for the movement of light and shadow in one direction, and does not hold for movements at right angles to that direc- tion. The observer has now brought his eye to one point of reversal where the band-like appearance can be best per- ceived. But, as he has been working with the original source of light in the position most favorable for the meas- THE BAND-LIKE APPEARANCE. 49 urement of hyperopia and myopia, the position that brings the immediate source of light as close as possible to the mirror (see Chapter III), he will probably see very little ap- pearance of the band in the pupil, even with the higher degrees of astigmatism. The reason for this is that with the immediate source of light in this position, the light is most accurately focused on the retina in the direction that the band should take. And, in the direction at right angles to the band, the focusing is quite incomplete, so that the diffusion at what should be the sides of the band partly or entirely neutralizes the effect produced by the magnification of the retina, which, otherwise, would cause the band-like appearance. In order to bring out this band-like appearance, it is necessary to make the focusing from side to side of the band as perfect as possible. And, to secure the perfect focusing in the principal meridians at right angles to the one in which the band is sought, the immediate source of light must be brought to the point of reversal for that other principal meridian. The band-like appearance is most per- fectly developed luhen the observer's eye is at the point of reversal for one principal meridian, and the immediate source of light at the point of reversal for the other principal meridian. Fig. 9. In figure 9, the solid lines represent the vertical merid- ian of an astigmatic eye and^ the rays emerging, so turned 4 50 REGULAR ASTIGMATISM. in that meridian, as to give the point of reversal at V. The broken lines represent the less curved horizontal meridian of the cornea, and the rays so turned in that meridian as to give a point of reversal at H. The dotted lines represent a plane mirror, P P, with the eye of the observer at V, and the light h pushed off from the mirror, so that the rays enter the eye as though they came from H, and are per- fectly focused on the retina in the horizontal meridian, ren- dering most distinct the appearance of a vertical band. For illustration, suppose a case (which the student will do well to reproduce for actual study, either in the artificial eye or by lenses placed before the living eye) having com- pound myopic astigmatism, the vertical meridian of the cornea being 2 D. myopic and the horizontal meridian i D. myopic. When, with the plane mirror, the observer's eye is one-half metre from the observed eye, it will be at the point of reversal for the vertical meridian, and in a position to see a vertical band of light. But, if the source of light be placed as close to the mirror as possible, the rays from it will be the more accurately focused upon the retina in the vertical meridian and more diffused horizontally, so that the I'eal form of the retinal light area will be rather that of a horizontal line or band. Now, from the observer's position, the retina is most magnified in the vertical direction, and this vertical magni- fication would cause a point of light on the retina to appear as a vertical band in the pupil ; but, with the light area really in the form of a horizontal band, the effect of the magnification is largely neutralized and the appearance in the pupil may be quite indefinite. To bring out the band-like appearance : While keeping the observer's eye and mirror in the same position, the the original source of light must be pushed off from the mir- THE BAND-LIKE APPEARANCE. 51 ror one-half metre, the immediate source then retreats cor- respondingly behind the mirror, and approaches the posi- tion of the point of reversal for the horizontal meridian, one metre from the eye. With the light and mirror in this relation to the eye, the rays are focused upon the retina perfectly in the hori- zontal meridian and diffused in the vertical meridian, so that the real form of the retinal area of light is a vertical line or band. This vertical line or band being viewed from the point of reversal of the vertical meridian (where it will be greatly magnified in the vertical direction and but slightly magnified in the horizontal direction), gives rise to the appearance of the most distinct vertical band of light in the pupil. And, under these conditions, the presence of the astigmatism and the direction of one of its principal meridians is most clearly and accurately revealed. Taking the same case and using the concave mirror at a distance of one metre, which is the point of reversal for the horizontal meridian, the appearance of a horizontal band of light in the pupil should be most distinctly visible. But, in order to develop it clearly, it will be needful to bring the original source of light so near to the mirror that the immediate source will be one-half metre in front of the mirror, that is, one-half metre in front of the observed e}'e, at the point of reversal for the vertical meridian. For it is from this position the light will be most perfectly focused on the retina in the vertical meridian, while diffused in the horizontal meridian, and the horizontal magnification of the retina at the point of reversal for the horizontal merid- ian where the observer's eye is placed, will emphasize and increase the appearance of the horizontal band of light then thrown on the retina. Since, with the plane mirror, the immediate source of 52 REGULAR ASTIGMATISM. light is always back of the mirror, and cannot be brought in front of it, the direction of the band can only be accu- rately determined for the meridian whose point of reversal is nearest the eye. It is only with the eye and mirror at this point of reversal that one is able, with the plane mir- ror, to bring the immediate source of light to the other point of reversal. And, with the concave mirror, since the immediate source of light is always in front of the mirror, the band-like appearance can only be distinctly brought out in the meridian which has its point of reversal the farther from the eye, as only with the eye at that point of reversal can the immediate source of light with the concave mirror be brought to the other point of reversal. With either the plane or concave mirror, only the band in one of the principal meridians can be most distinctly developed. But it is unnecessary in practice to bring out the bands in both meridians, since, by knowing the direc- tion of one principal meridian, the other being always per- pendicular to it, is also known. The measurement of the refraction in either of the principal meridians of astigmatism, is quite similar to the measurement of refraction in hyperopia and myopia. To determine whether the movement of light in the pupil in a certain meridian is with or against the movement of light upon the face, it is necessary that the focusing of the light on the retina be as perfect as possible in that particular meridian. To secure this, the immediate source of light must be as close as possible to the position of the observer's eye (see Chapter III). Hence, having determined the ex- istence of the astigmatism and the direction of its princi- pal meridians, the measurement in these meridians will proceed as the measurement of myopia or hyperopia. CHANGES IN THE LIGHT AREA. 53 Changes in the Light Area at Different Distances. — In regular astigmatism, supposing the eye to be myopic in all meridians, or a convex lens placed before it sufficiently strong to over-correct the hyperopia in all meridians, the observer using a plane mirror and viewing the eye from different distances, will be able to recognize the following changes in the appearance and movement of the light in the pupil. From a position within the point of reversal of the more myopic meridian, the light will be seen to move with the light on the face, in all directions. As the observ- er's eye is withdrawn from the observed eye, and approaches the point of reversal for the more myopic meridian, the light area in the pupil becomes elongated in this meridian ; and, while the movement is still with the light on the face in all meridians, it becomes more rapid in the direction of this elongation than in the direction perpendicular thereto. The observer, withdrawing his eye still farther on reaching the point of reversal for the more myopic merid- ian, [V, in figure 9,] is unable to distinguish the movement in this meridian, while the movement in the meridian at right angles to it is still with that of the light on the face. This point being reached, if the original source of light be pushed away from the mirror, so that its reflection, the immediate source of light approaches the point of reversal for the less myopic meridian, the form of the light in the pupil becomes a distinct band running in the direction of the more myopic meridian, readily seen to move from side to side, but without perceptible movement in the direction of its length. Bringing the source of light back to its usual position close to the mirror, and withdrawing his eye still farther from the eye under observation, the observer again sees the 54 REGULAR ASTIGMATISM. movement of the light in the pupil in all directions. But in the direction of the most myopic meridian, it is now against the light on the face ; while in the meridian at right angles to this, it is still with the light on the face. The band-like appearance is now lost entirely ; the area of light in the jDupil taking at one distance the same shape as though no regular astigmatism were present. But, as the point of reversal for the less myopic merid- ian is approached, elongation in the direction of that me- ridian may be noticed, and the erect movement of the light in that meridian becomes more rapid than the inverted movement now seen in the more myopic meridian. When the point of reversal for the less myopic meridian [H, figure 9] is reached, the movement in its direction ceases, but it is impossible, at this point (with the plane mirror), to bring out so distinct a band as was seen in the direction of the other meridian. Withdrawing still farther, the light in the direction of the less myopic meridian begins to have an inverted move- ment, at first very rapid as compared with the movement in the more myopic meridian ; but, as the observer with- draws farther from this second point of reversal, the differ- ence in rate of movement in the two meridians becomes less noticeable. With the concave mirror, the same series of appearan- ces are present, except that the directions of movement are reversed — " erect movement " meaning movement of the light in the pupil against the movement of the light on the face, and against the mirror ; and " inverted mo\'ement " meaning the movement of the light in the pupil with the mirror and with the light on the face. With the concave mirror the meridian in which it is possible to bring out the band-like appearance of the light most distinctly is the MOVEMENT OF THE BANDS IN ASTIGMATISM. 55 meridian of less myopia ; and it will be necessary to bring about the series of changes in the movement of the light area, which has been referred to, by changes of the lens placed before the eye, and not by changes in the observer's distance from the eye studied. Direction and Movement of the Bands in Astigma- tism. — The reason for the constant conformity of the di- rection of these bands of light to the principal meridians of refraction is obvious from their dependence on the magnifi- cation of the retina. That conformity sharply separates them from the somewhat similar appearance seen near the point of reversal in eyes free from astigmatism (page 32 ). O/^r:;- — ^o The apparent movement always -^^ at right angles to their direction is / dependent on an optical illusion, of / which one may satisfy himself by .'^ making a hole in the centre of a / sheet of paper, holding behind this hole the edge of a card, and moving it in a direction oblique to this edge. iG. 10. rj^-^^ motion will appear to be in a direction nearly or quite perpendicular to the edge seen. Thus, in figure 10, the real movement of the card be- hind the opening, or the band of light behind the pupil, may be in the direction O o. But the movement will appear to be in the direction P p. CHAPTER V. ABERRATION AND IRREGULAR ASTIGMATISM. In astigmatism, strictly regular, though the refraction differs in different meridians, in any given direction or meridian it is the same at all parts of the pupil. In aber- ration and irregular astigmatism, the refraction differs in different parts of the pupil, even in the same meridian. All eyes present variations of this kind, and these varia- tions constitute an obstacle to the measurement of refrac- tion b}- skiascop}' or by any other method. Appearances of Irregular Astigmatism. — To the be- ginner with skiascopy, they constitute the most serious obstacle he has to encounter. For one who has thoroughly mastered the principles of the test and become familiar with the various appearances of light and shade in the pupil, mistakes due to aberration or irregular astigmatism are readily avoided, while the reason for any uncertaint}' as to the results obtained by other methods, or any failure to secure perfect vision, on account of these defects is revealed. If we suppose two parts of the pupil, one of which has its point of reversal at the observer's eye, while the other is at a considerable distance therefrom, the illumination of the former will be the more feeble, of the latter the more brilliant ; the movement of the light in the former, if per- ceptible, will be rapid, in the latter, slow. If one watches two parts of the pupil, one of which has its point of rever- sal back of the observer's eye, and the other in front of it ; (5(5) IRREGULAR ASTIGMATISM. 57 in the former the light will have a direct and in the latter an inverted movement. With the irregular astigmatism due to preceding cor- neal inflammation, or to the changes in the refraction of the lens that sometimes precede cortical cataract, the pupil ap- pears broken up into a considerable number of distinct areas, each of which has its separate movement of light and shadow, constituting the typical ophthalmoscopic or skia- scopic picture of irregular astigmatism. The appearance Fig. II. Fig. 12. caused by irregular astigmatism following corneal disease is shown in figure ii. That due to changes in the lens- such as may precede cortical senile cataract is shown in figure 12, in which the black lines represent fixed spicules- of actual opacity, while the other parts of the pupil indicate- merely refractive differences, and change from light to dark or dark to light, as the inclination of the mirror is varied. Some such appearance is sometimes presented by young- persons, indicating a congenital defect which may not noticeably increase in many years. If the differences of refraction in the different parts of the pupil are slight — that is, if the aberration or irregular astigmatism is of low degree — these differences of illumina- tion and movement will not be perceptible until the ob- server puts his eve close to the point of reversal. Bfit at 5 58 ABERRATION AND IRREGULAR ASTIGMATISM. the point of reversal, they become perceptible and consti- tute a striking phenomenon in almost all eyes ; and, to the observer who does not understand their significance, one that is extremely confusing. In the nature and arrange- ment of its irregular astigmatism, every eye is peculiar. The number of varieties of play of light and shade that are obtainable as the point of reversal is reached, is equal to the number of eyes examined. Even the two eyes of the same individual differ. The only practical way to deal with irregular astig- matism by skiascopy is to understand thoroughly the gen- eral optical principles of the test, and apply them, so far as is needful, in the individual case. Certain peculiar forms of variations of the refraction of the eye in different parts of the pupil are, however, of sufficient constancy, regular- ity and practical importance, to warrant their separate classification and study. The most important of these is the regular, or symmetrical, aberration of the eye. Symmetrical Aberration.' — This is an error of the refraction of the eye which causes the rays of an incident pencil falling on the same meridian of the cornea, but at different distances from the axial ray, to meet at different distances behind the cornea ; while rays piercing different meridians of the cornea, at the same distance from the axial ray, intersect it at the same point. It is a defect similar to the aberration of convex and concave spherical lenses. It is readily recognizable in almost all eyes by skiascopy. In the majority of cases, it is in the same direction as ordinary spherical aberration ; that is, the margin of the pupil has a stronger lens action than the centre. The rays entering through the margin are brought to a focus first ; the rays ^ For an account of this error of refraction see paper by the author, Trans. Amer. Ophthalmological Society, 1888, p. 141. SYMMETRICAL ABERRATION. 59 entering nearer the centre being focused farther back. This is called 'positive aberration. In a certain proportion of cases, however, the defect is in the opposite direction ; the rays passing near the centre of the pupil being brought first to a focus, and those pass- ing through the periphery being focused farther back. The centre of the pupil has the stronger, and the periphery of the pupil the weaker, lens action. This is 7iegative aberra- tion. The Visual Zone.— The variation of refraction, how- ever, does not usualh' proceed regularly from the centre of the pupil to the margin. But, as with spherical lenses, and to a greater degree, the central refraction is compara- tively uniform over a considerable area ; and, towards the margin the change of refraction becomes progressiveh^ more marked. This area in the centre of the pupil of compara- tively uniform refraction is the usual visual zone. It is the portion of the pupil that is of practical importance for pur- poses of distinct vision. Its size varies considerably. Some- times it includes almost the whole of the dilated pupil, in other eyes an extremely small area near the centre of the pupil will be regular, and the remainder of the pupil use- less for accurate vision. If a high degree of irregular astigmatism be present, the visual zone, instead of behig a central area of considerable size, will often be some particu- lar portion of the undilated pupil, which happens to have the most regular curvature. In dny case, for the correction of ametropia, it is the behavior of the light and shade in the visual area which has to be studied. Its behavior elsewhere may be disre- garded. It is often much easier to watch the movement of light and shade in some other portion of the pupil— some part of the extra-visual zone. And, if the obser\-er does 60 ABERRATION AND IRREGULAR ASTIGMATISM. not understand their relative importance, he will be apt to fix his attention on this latter and be led away from the true refraction of the eye he is examining. This is the more likely to happen, because in that part of the pupil, which has its point of reversal at, or near, the observer's eye, the direction of the movement of light and shade is difficult to see, while in other portions, the movement is more striking. The Appearances of Positive Aberration. — The ap- pearances presented by an ordinary case with positive aberra- tion may be considered in the order in which they will be developed with the plane mirror, the observer starting to examine the eye from within the point of reversal for the most myopic part of the pupil, and gradually withdrawing his eve until it is beyond the point of reversal for the least myopic part of the pupil. From the first position, the light area in the pupil is seen to move with the light on the face entirely across the pupil ; its motions in the edges of the pupil being more rapid and indefinite than in the centre. If, now, the observer's eye is withdrawn to the point of re- versal for the margin of the pupil, there appear in the mar- gin points in which no movement of the light can be seen. Some of these may be points of stationary light, and others, points of stationary shadow. As the obser\'er's eye is still farther withdrawn, the points of stationary light run together and form a complete ring of light in the peripher}- of the pupil, shown in figure 13, which is presently seen to have an inverted motion, to move against the light on the face. Within this is a ring of comparative shadow where the movement is swift and difficult or impossible to recognize ; and still within this lies an area of light, similar to that first seen, but now con- siderably reduced in size, which still moves with the light on the face. POSITIVE ABERRATION. 61 As the observer draws still farther back, this area of light at the centre of the pupil, as shown in figure 14, grows smaller, and its movement more difficult to certainly distinguish. The ring of comparative shadow around it encroaches upon it, and the ring of light in the margin of the pupil in turn encroaches upon the shadow, and becomes brighter and its movement more readily noticeable, fig. 14. Fig. I- FlG. 14. Withdrawing still farther, the point of reversal for the •centre of the pupil is reached. The central area of light hecomes faint and its movement ceases to be noticeable, the ring of feeble illumination surrounding it having swal- lowed it up. But, around this feeble light area, the ring of inverted movement has now grown broad and distinct. And, as the observer withdraws still farther, this ring of inverted movement closes in until it occupies the whole of the central area, and the observer sees an area of light moving across the whole pupil, having an inverted move- ment, that is against the mirror or light on the face. The movements of these erect and inverted light areas in the pupil are illustrated by figures 15 and 16. Figure 15 shows the plane mirror turned to the left, or the concave mirror turned to the right, the central erect area being dis- placed toward the left, and the peripheral inverted area to- ward the right of the space it occupies. Figure 16 repre- 62 ABERRATION AND IRREGULAR ASTIGxMATISM. sents the light areas displaced in the opposite directions by an opposite inclination of the mirror. Fig. 15. Fig. 16. With the concave mirror, a similar series of changes may be brought about by placing before the eye successive strengths of the lenses, beginning with the weakest convex or strongest concave. The first should allow the points of reversal for all parts of the pupil to be back of the observer, and the successive changes bring these points closer and closer to the e)'e until all are in front of the observer. The movement is at first against the light on the face. Then appears the ring of illumination and swift movement in the margin of the pupil with the light on the face. The cen- tral area of light is then encroached upon by the ring of faint illumination, and this in turn by a ring of more bril- liant illumination in the margin moving with the light on the face, which latter finally occupies the whole area of the pupil. If the point of reversal be approached from the oppo- site direction, that is, starting with the observer's eye beyond it, we have, with the plane mirror, at first, inverted move- ment across the whole pupil. Then, as the point of re- versal for the centre of the pupil is approached, the light in the central zone becomes feeble and its movement indefi- nite. When the point of reversal for that part is passed^ POSITIVE ABERRATION. 63 there appears, in this central zone, an erect movement of light and shade, at first rapid and hard to see, but growing slower, gaining in distinctness, and occupying a larger and larger part of the pupil as the patient's eye is approached, until, finally, it occupies the whole area. With the concave mirror, starting with the strongest convex or weakest concave lens, the movement is first with the mirror throughout the pupil, then, as the lens is changed, it becomes indefinite at the centre ; presently it is against that of the mirror at the centre, while still with it at the margin ; and, with still weaker convex lenses, or stronger concaves, it becomes against throughout the whole pupillary area. Appearances of Negative Aberration — With nega- tive aberration, the series of changes is apt to be less regu- lar and complete, and the picture presented by the pupil is less characteristic. But the succession of appearances is the reverse of what has been described for positive aberra- tion. With a plane mirror starting closer to the patient's eye than the point of reversal for the most myopic part of the pupil, the movement is with that of the light on the face throughout the whole pupil. As the observer's e}-e is with- drawn to a greater distance, this movement becomes indefi- nite, and the light feeble near the centre of the pupil. Presently, the movement at the centre of the pupil is lost, while still quite distinctly with that of the light on the face in the irregular ring-shaped area of the periphery. With- drawing still farther from the eye, the inverted movement at the centre of the pupil becomes distinctly visible, and the direct movement near the margin becomes more and more encroached upon and less and less distinct, until finally all erect movement is lost and we have only the in- 64 ABERRATION AND IRREGULAR ASTIGMATISM. verted movement, which extends across the whole pupil. Before the erect movement entirely disappears, it is apt to break up into small areas detached from one another by spaces of comparative shadow, but still presenting some remnant of the erect movement. With the concave mirror, starting with a convex lens so •weak, or a concave lens so strong, that the point of reversal is back of the observer, we have the direct movement .against the light on the face throughout the pupil. The strengthening of the convex lenses or the weakening of the •concaves, so as to bring the point of reversal closer to the •obser\'er's eye, causes : first, the fading of the light and the indefiniteness of its movement in the centre of the pupil, then the inverted movement, or with the light on the face, at the centre, and the area of this movement extending until it includes the whole of the pupil. Approaching the point of reversal from be}'ond it, we have, with the plane mirror, inverted movement through- out the whole pupil, giving place to indistinctness first at the margin. Then the indirect movement confined to the central area of the pupil and direct movement appearing at certain parts of the marginal area. This direct movement becomes more distinct and its area increases as the patient's eye is approached, until, at the point of reversal for the cen- tre of the pupil, all inverted movement is lost, and the erect movement is seen in all parts of the pupillary area. With the concave mirror, starting with the point of reversal between the observer and patient, and removing it successively farther from the patient, by the use of weaker convex or stronger concave lenses, we have first the move- ment with the light on the face throughout the pupil ; then indefiniteness at the pupillary margin, changing, in turn, to movement against the light on the face. The area NEGATIVE ABERRATION. 65 of this peripheral movement then encroaches npon the central area until that is obliterated, and the movement against the light on the face occupies the whole width of the pupil. While the order of their development remains the same, the exact character of the appearances presented var- ies considerably with the degree of aberration. Generally, in the higher degrees, the areas of light occupy the greater part of the pupil and the area of feeble illumination separ- ating them is comparativeh^ narrow. While in very low degrees of aberration, the area of feeble illumination is broad, and it may be difficult to recognize more than one of the light areas at one time. That is, when the area of erect movement is visible, the remainder of the pupil is occupied by the area of feeble illumination ; and when the area of inverted movement is developed, the area of feeble illumination so encroaches upon the area of direct move- ment that it can no longer be identified. In some eyes, the variation of refraction from point to point which constitutes symmetrical aberration, is almost or entirely confined to the periphery of the pupil. In these, the appearances characteristic of aberration are hard to de- velop. Appearance of Conical Cornea. — In other eyes, an ■opposite condition is present. The variations of refraction, instead of being confined to the periphery of the pupil, encroach upon the normal visual zone, confining it to a very narrow area. In these eyes, the skiascopic appear- ances of aberration are striking and characteristic, and one of them is that which has been regarded as peculiar to con- ical cornea. The error of refraction produced by conical cornea is a high degree of negative aberration. At the apex of the 66 ABERRATION AND IRREGULAR ASTIGMATISM. cone, the curve is sharp, causing, usually, very high myopia in the corresponding part of the pupil. The sides of the cone, on the other hand, are comparatively flat, causing diminished myopia as the region of the apex is departed from and often running into h}'peropia near the edge of the pupil. If the observer's eye be placed somewhere near the point of reversal for the periphery of the pupil, the move- ment of light in that portion of the pupil will be rapid, but the movement in the portion of the pupil corresponding to the apex of the cone will be slow. On account of the high myopia, the point of reversal for this part of the pupil is very close to the eye, and, generally, many dioptres removed from the observer's eye. The movement of light in the pupil, then, is slow near the centre and rapid towards the periphery, causing the area of light to appear to wheel around a fixed point corresponding to the apex of the cone. The light area is first seen on one side of the pupil, then on the other, but always rests upon the central fixed point. In certain positions of the light, the form of this area will be somewhat triangular, its base resting on the margin of the pupil and its apex at the apex of the corneal cone. Sometimes the triangle covers almost half of the pupil, in other conditions of light it is considerably narrower, but the constant and characteristic phenomena is the wheeling of the light area about the fixed point at the apex. This is shown in figures 17 and 18, which represent the appearance of the pupil with the mirror inclined in opposite directions. It was for the detection of these appearances, to which attention was called by Bowman, in 1857, ^^^^ ^^^ test was first employed. Bowman mentions that he was able by means of it to detect low degrees of conical cornea, which APPEARANCE OF CONICAL CORNEA. 67 would not be detected in any other way. It is certain that among those cases that have been classed as low degrees of conical cornea, on account of their presenting such appear- FiG. 17- Fig. i8. ances, a considerable proportion were not of conical cornea at all, but were cases of high aberration from other forms of defect in the dioptic surfaces. The appearances in question occur in all cases of high aberration. Where the aberration invades the central por- tion of the pupil, and is not confined to the periphery, the phenomena are quite as striking and characteristic, and of very much more frequent occurrence in cases of high posi- tive aberration than in cases of true conical cornea. The conditions for their recognition are that the observer's eye shall be comparatively near the point of reversal for the margin of the pupil, and comparatively far removed (esti- mating by dioptres) from the point of reversal for the cen- tre of the pupil. By careful management of the light and relative position of observer and patient, such appearances can be demonstrated in the majority of eyes. Like the band-like appearances of the light in astig- matism, those of conical cornea reveal the presence of the condition and the location of the apex of the cone, but be- yond this, they are of little value. The measurement of the difference of refraction between the margin and the 68 ABERRATION AND IRREGULAR ASTIGMATISM. centre of the pupil, or the measurement of the refraction in the portion of the pupil best suited to purposes of vision, must be accomplished by the same application of skiascopy as serves to measure the amount of hyperopia or myopia in an eye free from astigmatism and aberration. The series of movements presented in positive aberra- tion can be well studied in one of the numerous forms of artificial eyes, in which spherical lenses are used to repre- sent the dioptric surfaces, and it is well by such study to become thoroughly familiar with them. They may, of course, be studied in living eyes presenting positive aberra- tion ; but, in many of these, the appearances are not so typical and regular in order of sequence, as with the ordi- nary strong spherical lenses. The appearances presented by negative aberration can only be studied in eyes in which this condition of the refraction is present, but their recog- nition and observation will be comparatively easy to one who has mastered the corresponding appearances of positive aberration, and who understands the optical conditions on which these appearances depend. Scissors-like Movement. — A special form of irregular astigmatism exists of sufhciently frequent occurrence and striking character to merit special description. It is also of some practical importance. In it, one portion of the pupil, as an upper or lower half, is more myopic in a cer- tain meridian than is the other part of the pupil. This causes an inverted movement of light in the one portion of the pupil, while there is an erect movement in the other. These two areas are distinct and separated by an intermedi- ate zone of feeble illumination. As the light is made to move back and forth in this meridian, the two areas of light in the pupil are seen alternately to approach and sep- arate, narrowing or widening the intermediate zone. As SCISSORS- LIKE MOVEMENT. 69 the areas, under these circumstances, are generally band- like, or have comparatively straight margins, the effect is similar to that of the opening and closing of a pair of scissors. These appearances are represented in figure 19, which shows the mirror so turned as to separate the two areas ; and figure 20, which represents them brought to- gether by an opposite inclination of the mirror. Suppos- ing the upper part of the pupil to be more myopic, figure 19 corresponds to the plane mirror facing down or the con- cave mirror facing up ; and figure 20 shows the plane mir- ror facing up or the concave mirror facing down. Fig. 19. Fig. 20. The relative size of the two areas will depend on the distance of the observer from the eye or upon the strength of the lens employed. As the observer withdraws to a greater distance, or the convex lens is made stronger, or the concave lens is made weaker, the area of inverted move- ment encroaches upon the zone of feeble illumination sep- arating the areas of light and the area of erect movement diminishes. As the observer comes closer to the eye, or the convex lens is made weaker or the concave lens stronger, the area of inverted movement diminishes. Always the observer's eye is at or near the point of reversal for the por- tion of the pupil occupied by the intermediate zone of feeble illumination ; and, in making the determination of 70 ABERRATION AND IRREGULAR ASTIGMATISM. the refraction for practical purposes, care must be taken to see that this zone occupies a portion of the pupil that is available when the pupil is contracted, as under ordinary conditions of illumination and near work. The scissors-like movement may be produced in an artificial eye by placing the lens which represents the diop- tric surfaces, so that the light passes through it obliquely. It may also be developed in most eyes by applying skias- copy^ from some direction at a considerable angle to the optic axis. Its presence in the eye indicates obliquity of one or more of the dioptric siirfaces. Probably it is often due to some obliquity in the position of the crystalline lens. Perhaps, because of such obliquity, this appearance of light and shadow in the pupil is apt to co-exist with a consider- able degree of regular astigmatism, which, on account of it, becomes more diihcult to recognize and measure than it would otherwise be. Eyes presenting it, therefore, demand special care and attention on the part of the observer, to develop their best vision possible with correcting lenses. CHAPTER VI. PRACTICAL APPLICATION WITH THE PLANE MIRROR. Position and Arrangement of Light. — The room being thoroughly darkened, the patient and surgeon take posi- tions facing each other at a distance of about one metre with the original source of light close to the surgeon on the side of the eye he desires to use, that is on the right if he intends using his right eye for the test. He can really use but one eye at a time, although he will find it much more pleasant to work with both eyes open if he once learns to do so. The source of light should be freely movable over a space of about one metre, a movement obtainable with a double jointed bracket of over one-half metre total length. The light is covered from the patient's face and also from the surgeon's except at the aperture of five [or ten] millimetres opposite the brightest part of the flame. The mirror is held so that with the eye he is using the surgeon can watch through the sight hole the movement of light on the patient's face and turned until the area of light that it reflects falls upon the eye to be tested. If dif- ficulty is experienced in properly directing the light, the surgeon may hold his hand a few inches in front of the mirror and upon it find the light area and get it properly directed towards the patient's eye. If the mirror be large it is necessary that the central portion of the light area be made to fall upon the patient's eye, the centre being marked (71) 72 APPLICATION WITH THE PLANE MIRROR. by a spot of feeble illumination, corresponding to the sight hole of the mirror. With the light properly directed, the pupil will be seen to be occupied by a red glare, the light area with which skiascopy is especially concerned. In first attempt-^ ing the test, care must be taken to discriminate clearly be- tween this general red glare and the reflection from the cornea or from the surfaces of any lens that may be placed before the eye. These reflections have the same color as the ligfht used for the test. The one from the cornea is small and brilliant, a mere point of light if the room be thoroughly darkened and the original source of light prop- erly shaded. The reflections from the lenses employed are larger and more confusing. They may be avoided by tilt- ing the lens slightly, in which case, they pass off to the peri- phery, leaving the centre of the lens free from reflection. Hyperopia. — If the mirror be rotated about a vertical axis, that is if it be made to turn more to the right or left, the area of light in the pupil will be seen to move with the light on the face to the right or left as the inclination of the mirror changes. If the rate of movement be slow, the h)peropia is of high degree, if more rapid, it is lower, A convex lens is now to be placed before the eye and this rate of movement of light in the pupil is the guide to the probable strength of lens required. If the observer has not sufficient practice with skiascopy to judge in this way about the strength of the lens required, he will save time by placing before the eye rather a strong lens, one of say 5 D. With this the light is again thrown upon the eye, and if the lens be not sufiicient to correct the hyperopia present, the movement of light in the pupil will still be found with that of the light on the face. In this case a still stronger lens must be used. This strengthening of HYPEROPIA. 73 the convex lens before the eye is continued until one is found which causes the reversal of the apparent movement of light in the pupil — until the light in the pupil moves against the light on the face. Then the surgeon is to approach the patient, mean- while rotating the mirror and watching for the nearest point at which he still sees the inverted movement in the visual zone. Near this distance, the illumination of the pupil be- comes quite feeble, and the movement being rapid requires the closest watching. Approaching still nearer to the patient the light in the visual zone is seen to move with the light on the face, and the greatest distance at which this can be distinguished is to be noted. Between these two, the least distance of inverted movement, and the greatest distance of direct movement lies the point of reversal. But, for reasons given page 40, it is better to take the lat- ter, the greatest distance at which direct movement can be perceived as the point sought. The distance from the surgeon's to the patient's eye- is then measured. It is the focal distance of the amount of myopia produced by the convex lens employed. That amount is to be subtracted from the total strength of the lens to ascertain the portion of its strength which has. been necessary to correct the existing hyperopia. Having made such a determination of the refractiom and having repeated the various observ^ations until no- doubt is left as to their correctness, the lens before the eye is to be changed for one sufficiently weaker to carry the point of reversal to as great distance as the size of the visual zone will allow the accurate determination of the movements of light and shade within it. At this distance the final estimate of the ametropia is to be completed. For example, suppose the eye under examination to 6 74 APPLICATION WITH THE PLANE MIRROR. have hyperopia of 3 D. When the 5 D. lens is placed be- fore it, the point of reversal will be brought to one-half metre. As the surgeon's eye is made to approach that of the patient, the inverted movement in the visual zone will cease when they are about 60 centimetres apart. Going still closer, the erect movement will be distinguished at about 40 centimeters. These observations are to be repeated until the surgeon makes sure that the point of reversal lies somewhere between 40 and 60 centimetres. The 5 D. lens is then replaced by the 4 D. lens. Repeating the test, the inverted movement is seen at one and one-quarter metres and the direct movement as far away as one metre, thus locating the point of reversal at about i metre from the eye, and determining the myopia caused by a 4 D. convex lens to be i D. and the refraction of the eye to be 4 D.- I D.^3 D. of hyperopia with less than 0.25 D. of possible error either way. Myopia. — In myopia the first rotation of the mirror will usually cause a movement of light in the pupil against that of the light on the face. The surgeon then approaches the patient, continuing the movements of the mirror and watching the apparent movement of light in the pupil, until this apparent movement becomes rapid and indefinite and presently is entirely lost. Approaching still closer to the patient's eye, the movement of the light area in the pupil again becomes distinct, but is now with the move- ment of the light on the face. Drawing back again, the surgeon notes the greatest distance at which this erect movement can be observed, and the shortest distance at which the inverted movement is distinguishable, and takes a point midway between these to be the point of reversal. The distance of this point of reversal from the patient's eye is the focal distance of the lens that will be required to MYOPIA. 75 correct the myopia. To complete the test, however, a lens about I D. weaker than this is placed before the eye to bring the point of reversal to the distance of a metre and the test is repeated, the surgeon noting carefully the great- est distance at which the erect movement is visible, and the shortest distance at which the inverted movement is perceived, always in the visual zone. The distance of the point of reversal as thus determined is the focal distance of the lens required to correct the remaining myopia. The strength of such a lens added to the strength of the lens already before the eye, gives the total amount of myopia present. Suppose the eye to be 6.5 D. myopic. With the first test the inverted movement will be perceived up to about eight inches from the patient's eye ; and at five or six inches from the eye an erect movement will begin. From this, the surgeon assumes that the m}'opia is about 7 D. [focal distance 6^ inches] and he will, therefore, place before the eye, for the more accurate test, a concave 6 D. lens. On trying the movement of light in the pupil through this lens, it will be found at the distance of one metre to be with that of the light on the face. The surgeon then with- draws still farther from the patient until the direct move- ment becomes indistinguishable and at two metres is entirely lost. Drawing back still farther from the patient, he will in a favorable eye be able to distinguish the inverted move- ment in the pupil, and in this way fix the point of reversal at a distance of two metres, indicating with great accuracy an uncorrected myopia of 0.5 D. Sometimes, however, the distance of two metres will be found so great that it is difficult or impossible there to be sure of the movement in the visual zone. In such a case the 6 D. lens will need to be replaced by a weaker lens as 76 APPLICATION WITH THE PLANE MIRROR. a 5.5 D., with which the erect movement will be seen to almost a metre, and the inverted movement again a few inches beyond that point. If the myopia be very low, the first inspection of the pupil without a lens may show a movement of light in it ivith the light on the face. In such a case, the surgeon will draw back as far as he can readily distinguish the movement of light in the visual zone. If the movement still appears to be ivith that of the light on the face, he will place before the eye a convex lens, and with it determine the point of reversal as for a case of hyperopia. The final result of testing, however, will show that the myopia caused by the lens is greater than the strength of the lens, and, therefore, that some myopia must have been present before the lens was placed in front of the eye. For example, suppose that before reaching that dis- tance of two metres the erect movement in the pupil becomes indistinct, but that the visual zone, where the movement must be watched, is so small that beyond this the direction of movement in it cannot be recognized with certaintv. A 0.5 D. convex lens being placed before the eye is found to cause an inverted movement up to 125 cen- timetres, and to confine the erect movement to within 85 or 90 centimetres of the eye. The point of reversal then, is at one metre. The amount of myopia corresponding to this is I D., of which 0.5 D. was the amount originally present in the eye. Emmetropia. — On first inspection, without a lens, the surgeon sees an erect movement in the pupil, the rapidity of which indicates that if there be hyperopia it is of low degree. Drawing back from the patient's eye as far as pos- sible, however, this erect movement still continues. He places before the eye under observation a convex lens of i EMMETROPIA. or 2 D., and viewing the movement of light in the pupil through this lens, finds where the inverted and the erect movement come together. On measuring the distance of this point of reversal from the patient's eye, he finds that it exactly corresponds with the focal distance of the lens he has been using. That is, the lens has caused myopia just •equivalent to its own strength, showing that before they passed through the lens, the rays emerging from the cornea were parallel. Regular Astigmatism. — Whether it be known that the eye under examination is astigmatic or not, the test will proceed at first as for simple hyperopia or myopia. Sometimes if the astigmatism be high and one meridian nearly emmetropic or slightly myopic, the first inspection, without any lens, will reveal an unmistakable band of light, or that there is erect movement in one meridian and in- verted movement in another, or that the movement of light in the pupil is more rapid in some one meridian than in the meridian at right angles to it, indicating that these meridians have different points of reversal, and that the surgeon is nearer the point of reversal for the former than for the latter. But, commonly, the first appearance will give no posi- tive indication of the presence of astigmatism, and the test goes on until a point of reversal is found. Then, on trying the movement of light and shade in different meridians, as should alwa}'S be done from the neighborhood of the point of reversal, it is discovered that it is the point of reversal for only one meridian ; and that for the meridian at right angles to that one, there is a distinct movement of the light either erect or inverted. If the movement still noticeable from the point of reversal first discovered be an inverted movement — against 78 APPLICATION WITH THE PLANE MIRROR. the light on the face — the surgeon should bring his eye still closer to the patient until this inverted movement ceases. He will then be near the point of reversal for the meridian in which the inverted movement was before noticed, and will be able to see in the other meridian an erect movement. Such a lens is now to be chosen and placed before the eye as will bring this point of reversal for the more myopic meridian — the point of reversal from which an erect move- ment is seen in the other meridian — to a convenient distance from the eye. The surgeon's eye is placed as nearly as pos- sible at this point of reversal. Then the original source of light [which has up to this stage of the test accompanied the mirror in its movements to or from the patient's eye] is pushed away from the mirror, and while it is pushed away, the mirror is rotated and the light area in the pupil watched. This light area will be seen to assume the band- like appearance characteristic of astigmatism. At a certain distance this band-like appearance will be most distinct. With the source of light nearer the mirror or farther from the mirror, it will be less characteristic. The distance of the light from the mirror at which the band becomes most distinct is the distance between the two points of reversal. The surgeon's eye (with the mirror) is now at the point of reversal for the more myopic meridian, and the immediate source of light is at the point of rever- sal for the less myopic meridian. With the light in this position, the direction of the band is to be carefully studied and noted as the direction of one of the principal meridians of astigmatism. It is the direction of the axis of the convex cylinder that will cor- rect the astigmatism. The other principal meridian will,, of course, be perpendicular to this. REGULAR ASTIGMATISM. 79 Having now fixed the direction of the principal me- ridians of astigmatism, the surgeon should again bring the original source of light as near to the mirror as possible, and proceed to measure the refraction, first in the one prin- cipal meridian and then in the other, just as he would measure the refraction in cases of hyperopia or myopia. The difference between the refractions of the two being the amount of astigmatism present. To measure the refraction in a certain meridian the light is made to move on the face and on the retina in the direction of this meridian by rotating the mirror about an axis perpendicular to it. Thus for the vertical meridian the light is made to move vertically by turning the mirror about a horizontal axis. For the horizontal meridian the light is made to move horizontally about a vertical axis. Great care is necessary in the higher degrees of astigmatism to make the movement conform accurately to the meridian to be tested, since any oblique movement will appear (see page 55) as though perpendicular to the band. When the astigmatism is of very low degree, 0.5 D. or less, it becomes correspondingly difficult to distinguish be- tween the points of reversal for its principal meridians. The band-like appearance of the light in the pupil becomes less characteristic, and there is no space between the two points of reversal where an erect movement can be obtained in the direction of one meridian, and a reverse movement in the direction of the meridian perpendicular to it. In these cases, the astigmatism is to be recognized by the fact that when near one point of reversal, the movement in one meridian has become indistinguishable, it can still be per- ceived in the other principal meridian. And, if the sur- geon places his eye at the point of reversal for the more myopic meridian and pushes the source of light a little 80 APPLICATION WITH THE PLANE MIRROR. away from the mirror, the erect movement in the meridian of less myopia, and absence of movement in the more myopic meridian becomes most distinct. It is upon the behavior of light in the pnpil under these conditions that the diagnosis of the very low degrees of astigmatism must principally rest. The final test in any case will be made with the points of reversal brought together, usually at a distance of i metre or more. To do this, it will be necessary to place "before the eye such a cylindrical lens as will correct the .astigmatism, together with the spherical lens which will bring the point of reversal to the desired distance. With these lenses before the eye, the test is again applied. If the light in the pupil is found to move with the light on the face, the surgeon withdraws to a greater distance until that movement becomes indistinct. If the movement in the pupil is found against that of the light on the face, the surgeon approaches the patient until the movement be- comes indistinct. The apparent movement is to be care- fully inspected from the point of reversal and from a little within and a little beyond it. If it is found that the reversal occurs at the same dis- tance from the eye for all meridians, the cylinder chosen is known to be correct, both as to strength and as to the placing of its axis ; and the distance of this point of rever- sal from the eye indicates the amount of myopia which the spherical lens employed has caused or has left uncorrected. If, however, the movement of light is found to cease in some meridian, but to continue (either direct or inverted) in a meridian at right angles thereto, it becomes evident that the cylinder chosen does not perfectly correct the astigmatism. If the astigmatism thus found to remain has the same principal meridians as those already fixed upon, REGULAR ASTIGMATISM. 81 the direction of the axis of the lens is correct, but its strength is not exactly right. Whether the strength needs to be increased or to be diminished will appear from the fact that the more myopic meridian continues to be the more m}'opic ; or that what was originally the less myopic meridian has become the more myopic. If the astigmatism remaining after the cylindrical lens has been placed before the eye has principal meridians that do not correspond with those for which the lens is placed, the placing of the lens is incorrect, and the direction of its axis needs to be slightly varied, until the remaining astig- matism disappears or its direction corresponds with that of the lens before the eye. Where the cylindrical lens before the eye is of the right strength or is too weak, its axis needs to be turned slightly toward the axis of a similar cylinder which would correct the remaining astigmatism. If the cylindrical lens already before the eye is too strong, its axis needs to be turned slightly toward the axis of a cylindrical lens of the ■opposite kind that would correct the astigmatism. The effect of such combinations of cylindrical lenses may be more fully understood by the study of the writer's paper upon " The Equivalence of Cylindrical and Sphero- cylindrical Lenses " in the Transactions of the American Oph- thalmological Society for 1886, page 268, or " Some Remarks on the Refractive Value of two Cylinders " by Carl W^eiland Archivef, of OphthalmoJogy, 1893, p. 435, and 1894, page 28. When the meridians of any remaining astigmatism hiave thus been made to conform to the direction of the cylindrical lens before the eye, this remaining astigmatism has to be corrected by a change in the strength of the cylin- drical lens. For example : suppose an eye having a compound 82 APPLICATION WITH THE PLANE MIRROR. hyperopic astigmatism corrected by + i sph. 3 ^~ ^ ^y^ axis 95°. The first inspection of the movement of light in the pupil shows a movement with that of the light on the face in all meridians ; and the difference in the rate of movement in the different meridians will be so slight as probably to escape notice. A convex 3 D. spherical lens will cause the movement in the pupil to be against the light on the face in all meridians when the eye is viewed from a greater distance than one metre. But it will also be noticed that the light moves more swiftly from side to side than it does upward and downward. If now the surgeon brings his eye closer to the patient, when the distance of one metre is reached, the movement of the light from side to side becomes indistinguishable, while there is still a very distinct movement against the light on the face upward and downward. Approaching still closer, the movement from side to side is seen to be witJt the movement of the light on the face, the inverted movement still continuing in the vertical meridian. The movement horizontally tvitli the light on the face, at first very rapid, grows slower as the patient's eye is approached, and the movement — against the light on the face — in the vertical meridian grows more rapid, until at a distance of one-half of a metre, the movement in the vertical meridian becomes indistinguishable, although there is a very clear movement of light ivitJt the light on the face from side to side. The point of reversal for the more myopic meridian (more myopic with the lens) has now been reached, and the the surgeon keeping his eye at this position pushes the source of light away from the mirror. As he does so, the area of light in the pupil assumes more and more the appearance of a distinct vertical band, readily moved from REGULAR ASTIGMATISM. 83 side to side, but without apparent movement in the direc- tion of its length. This band continues to become more and more distinct until the original source of light is one- half metre from the mirror ; and the immediate source consequently one-half metre back of the mirror, and one metre from the patient's eye — at the point of reversal for the less myopic meridian. In this position careful obser- vations will show that the band of light in the pupil is not exactly vertical, but has the direction corresponding to the more myopic meridian of 95°. The principal meridians then are located at 5° and at 95°. Having determined this, the light is brought back as close to the mirror as possible, and the point of reversal for the 95° meridian is determined. To do this it may be advisable to change the convex spherical lens before the eye, but whatever lens is employed, from the results obtained with it, the surgeon deduces the fact that in that meridian the refraction of the eye is hyperopic i D. He then proceeds to measure in the same manner the refraction of the eye in the other principal meridian, finding with the convex 3 D. lens that this point of reversal is at one metre, and its refraction, therefore, hyperopic 2 D. The differ- ence between these meridians will be i D., the amount of astigmatism present. To make the final determination there, there should be placed before the patient's eye the i D. convex cylinder with its axis at 95° and a 2 D. convex spherical lens, with which the point of reversal for all meridians will be found to lie one metre from the eye. If, in the placing of the cylinder, its axis is made to not correspond exactly with the meridian of least hyperopia, there will be found by this test a remaining astigmatism of low degree. Suppose through carelessness or inaccuracy in the earlier observa- 84 APPLICATION WITH THE PLANE MIRROR. tion, the axis of the cylinder should be placed at 105° in- stead of 95°, the remaining astigmatism then would be found to be such as would be corrected by a convex cylin- der with its axis at about 70°. But on turning the cylin- der before the eye 10° in that direction, that is, to its proper direction at 95°, the remaining astigmatism would disap- pear. If, however, instead of the i D. cylindrical lens, a lens of 1.5 D. had been placed with its axis at 105°, there would remain an astigmatism which might be corrected by a concave cylinder with its axis at about 70°, and the turn- ing of the cylinder before the eye 10° in that direction [to 95°] would cause the remaining astigmatism to so change that its meridians would be at 5° and 95°, where a measurement of it would reveal the fact that the cylindri- cal lens employed was 0.50 D. too strong. Aberration and Irregular Astigmatism. — The differ- ence in the refraction of different parts of the pupil is to be ascertained by measuring the refraction for each part sep- arately, just as though it were a case of simple hyperopia or myopia, care being taken to confine each observation strictly to the little portion of the pupil the refraction of which it is desired to ascertain. The amount of aberration or irregular astigmatism that is thus ascertained is of some scientific interest and occasionally of practical importance as bearing on the prognosis of conical cornea, or of the changes of refraction in the lens which precede cataract. Generally, however, the important practical point about aberration or irregular astigmatism is its distribution. For practical purposes, the surgeon desires to ascertain which part of the pupil is free from any such defect, as that part will furnish the best visual zone ; and by what lenses that visual zone can be made most useful to the ABERRATION AND IRREGULAR ASTIGMATISM. 85 patient. The need for careful study to develop these points is sometimes great. Figures 21 and 22 represent the appearances brought out by thorough investigation of a case of considerable astigmatism, coincident with equally pro- nounced positive aberration. Without careful study of the visual zone at the proper distances, it would have been easy to set the case down as one of aberration, and to have over- looked the astigmatism entirely. If the aberration en- FiG. 21. Fig. 22. croaches decidedly upon the area of the pupil as deter- mined by a moderate light, it may be necessary to give a correcting lens, for use at near work and on exposure to bright light, different from the one required when the pupil will be somewhat larger. Or the surgeon may need to caution the patient that under certain conditions of light he must expect the correcting glasses to give slightly im- perfect vision. A fact to be borne constantly in mind in the applica- tion of skiascopy is that it is not the high degrees of aber- ration and irregular astigmatism that are of most practical importance, requiring the surgeon to take into account their bad effects. More frequently it is the slight imper- fections of this kind situated within the portion of the pupil used for accurate vision that need to be recognized and taken into account, when prescribing glasses or in giving 86 APPLICATION WITH THE PLANE MIRROR. an opinion as to the value of glasses. These low degrees of imperfection are to be recognized and studied only when the surgeon's eye is close to the point of reversal for the visual zone after the effects of hyperopia, myopia and regu- lar astigmatism have been excluded by placing the proper glasses before the patient's eye. The investigation of aberration and irregular astigma- tism is the last step in skiascopy. A step very frequently not taken, yet essential to complete certainty and accuracy in the objective measurement of refraction. In a small pro- portion of cases, it will lead to modification of the glasses previously selected as best, and in a much larger proportion of cases it will discriminate sharply between the lenses which really best correct the ametropia and others which appear to give equally good, or almost equally good sub- jective results. To carry it to completion will often require more time and effort than has been necessary for all other parts of the skiascopic examination. Nor is this strange, for it often includes the measurement of hyperopia or myopia with astigmatism in two or more areas of distinctly, though slightly, different refraction. It is not a distinct application of the test but its application to parts of the pupil, instead of to the pupil as a whole. It requires no special directions and cannot be much elucidated by examples. It is to be mastered by a full understanding of the optical principles of the test. Chapter II, strict observance of the conditions of accuracy set forth in Chapter III, and the exercise of the needful care and patience. Measurement of Accommodation. — The objective determination of the nearest point for which the eye can be focused is possible only by skiascopy. It is sometimes of importance as in cases of suspected cycloplegia in child- MEASUREMENT OF ACCOMMODATION. 87 Ten, or others for whom the subjective test cannot be relied on. In determining the condition of the accommo- dation in an eye with imperfect vision, or in recognizing any slight remaining accommodation after the use of a mydriatic, the test is also of practical value. To make it, the surgeon first ascertains the refraction of the eye, and then places before it such lens or lenses as will correct astigmatism and bring the point of reversal to a distance of one metre or a little less. He then places himself at this distance from the patient, and directs the patient to fix his gaze upon some object on the farther side of the room in such a position that the visual axis of the eye under examination shall pass as close as possible to the surgeon's eye. The point of a finger or pencil is then held close to the patient's eye, about the near limit of conver- gence and in the visual axis, so that the direction of the visual axis shall not be changed during the test. The patient is then directed to look first at the object across the room, then at the point of the pencil close to his eye. The surgeon by watching his other eye can ascertain whether the movements of convergence are really executed. Very strong convergence being impossible without strong accommodative effort, if any power of accommodation remains the eye will be seen to grow more myopic when the pencil is looked at, and less so when the distant object is fixed, an inverted movement of the light in the pupil becoming apparent in the one position and disappearing in the other. To measure the amount of accommodation the sur- geon may approach the observ^ed e}'e until the point of reversal is reached for the eye during \'ery strong conver- gence, or the lens before the eye may be modified in the direction of weaker convex or stronger conca\'e until the 88 APPLICATION WITH THE PLANE MIRROR. point of reversal is brought with a new lens, with the accommodation, to the same distance as it was brought by the original lens without accommodation. The difference between the lenses in this case representing the amount of accommodation present. For example : in a case of hyperopia 2 D. to ascertain if accommodation was present a convex 3 D. lens would be placed before the eye. This would bring to one metre the point of reversal of the eye with its accommodation relaxed. The surgeon at a little less than a metre could get through it an erect movement of the light in the visual zone when, the patient was looking across the room. If now, on look- ing at the point of a finger held two inches in front of the eye, the movement becomes distinctly inverted, the light in the pupil moving against the light on the face, it is known that accommodation is present. In place of the 3 D. lens a weaker convex may be substituted, and if the strong convergence of the visual axis still brings an in- verted movement of light in the pupil, a still weaker lens for this. In this way if it be found that with the i D. lens the patient is able by strong convergence and coincident accommodative effort, to bring the point of reversal to just one metre, the difference between the 3 D. lens and the I D. =2 D. will be the amount of accommodation pres- ent. Instead of changing the lens, the surgeon can approx- imately estimate the amount of accommodation by bring- ing his eye closer to that of the patient and finding the new position of the point of reversal caused by the exer- tion of the accommodation. Where much accommodation is present, such an approximate determination should first be made, but it is open to the inaccuracies attendant on any skiascopic determination at a short distance. CHAPTER VII. PRACTICAL APPLICATION WITH THE CONCAVE MIRROR. The Source of Light. — The room is to be thoroughly darkened, and to secure this, it is well to have the original source of light shaded. This light will, however, usually be back of the patient and except for the determination of the principal meridians of astigmatism, the farther it is behind the patient the better, so that it is less essential to have the light thoroughly shaded. It is also of less import- ance that the original source of light should be small ; still the separation of light and shade should be as sharp as possible, so that the opaque shade with an opening oppo- site the brightest part of the flame will be found service- able. The opening in the shade may be two or three cen- timetres in diameter, so long as the original source of light is a metre or more away from the mirror. But when this is brought near the mirror to bring out the band of light in astigmatism, the opening should be one centimetre in diameter or less. The surgeon places himself a little over one metre from the patient. On throwing the light upon the patient's face with the mirror, it is found that the area of light on the face moves with the mirror just as in the case of the plane mirror. The same reflections from the cornea and trial lenses are to be recognized and provided for, and within the pupil lies a similar portion of red fundus reflex bounded by shadow, which is the subject of observation 7 (89) 90 APPLICATION WITH THE CONCAVE MIRROR. during the test. If, however, the light in the pupil be seen to move ivith the light on the face, the eye is myopic more than I D. If the light in the pupil be seen to move against the light on the face, the eye is hyperopic, emmetropic, or myopic less than i D. Hyperopia. — If the mirror be rotated about a vertical axis from right to left, the area of the light in the pupil w^ill be seen to move from left to right, that is, against the mirror and against the light on the face. That this is really an erect movement, we know from the demonstrations as to the real direction of the movement of the light on the retina in Chapter II. The difference between erect move- ment and movement with the lighten the face must be borne in mind. With the concave mirror, the one is just the opposite of the other. The movement with the light on the face being an inverted movement, and the movement in the pupil against the light on the face the erect movement. As with the plane mirror, the movement will be swift if the hyperopia be of low degree, slower if of higher amount. The convex lens is now to be placed before the eye, the swiftness of the movement in the pupil being the guide to the strength probably required. If the observer is not able to judge by this movement, let him at first employ in suc- cession lenses that differ considerably in strength, as the 2, 4 and 6 D., increasing the strength as long as the move- ment in the pupil is against the movement on the face. When a lens is reached that causes movement of light in the pupil witli the light on the face, slightly weaker lenses are to be tried until two consecutive lenses are found, of which one gives the movement against the light on the face and the next stronger causes movement with the light on the face. Between these two lies the lens strength which would bring the point of reversal to the surgeon's HYPEROPIA. 91 eye. This being one metre from the patient, it is the lens which causes i D. of myopia ; and by subtracting i D. from its strength the hyperopia of the eye is obtained. For example : suppose hyperopia of 4 D. The light in the pupil will move against the light on the face at the first inspection, and also with the convex 2 D. and 4 D. lenses. With the convex 6 D. it is found to move witlt the light on the face. On trying the convex 5 D. the move- ment is indeterminable. With the 4.75 D. it is very rapid, but still against the light on the face. With the 5.25 D. it is equally rapid but loith the light on the face. The lens strength between the two, or the 5 D., is then the one which causes i D. of myopia, and 5 D. the strength of the lens minus i D. the myopia caused by it, leaves 4 D. the lens strength required to correct the hyperopia present. Myopia. — In the mass of cases, inspection without a lens will show movement of light in the pupil with the movement of the light on the face, indicating that the point of reversal is between the surgeon and the patient. When this is the case, concave lenses are to be tried, their strength being indicated by the rate of movement, or if this be not a sufficient guide, they may be tried in series with an interval of 2 D., or more, until one is found which causes the light in the pupil to move against the light on the face. As the point of reversal is thus brought nearer to the observer's eye, the light area in the pupil becomes more brilliant and its movement more rapid. When the lens has been found which causes the light in the pupil to move against the light on the face, slightly weaker lenses are to be tried until it has been certainly ascertained which is the weakest lens that will cause the movement against that of the light on the face, and which is the strongest lens that 92 APPLICATION WITH THE CONCAVE MIRROR. still allows movement in the pupil witJi the light on the face. Between these two lies the lens strength which leaves the eye i D. myopic ; this lens strength added to i D. will give the total myopia present. For example : suppose the myopia present to be 8.5 D. The movement in the pupil without any lens will be very slow, and the light area round and dim. Judging from this appearance, the first lens tried may be the concave 5 D. With it the light in the pupil will appear more brilliant and its movement will be more rapid, but it will still be witJi the movement of the light on the face. Next, the concave 8 D. will be tried. The movement of light will be found still more rapid, but now against that of the light on the face. With the concave 7 D. it will be found equally rapid, but ii'ttli the light on the face. With the 7.5 D. it will not be distinguishable. Hence the 7.5 D. lens leaves i D. of myopia still uncorrected, and added to that I D., gives 8.5 D. the total myopia present. If the myopia be of low degree, the test without a lens will show either no distinguishable movement of the light in the pupil [for myopia of i D.] or movement in the pupil against the movement of the light on the face [for myopia of less than i D.] . In the former case, the test is to be repeated with very weak convex and concave lenses [0.25 D. or 0.50 D.]. The convex will give a movement of the light in the pupil luith the light on the face, and the concave a movement against the light on the face. If the movement is found to be against the light on the face to start with, the convex lenses are to be tried, com- mencing with I D. lens which will cause the movement luith the light on the face, and will show, therefore, that the refraction is myopia and not emmetropia, or low MYOPIA. 93 hyperopia. The weaker lenses are then to be tried and the one which causes i D. of myopia thus ascertained. Since this lens is added to the myopia of the eye to cause i D. of myopia, it must be subtracted from i D. to find the amount of myopia originally in the eye ; the difference between it and i D. being the myopia present. Thus, in a case of myopia of 0.50 D., the light will be found to move against the light on the face, without any lens or with a 0.25 D. convex. But it will be found to move ivitli the light on the face with a convex i D. or 0.75 D., and with an 0.50 D. the movement should be in- distinguishable. The convex 0.50 D. then, causes i D. of myopia ; and subtracting it from i D., leaves 0.50 D. the degree of myopia existing in the eye. Emmetropia. — In emmetropia, on the first trial, the light in the pupil is found to move against the light on the face, and rapidly. With convex lenses it is found that the 0.75 D., or anything weaker still allows this movement against the mirror; but the 1.25 D. or anything stronger causes motion in the pupil with the light on the face ; and that the convex i D. causes no perceptible movement. Hence, the convex i D. lens causing i D. of myopia, the eye without a lens must be emmetropic. Regular Astigmatism. — The test beginning as for simple hyperopia or myopia, as the point of reversal for one of the principal meridians is brought near the ob- server's eye, the movement of light becomes notably more rapid in one meridian than in the other, indicating the presence of this form of ametropia. When this is recog- nized, the lenses used are to be such as give a movement of the light in the pupil with the light on the face in all meridians. Thus, if the eye has been hyperopic, the con- vex lenses used before the eye must be increased in strength 94 APPLICATION WITH THE CONCAVE MIRROR. until the movement ivith the light on the face occurs in all directions. Or, if the eye is myopic, the increase of strength in the concave lenses must stop so soon as any movement is seen in the pupil against the movement of ligfht on the face. And the lens which causes this must be replaced by a weaker one that just allows movement with the light on the face in all meridians. The lens aimed at is the one which will bring the point of reversal for the least myopic meridian, just to the surgeon's eye, i metre from the patient. If this is exactly attained, there will be in that meridian no perceptible movement of light and shadow, but the movement in the other princij^al meridian will still be with that of the light on the face. When this lens has been found, the original source of light, which, up to this time has been kept at as great a distance as possible from the mirror, is to be brought closer to the mirror, so that the image of it formed at the conju- gate focus in front of the mirror will be removed farther from the mirror and closer to the patient's eye. The lens before the patient's eye brings the point of reversal for the least myopic meridian to the eye of the sur- geon and necessarily places the point of reversal for the more myopic meridian somewhere between the surgeon and patient. The object of bringing the original source of light nearer to the mirror is to carry the immediate source of light to the point of reversal for this more myopic meridian. As the light approaches its proper position, the area of light in the pupil becomes more and more band-like in form, being most distinctly so when the immediate source of light corresponds with the point of reversal for the more myopic meridian. When this "is attained, the direction of the band is to REGULAR ASTIGMATISM. 95 be carefully noted as indicating the direction of the princi- pal meridian of least myopia. This direction having been determined and recorded, the original source of light is again moved as far away from the mirror as possible, and measurement of the refraction in the least myopic meridian completed as for a case of simple myopia. Then, the lenses are so changed as to bring the point of reversal for the more myopic meridian to the surgeon's eye i metre distant from the patient : and the lens that is found to do this, shows by the addition of i D. to a con- cave, or the subtraction of i D. from the strength if convex, the amount of myopia or hyperopia in the second meridian. The difference between the two meridians is the amount of astigmatism. When it has thus been ascertained, the cylindrical lens correcting it is to be placed before the eye and with it, the spherical lens, which will bring the point of reversal to a distance of one metre. The trial is then repeated and if the point of reversal be found at the surgeon's eye for all meridians of the pupil, the determination already made is accurate. If, however, there be found distinct movement in the visual zone in some one direction, while movement in the principal meridian perpendicular thereto is abolished, the cylinder selected does not perfectly correct the astig- matism. If this movement be in one of the principal meridians as previously determined, [in the direction of the axis of the cylinder placed before the eye, or at right angles to that axis] the cylinder has been properly placed, but is too strong or too weak, and its strength must be diminished or increased according to the indications of the movement. If, however, the movement appears to be in a meridian different from either of the principal meridians as at first determined, 96 APPLICATION WITH THE CONCAVE MIRROR. [different from the direction of the axis of the cylindrical lens before the eye, or the principal meridian at right angles to that axis] the axis has not been properly placed — does not conform exactly to the direction of the princi- pal meridian. If the cylindrical lens before the eye is of the right strength or too weak, its axis needs to be turned slightly toward the axis of a similar cylinder, which will correct the remaining astigmatism. If the cylindrical lens already before the eye is too strong, its axis needs to be turned toward the proper position the axis for a cylindrical lens of the opposite kind that would correct the astigmatism. Such a change in the direction of the axis of the cylinder is to be made, and the test repeated until the correction of any remaining astigmatism conforms exactly with the direction of the lens before the eye. This remaining astigmatism must be corrected by a change in the strength of the lenses employed. For example : suppose an eye to have compound hy- peropic astigmatism corrected by +4 sph. O +2 cyl. axis 90°. The first inspection of the pupil shows the light moving against the light on the face in all meridians. Convex lenses 2 D. and 4 D. placed before the eye show the same thing. Convex 6 D. shows the light moving against the light on the face from side to side, but ivith it in a ver- tical direction. It thus becomes evident that astigmatism is present. Still stronger convex lenses are to be tried. The 8 D. lens shows movement in the pupil with the light on the face in all meridians. The 7 D. lens shows move- ment very indefinite or indistinguishable in the horizontal meridian, but clearly with the light on the face in the ver- tical meridian. This lens then, brings the point of rever- sal for the less myopic [more hyperopic without the lens] meridian to the surgeon's eye. REGULAR ASTIGMATISM. 97 The next step is to bring the original source of light closer to the mirror so as to cause the immediate source of light to fall at the point of reversal for the more myopic [less hyperopic] meridian, which will now be one-third of a metre from the patient's eye. To do this, [supposing that the mirror has a focal distance of one-quarter of a metre, ten inches] it will be necessary to bring the source of light to within two-fifths of a metre of the mirror. That is, the immediate source of light to be at one-third of a metre from the patient, must be at two-thirds of a metre from the mirror corresponding with 1.5 D. of focusing power. The total focusing power of the mirror being equal to 4 D., the light must be so placed that the divergence of its rays will correspond to 4-1.5^2.5 D. That is, the light must be two-fifths of a metre from the mirror. When the light is in this position, the area of light in the pupil will assume the most distinct band-like appearance, run- ning in the direction of the principal meridian of least myopia [greatest h^^peropia] in this case horizontal. Having thus determined the direction of the principal meridians, one being known from the direction of the other, the original source of light is again placed back of the patient as far as possible, and the refraction in the horizon- tal meridian carefully tested by trying first the +6.5 D. spherical lens, and then the +7.5 D. spherical lens before the eye, the former of which shows the movement in a horizontal meridian against the light on the face, and the latter a movement in the same meridian luith the light on the face, thus fixing the refraction of that meridian as 7 D. — I D. =6. D of hyperopia. Weaker convex lenses are then to be tried until it is found that with the 5.5 D., the light moves ivith the light on the face in the vertical meridian, and with the 4.5 D. it 98 APPLICATION WITH THE CONCAVE MIRROR. moves against the light on the face in the vertical meridian, while the 5 D. gives no distinguishable movement in that meridian, showing that 5 D. — i D.=4 D. is the amount of hyperopia in the less hyperopic meridian. The difference between the two then is found to be 2 D., the amount of regular astigmatism present. The surgeon will then place before the patient convex 5 D. spherical with convex 2 D, cylindrical axis vertical ; and on again trying the test, will find that he is at the point of reversal for all meridians. But if on placing the cylindrical lens he makes a slight error in the direction of its axis, placing it say at 5° one side from the vertical, he will find on testing the eye some appearance of astigmatism with its axis inclined several degrees in the other direction from the vertical. And, to get rid of this astigmatism, he has to move the axis of the cylindrical lens to its proper position, pushing it towards the axis of a convex cylinder that would be required to correct this remaining astigma- tism. If the case be one of slightly myopic or high mixed astigmatism, the first inspection may show a movement witJi the light on the face in one direction, while the move- ment is against the light on the face in the other meridian. This, of course, will indicate at once the presence of astig- matism. The fact that it may occur makes it important that the first observation on the pupillary movements should include the movements in different meridians. With the concave mirror [the immediate source of light necessarily lying as far in front of the mirror as its principal focus or even farther] if the astigmatism be of quite low degree, when the least myopic point of reversal is at the surgeon's eye, the more myopic point of reversal will be at the immediate source of light, or even closer to REGULAR ASTIGMATISM. 99 the mirror without any change in the position of the origi- nal source. Thus the most distinct band-like appearance of the light in the pupil, the clearest difference between the movement against the light on the face in one meridian and the indefinite movement in the other meridian will be attained without bringing the original source of light any nearer to the mirror than its usual position. This must be borne in mind for low degrees of astigmatism. Aberration and Irregular Astigmatism. — With the concave mirror and the need of bringing the point of reversal to a fixed distance from the patient's eye, the measurement of the amount of aberration and irregular astigmatism becomes very much more tedious and difficult than with the plane mirror, though not impossible. It is, however, not difficult to detect the presence of such defects, and to ascertain which portion of the pupil they occupy, and which portions being comparatively free from them are available as a visual zone. As to the importance of such a study of the pupil, what has been said in the chapter on the plane mirror will equally apply here. Measurement of Accommodation. — With the aid of lenses, usually concaves, the near point of accommodation may be brought to the required distance of one metre from the eye and the amount of accommodation thus measured. The arrangement of the patient's and surgeon's eyes, and of the points to be looked at is the same as that described in connection with the measurement of accommodation with the plane mirror. It is, of course, impossible to make the approximate determination of the accommodation with the concave mirror by the surgeon approaching the eye of the patient. He must rely on a change of lenses to bring the point of reversal to the fixed distance of one metre. CHAPTER VIII. GENERAL CONSIDERATIONS. Apparatus. — In the chapter of the Conditions of Accu- racy, something has already been said as to the apparatus by which these conditions are best complied with. The requirements, to meet which the apparatus for skiascopy is to be adapted, are that it shall furnish the conditions necessary to the greatest accuracy, and that it shall facili- tate the finding of the lens that will bring the point of reversal to the surgeon's eye. The Mirror. — As has been stated, the essential point, in- the mirror is to have the sight hole free from reflections. This may be obtained by having the glass thin if the sight hole is cut through it, having its margin free from chip- ping, beveled as little as possible, and thoroughly blackened with a dead black. If the sight hole is not cut through the glass, but is merely an aperture in the silvering, the glass may be much thicker and there is no ground glass to deal with. The difficulty with such a mirror is in keeping the exposed glass at the sight hole clean. Unless great care is taken in preserving it from dust, and care in removing any that falls upon it, there will be a ring of dust in the periphery of the sight hole, which will reflect more light than would the ground glass. And, it is difficult to keep this space entirely clean without chipping into the backing of the mir- ror in such a way as to cause annoving reflections. But, (100) THE MIRROR. 101 however difficult, it is essential to have the sight hole free from reflections. The size of the mirror will depend somewhat upon the purpose for which skiascopy is to be used. If the mir- ror is to be employed to measure refraction of all kinds ; to show the movement of light in the pupil with high uncor- rected hyperopia or myopia, it must be large ; to give the range of movement for the immediate source of light that is necessary to render evident the direction of movement in the pupil, when that movement is slow and the illumi- nation of the light area is comparatively feeble. The disadvantage of a large mirror is that it gives a large area of light on the face, especially when, as with the plane mirror, the original source of light is brought close to it. And in this large area of light on the face only the light reflected by a small portion of the mirror immediately surrounding the sight hole is of any use when the point of reversal is near to the surgeon's eye [see page 30 for discus- sion of limits of the part of the retina visible in the pupil] . With a small mirror, making a small area of light on the face, it is easier to keep this upon the eye than it is to keep properly directed the similarly limited portion of a largearea. On this account, where skiascopy is used after an ap- proximate estimate of the refraction has been made by the ophthalmoscope or other means, quite a small mirror is found convenient. By a large mirror is meant one from 35 to 50 mm. in diameter. By a small mirror is meant one under 20 mm. in diameter. The mirror, or at least the opaque back that carries it cannot be well reduced to less than 20 or 25 mm., because, if smaller than this, it w^ill admit light to the e}'e from the original source through the space around the mirror, and such light, though not so annoving as a reflection at the sight hole, is a serious hin- 102 GENERAL CONSIDERATIONS. derance in the application of the test. The mirror plate then, must be large enough to shade the eye. A large mirror having a metal cap with an aperture of from lo to 15 mm. in diameter, that can be slipped before the face of the mirror or turned back at pleasure, will an- swer for all sorts of testing. Such a mirror- is shown in figure 23. As already indicated in Chapter III, the sight hole should be from 2 to 3 mm. in diameter. The handle of the mirror should be rather broad, so that a ver>' slow, even rotation can be secured ; for as the point of reversal is approached, the magnified movement in the pupil becomes so rapid that only by moving the mirror slower, and making excursions of very slight extent can this apparent motion in the pupil be readily followed. This difficulty of causing the immediate source of light to move slowly enough is diminished in proportion as the immediate source of light is brought closer to the mirror. The Shade. — The shade that covers the original source of light should extend far enough above and below to pre- vent the escape of any considerable amount of light into the room. Where an argand burner is used as the source, "^ Made at my suggestion by Wall and Ochs of Philadelphia. Another form is described by Dr. James Thorington, Philadelphia Polyclinic, 1893, page 329. THE SHADE. 103 a cylindrical shade is needed, 20 to 25 cm. long, and having a diameter of 6 or 6.5 cm,, slightly greater than that of the chimney used so as to allow a free current of air between the shade and chimney, and thus diminish the heat from the flame. An asbestos shade as proposed by Dr. J. Thoring- ton (Ann. of Ophthalmology and Otology, 1895, p. 5) is to be preferred to metal on account of intercepting the heat of the flame. The aperture of about 5 mm., or larger, if the test is not intended to be very accurate, should be opposite the brightest part of the flame, which ought to be broad enough to allow of slight change of position of the surgeon with reference to it without its becoming hidden by the shade. The Lenses. — Ordinarily these are taken from the trial case and placed in a trial frame before the eye. It is important to have them clean and comparatively undam- aged by scratching. The trial frames should be such as to support the lenses well up before the eye and with their centres before the centres of the pupils. They must also be far enough away from the face to escape the touching of the lashes, and to prevent the condensation of moisture upon them. The interruption of the red reflex from the pupil by such an occurrence prevents the satisfactory ap- plication of the test, and may be quite puzzling, because the reason for the obscuration is not immediately apparent, and it may be ascribed to opacities within the eye. Support of Lenses. — The trial frames have the advan- tage over other supports for lenses to be presently men- tioned, that they keep a constant position with reference to the patient, so that a slight movement of the patient's head does not carry his eye away from the centre of the lens to its periphery or beyond. When the surgeon has learned to estimate by the 104 GENERAL CONSIDERATIONS. rapidity of movement of the light in the pupil the amount of ametropia remaining uncorrected, by following the plan here laid down of considerable intervals between the lenses until an approximation of the required lens has been made, the number of changes of lenses for any case is not neces- sarily great. So that for any one w^ho does not employ skiascopy on large 'numbers of patients daily, the trial frame and lenses will be found satisfactory. Special series of lenses mounted in revolving disks have been arranged by Haines {Ophthalmic Review, 1886, p. 282), Doyne, Couper, Burnett {Trans. Am. Ophthalmol. Soc, 1888, p. 223), Wurdemann and others, to save time by facilitating the changes to the lens required. Some of these have been designed for the patient to make the change of lens under direction of the surgeon, and others to give the surgeon himself constant control of their movements. One of the simplest arrangements is the " hand-skiascope " of Wurdemann {American Journal of Ophthalmologij, 1891, page 223) shown in figure 24. The lenses are inserted in a sheet of hard rubber which the patient holds by the handle, bringing before his eye the lens the surgeon may indicate. In an instrument suggested bv the writer the disk is rotated by a rod one metre long and |i:||U attached by a universal joint so that it drops out of the way wdien not in use. The lens series runs from 7 concave to 7 convex spherical with 0.5 D. intervals, requir- FiG. 24. -^g^ ^Q ^g supplemented by lenses in the trial frame for high hyperopia and myopia, or astigmatism. SUPPORT OF LENSES. 105 An ingenious piece of apparatus having a complete series of lenses, both spherical and cylindrical, arranged for the purpose has been devised by Lambert (Traiis. Amer. Ophthalmol. Soc, 1894, p. 196). It has the lenses arranged in two disks for the special lenses, and detachable slides for the cylinders, enabling the surgeon to reach the lenses wanted quickly. To be compelled to run over the whole lens series to find the one sought, would be a way of consuming time rather than saving it. A series sufficient for the approximate testing of the majority of eyes may save time where many are to be tested, especially if the conca^'e mirror be employed. Meridian Indicators. — In working with lenses in the graduated trial frame one may refer to its graduation to as- certain the direction of the bands of astigmatism. But in the darkened room this is not convenient. To meet this want, Thorington (Medical News, March 3rd, 1894) and Prince (Ophthalmic Review, July, 1894) have suggested disks specially graduated for the purpose. The former, figure 25, called an axonometer ; the latter, figure 26, called an inclinometer. Fig. 25. Fig. 26. A Distance Measure. — Where the concave mirror is employed, the distance remaining fixed throughout the test, 106 GENERAL CONSIDERATIONS. it is only necessary that the surgeon should properly place himself at the beginning, and retain his position. He can then dismiss the consideration of the distance, and consider simply the changes made in the lens before the eye. With the plane mirror, no measure is necessary where the test is used only to approximate the refraction, the sur- geon soon learning to guess at the distance close enough to be within 0.25 D. of the amount of myopia present with the lens fixed upon. But, for exact measurement, it is con- venient to have something to measure from the patient's eye to the surgeon's. This may be either a tape attached to the trial frame or lens disk (Burnett), picked up and held to the surgeon's eye when the test is completed, or the ordinary metre stick. In either case, it is convenient to have the measure graduated in dioptric focal lengtlis de- scribed by the waiter in the Mcdic(d Neivs, June 27, 1885. The graduation should begin from the end that is applied to the patient's eye. Mydriatics. — In making any measurement that is to have positive significance, the first essential is that the quantity to be measured should be fixed. When the refrac- tion of the eye varies from moment to moment, it is impos- sible to make a valuable measurement of it by any method. When it is liable to vary from moment to moment, there is a liability to error, due to such variations. Believing that when consulted as to an error of refraction or its effects, the ophthalmic surgeon should ascertain its degree with exact- ness and certainty, the writer is accustomed to employ a mydriatic so as to secure complete paralysis of accommoda- tion in the great majority of cases under 50 years of age. While any of the true mydriatics, atropin, daturin, duboisin, hyoscyamin, or scopolamin, will give a satisfac- tory paralysis of accommodation, if a mydriatic is used solely MYDRIATICS. 107 for diagnostic purposes, homatropin should be selected on account of its briefer period of recover}^ Properly used, it is for practical purposes of diagnosis, as reliable as any mydriatic we possess. To secure paralysis of accommoda- tion, therefore, four to six drops of a two to four per cent, solution of homatropine hydrobromate are to be instilled, one drop at a time at interv^als of five minutes, about an hour before the test is to be applied. To apply skiascopy with the greatest ease, requires a pupil moderately dilated. Like other methods for the measurement of refraction, it will not give as accurate re- sults if the pupil be narrow ; and, on account of the aber- ration and irregular astigmatism that usually exist near the margin of the lens and cornea, the wide dilatation of the pupil introduces factors of confusion. The need, then, for a dilated pupil is about the same with skiascopy as for the use of the refraction ophthalmoscope or the test lenses, ex- cept that skiascopy is slightly more at a disadvantage when the pupil in the dark room is less than four millimetres in diameter. When this is the case, sufficient dilatation can be obtained, with the least inconvenience to the patient, by placing in the eye a drop of a two or four per cent, solution of cocaine, thirty to fifty minutes before using the test. Relative Advantages of Plane and Concave Mirrors. — The difference in methods of using most efficiently the plane and concave mirrors have caused most surgeons to habitually employ the one or the other, and to depend upon it almost entirely for practical purposes. Either, properly used, will meet the requirements of practice. In Astigmatism, the plane mirror is capable of de- termining with greatest accuracy the meridian of greatest myopia, but not the meridian of least myopia. On the other hand, the concave mirror fixes with greatest accuracy 108 GENERAL CONSIDERATIONS. the meridian of least myopia, but not that of greatest myopia. But, for regular astigmatism — the astigmatism that can be fully corrected by a cylindrical lens, or by any com- bination of cylindrical lenses — the principal meridians are always perpendicular to each other, so that for practical purposes, it is only necessary to accurately locate one of them ; and it is a matter of indifference which one this shall be. In positive aberration^ the focusing of the light upon the retina being such that the light area has the sharpest out- line when the immediate source of light is closer to the eye than the point of reversal, this can only be effected by the concave mirror, which, therefore, has so much advantage over the plane mirror. It is of some practical importance in a few cases, in which the aberration invades the visual zone. With negative aberration, the advantage lies with the plane mirror, but is of still less practical importance on ac- count of the smaller number of cases of aberration of this kind. With a concave mirror, the distance from which it can be used with advantage is fixed, and the surgeon being able to readily check his position, by a mark on the neighboring wall or some similar device, there is no need of the meas- urement of the distance between the surgeon and the patient ; but all changes of the movement of the light in the pupil must be effected by a change of the lens before the eye. On the other hand, the plane mirror can be used from any fixed distance, but allows a variation of the dis- tance of the surgeon from the patient, and therefore requires the fewer changes of the lens before the eye. This latter advantage of the plane mirror over the con- cave mirror may not seem very great, but when it comes to ADVANTAGES OF PLANE AND CONCAVE MIRRORS. 109 the accurate determination of the refraction, requiring re- peated inspections of the light movement in the pupil from within and from beyond the point of reversal, it is really quite important. The disadvantage of the concave mirror may be lessened by the employment of some such arrange- ment of lenses in a disk as has been referred to in a preced- ing section. But, even in this case, the fact that there is a complete break between the appearances presented by one lens, and the appearances present by the use of the lens next stronger or weaker, makes the information obtained less valuable and satisfactory than that derived by the move- ment of the surgeon's eye from one position to another, which allows him to watch the different appearances of light and shade as they pass gradually into each other. Perhaps the greatest advantage of one over the other is that of the plane mirror over the concave in thus making possible the more complete study of aberration and irregu- lar astigmatism. INDEX. ABERRATION, 17, 41, 56, 59, 84, 99, 108. Accommodation, 86, 99, 106 Accuracy 36, 40, 45,49 Advantages, 5, 107 Apparatus, ^S, 37) 71, 100 Apparent movement,. ..20,22 26, 31, 50, 55, Applications, 71, 89 Area of light. ..23, 25, 26, 32, 47, 53 Artificial eye, 18 Astigmatism,... 7, 17, 38, 41, 46, 56, 76, 93, 107. Axonometer, 105 BAND appearance,... 7, 47, 78, 94 Bowman 7, 10, 47 Brilliancy of light, 33, 36 Burnett, 104 CHARNLEY, ; 9 Chibret, 9, n Concave mirror, 9, 24, 39, 40, 44, 51, 54,_ 62. 64, 89, 109. Conditions of accuracy, 36 Conical cornea, 7, 65 Contents, 3 Couper 7, 104 Cuignet 8, 9, 10 FACIAL light area 23, ''25 Fantoscopie, 11 Focal lengths, 106 Forbes, 9 Form of light area, 32, 47, 53 Fundus-reflex test, 11 r^ALEZOWvSKI, 8,':ii HAINES, 104 Hartridge, .11 History, 7 Hypermetropia 7, 24, 26, 72, 90 TLLUSTRATlONS,..2o, 23, 25, 31, ^ 32, 48, 49. 55. 57, 61, 62, 67, 69, 85, 102, 104, 105. Immediate source, 23, 38 Inclinometer 105 Indicators, 105 Inverted, , 20, 61 Irregular astigmatism, 7, 41, "'56, 84, 99- JACKSON, 9, 10, 81, 106 'J Juler, .'9 KERATOSCOPIE,., 10 Koroscopie, 11 pvARK ROOM, 36 I AMBERT, 105 L/ Derby, 37 L Landolt, 11 Learning the test, 13 Lenses, 103, 104 Light area,..22, 25, 26, 28, 32, ^8, 53 Light source, 23, 36, 71, 89, 102 •37 Difficulties 11 Dioptric Scale 1 06 Dioptroscopie, 11 Direction, 47, 55, 78, 97 Distance, 42,43, 49, 89, 105, 108 Donders, .-.7, 8 IWJAGNIFICATION of retina,. ..30 Doyne, 104 J'l Mengin 8 Meridians, 46, 81, 97, 105 CGGER II *-' Emmetropia, 24, 26, 76, 93- Enlargement, 30 Erect, 20, 61 Mirror 37, 100 Morton, 9 Movements of light, 23, 25, 26, 28, 31, 53, 61. (Ill) 112 INDEX. Mydriatics io6 Retinal area, 23, 25, 28, 32, 38 Myopia, 7, 19, 24, 26, 74, 91 Retinal enlargement, 30 Retinophotoscopie, 11 VjAME..... 10 Retinoscopy, 10 1> Negative aberration,... 42, 59, Retinoskiascopie 11 ^3? 108. Reversal, 19, 34, 46 r^BLIQUITY of lens, 70 QCISSORS movement, 69 ^^ Oliver, II O Shade, 37, 102 Optical principles,... 19 Shadows, 26, 32, 47, 56, 60, 69 Original source of light 23, 38 Shadow-test, 11 PARENT, 9, 10, II |!s^t-,^°l^' 37, 100 •I Plane mirror, ..9, 23, 38, 40, 44, 50, 53, 60, 63, 71, 109 Point of Reversal, 21, 34, 46 Size of mirror, loi Smith , Priestl ey , i r Skiascopy, ii Position, ';4o/43', 49,' 7i! 89 ff""^^ ^^ ^^SK". 23, 36, 38, 71. 89 Positive aberration,. .41, 59,60, 108 Practial application, i^, 71 Story, 9 Study of test, 13 Preface ' J' ' ' ^ Symmetrical aberration, 41, 42, 58, prince,::"!;""":"!:":;:.:::;::::::::io5 59. 60, 63, los. Principal meridians, 46, 81, 97 -THORINGTON, 102, 103, 105 Pupillary shadows,... 26, 32, 47, 56, 1 60, 69. Pupilloscopie 11 T | MBRASCOPY, 11 RANDALL, 43 Rate of movement,... 28, 31,66 W^^^^'^^ ZONE, 3. 59 Real movement, 22, 28 Regular astigmatism,... 7, 17, 38, 41, TT /EILAND, 81 46,76,93,107. VV Wiirdemann, 104 * UNIVERSITY OF CALIFORNIA LIBRARY Los Angeles This book is DUE on the last date stamped below. Form L9-Series 4939 t