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 [Photographic Eeproduction (Actual Size) of the Original Illustration in Helm- 
 holtz's "Beschreihung eines Angenspiegels."]
 
 THE DESCRIPTION 
 
 OF AN 
 
 OPHTHALMOSCOPE 
 
 BEING AN ENGLISH TRANSLATION OF 
 
 Von HELMHOLTZ'S 
 "Beschreibung eines Augenspiegels" 
 
 (BERLIN, 1851) 
 BY 
 
 Thomas Hall Shastid, A.B., A.M., M.D., LL.B., F.A.C.S. 
 
 SUPERIOR, WISCONSIN 
 And the first translation of this classic into any language 
 
 Chicago 
 
 CLEVELAND PRESS 
 
 1916 
 
 J
 
 Copyright 1916 
 
 By 
 
 THOS. HALL SHASTID 
 
 All rights reserved.
 
 Sebication 
 
 It is fitting that this, the first translation into any language of 
 
 Helmholtz's "Beschreibung eincs Augenspiegels," 
 
 should be inscribed to 
 
 3Br. Casftp a. aaioob 
 
 Ophthalmologist and almost universal scholar, who has contributed 
 so much to our knowledge of the visual apparatus, and who, in addi- 
 tion, has so indefatigably gathered together and preserved (as in the 
 American Encyclopedia of Ophthalmology and works of a similar char- 
 acter) the comprehensive literature of our subject. Because of a 
 friendship lasting now these many years, he consents to waive the 
 imperfections of my rendering, and to accept this very slight 
 performance — which is offered merely as a little token of a high regard. 
 May the fraternal reader be as lenient toward those imperfections as 
 is he, and — I may add — as appreciative (for many are not) of the 
 timeless, the deathless character of the little document which I have 
 had the temerity to try to translate. — T. H. S.
 
 Edition limited to 500 copies. 
 For private distribution only.
 
 DESCRIPTION 
 
 OP AN 
 
 OPHTHALMOSCOPE 
 
 FOR THE INVESTIGATION 
 OF THE RETINA IN THE LIVING EYE 
 
 BY 
 
 H. HELMHOLTZ, 
 
 PROFESSOR OF PHYSIOLOGY AT THE UNIVERSITY OP KOENIGSBERG. 
 
 BERLIN 
 
 THE A. FORSTNER PUBLISHING HOUSE 
 
 (p. jeanrenaud) 
 
 1851 
 
 [A translation of von Helmholtz 's title-page, treated typographically as nearly 
 like the original as possible.]
 
 THE OPHTHALMOSCOPE 
 
 The present treatise contains the description of an optical instrument, 
 by which it is possible in the living to see and recognize exactly the 
 retina itself and the images of luminous objects which are cast upon it." 
 
 * Even tho ancients, as a matter of course, had noticed that the ejes of cer- 
 tain animals are brilliant in the dark. Thus Pliny (Book XI, Chap. 55): "The 
 eyes of animals that see at night in the dark, cats for example, are shining and 
 radiant, so much so that we cannot bear to gaze upon them; those of the she- 
 goat, too, and the wolf are resplendent, and emit a light like fire." Pliny did 
 not, however, attempt to explain the phenomenon. 
 
 In 1704, Jean Mery, of Paris, performed his famous experiment with a cat. 
 Having immersed the animal in water, he first observed that the pupil dilated 
 (as a result of suspended respiration) and then he beheld in all its glory the 
 fundus of the animal 's eye — the entrance of the optic nerve and all the colors 
 and vessels of the choroid. Mery understood quite well enough that something 
 more than mere pupillary dilatation was necessary to account for the possibility 
 of observing the fundus of the eye when the eye was under water. His ex- 
 planation, however, of the "something more" was wholly erroneous. He 
 believed, that is to say, that the view of the fundus was rendered possible by 
 the water's filling up a multitude of tiny " unevennesses " on the anterior sur- 
 face of the cornea. Five years later, de la Hire stepped forward with the cor- 
 rect explanation. According to him, the water obviated the refraction of light 
 by the cornea, so that all rays leaving a given point upon the fundus emerged 
 from tho eye not as parallel, but as divergent, rays. De la Hire also observed, 
 incidentally, that the disturbing light-reflexes proceeding from a cornea in aero 
 are done away with by the water. 
 
 In 1796 Fermin observed a certain luminosity in the pupils of an Ethiopian 
 albino. In 1816 Scarpa remarked upon a similar phenomenon in a certain dis- 
 ease of the fundus, and, one year later, Beer described the same condition fully, 
 inventing therefor the expression "amaurotic cat's eye" — a term which is still 
 in use. In 1836 Hasenstein first produced a factitious luminosity by compress- 
 ing the eveball backward — making the eye, in fact, artificially hypermetropic. 
 In 1847 Babbage, an English mathematician, exhibited to Wharton .Tones, the dis- 
 tinguished London oculist, the model of an instrument invented by him for the pur- 
 pose of examininsr the interior of the eye. It consisted of a small plane glass 
 mirror, from which a portion of the silvering had been removed. This device, 
 however, was first made known to the world in 1854, by Wharton Jones (Brit, and 
 For. Medico Chir. Beview, Oct., 1854). The services of Briicke Hn 1845, published 
 in 1847) and of C'umming (in 1846) are adverted to herein by Helmholtz. 
 
 Tho earliest reception of the ophthalmoscope is decidedly interesting. Thus, 
 to quote from Koenigsberger, "Hermann von Ilelmholtc," (1906) p. 74: "The 
 ophthalmoscope was, however, some time in making its way, on account of the 
 mathematical and physical knowledge presupposed by the ' Description of an Oph- 
 thalmoncope for the Inve.itination of the Retina in the Livinrj Eye,' published in 
 the autumn of 1851, and people were at first very shy of employing it. One dis- 
 tinguished surgical colleague told Ilebnhnltz he should never use the instrument — 
 it wo\ild be too dangerous to admit the naked light into a diseased eve; another 
 was of opinion that the mirror might be of service to oculists with defective eye- 
 sight — he himself had good eyes and wanted none of it." — (T. H. S.) 
 
 7
 
 THE OPHTHALMOSCOPE 
 
 The instrument has, for this purpose, two different problems to solve. 
 First, everything which we can see of the backgi-ound of the uninjured 
 eye appears to us absolutely dark. The cause of this lies, as I will show, 
 in the light-refracting media of the eye, which, under ordinary circum- 
 stances, hinder us from seeing illuminated parts of the retina behind 
 the pupil. Therefore, the first question is to discover a means of il- 
 lumination whereby exactly that portion of the retina on which we gaze 
 through the pupil may be adequately lighted. Secondly, we view the 
 background of the eye only through the light-refracting media. These, 
 however, cast images of the retinal objects, which, in general, do not lie 
 for the observer within the limits of plain vision. We need, therefore, 
 together with a proper procedure for illumination, also further optical 
 expedients which will render possible for the observing eye a correct 
 accommodation for the objects which it should see. 
 
 I. Illumination. 
 
 In order to be able to find the essential conditions for the method of 
 illumination, we must first of all make clear to ourselves why, as a rule, 
 the ground of the eye behind the pupil appears to us to be of so deep 
 a black. 
 
 The cause of this is not the condition of the pigment of the chorioidea; 
 for, if the pigment layer absorbed the light which falls upon it even 
 more completely than any other known black substance, still, there lie 
 before the chorioidea parts which can reflect a quantity of light suffi- 
 cient to render them visible. That is true, first of all, of the substance 
 of the x-etina, which, to be sure, in the recent condition, is very trans- 
 parent, and marks itself off but little against the dark pigmentary 
 backgi'ound ; to a much higher degree, however, it is true of the blood- 
 vessels of this membrane, whose tiny stems carry blood enough to 
 exliibit a strongly red hue. Finally, there appears, even in the fundus 
 of the eye, a shining white spot, namely, the place of entrance of the 
 optic nerve, on which no pigment at all lies, and which, therefore, 
 reflects all the light that falls upon it. And yet we observe, under 
 ordinary circumstances, behind the pupil of the living eye, not the 
 slightest trace of the red color of the blood, nor of the white color of 
 the optic nerve. 
 
 It can be shown much better by a simple experiment, that not the 
 color of the background, but only the refraction of the light in the ocu- 
 lar media, is the cause of the deep blackening of the pupil. Let one take 
 any kind of small camera obscura well blackened within, and let him 
 bring to the place where the picture is produced an opaque white
 
 THE OPHTHALMOSCOPE 9 
 
 card, for example one from thick white drawiug-paper. Among other 
 kinds of camera may be employed the ocular tubes of most microscopes, 
 after the ocular glass has bceu removed therefrom, and the collective 
 glass has been inserted. These tubes are, as a rule, precisely as long 
 as the local distance of the collective glass. If one sets them with the 
 end which contains the ocular upward upon the white card, then they 
 form a camera obscura of the kind we need. There are thrown, in 
 this case, very bright images of the surrounding illuminated objects, 
 on the white card, and still the interior of the instrument, w'hen one 
 looks into the lens in any desired direction, appears absolutely black. 
 We have here a fac simile of the eye, where cornea and crystalline lens 
 are substituted by the objective lens of the camera, and the retina by 
 a clear white paper surface, but there occurs apparently the same 
 complete darkness of the internal space as in the eye, as long as the 
 paper surface lies precisely at the spot where the tiny images of ex- 
 ternal objects are produced. If one takes away the convex glass, or 
 if one materially alters its distance from the paper surface, there 
 appears to the beholder at once the clear white surface of the paper. 
 
 How, now, can the refraction of the light produce the phenomenon 
 described ? Let us consider the course which the rays of light must 
 take, according to the physical laws of the refraction of light in the 
 eye. 
 
 Let light fall from a luminous point upon a fittingly adapted eye, 
 concerning which we assume that it is formed with absolute accuracy, 
 that is, that all the incident rays from the point in question concentre 
 upon a single point of the retina. Of the light which, by the ocular 
 media, is caused to converge upon this membrane, the greater part 
 is absorbed by the black pigment, while the smaller is reflected partly 
 by the nerve elements and blood-vessels, partly by the layer of rod- 
 shaped corpuscles. That which is thrown back by the latter structures, 
 passes, as E. Briicke has shown, back out through the pupil, without 
 beeonung scattered to any other portion of the wall of the eye. In 
 this way is avoided the spreading of perceptible quantities of dispersed 
 light within the eye. The reflected rays, which, from the point of con- 
 vergence on the retina, pass back out divergently to the refracting 
 surfaces of the eye, follow then precisely the same path, in a reverse 
 direction, by which the incident rays of the luminous point converged 
 from the refracting surfaces of the eye until they reached the retina. 
 From this it follows that the returning rays, even after they have 
 passed clear through the refractive media and out of the eye, must 
 coincide completely with the incident rays, mu.st therefore finally all 
 betake themselves to the original luminous point.
 
 10 THE OPHTHALMOSCOPE 
 
 For, when two rays, which pass through several simply refracting 
 media in a reverse direction, coincide in one of the same [media] they 
 must do the same in all [the media] . On the limiting surfaces of the 
 medium, that is to say, within which they coincide, the angle of inci- 
 dence of the outcomiug rays is identical with the angle of refraction 
 of those which are entering. As, now, according to the laws of refrac- 
 tion, the proportion of the sine between the angle of incidence and the 
 angle of refraction of the former, is precisely as large as that between 
 the angle of refraction and the angle of incidence of the latter, so 
 must also, on the other side of the refracting surface, the angle of 
 refraction of the outcoming and the angle of incidence of the ingoing, 
 rays, be equal. As, at the same time, all these rays lie in one plane, 
 the plane of refraction, it follows that they also fall into one another 
 [coincide] in the second medium. In like manner it follows further 
 for the third, fourth medium, and so on. 
 
 Let us apply that to the case where any given system of refracting 
 surfaces produces an exact image of the luminous point a at the point 
 h, that is, where all the rays which proceed from a unite again in &, 
 then follows the well known fact that in this case, always, a will be 
 the image of b, if the latter sends out rays. Exactly upon the same 
 paths, that is to saj', on which rays from a proceed to b, they may 
 also return from h to a. If now o is a luminous point outside the eye, 
 and b its image, a point on the retina, then the ocular media will con- 
 centrate the returning light precisely at a into an image of i. The 
 image of the illuminated retinal point will coincide exactly with the 
 original point of luminosity. The same is still valid, also, when we 
 have to do not with a luminous point, but with a luminous surface or 
 a body, as soon as the eye is adapted for its outlines. All the incident 
 light which is thrown back can always only return to its place of 
 origin, and never can proceed in any other direction. 
 
 From this it follows that, without special expedients, we can see 
 nothing of the illuminated portion of the retina, because we cannot 
 bring our eye into the direction of the returning light without at the 
 same time cutting oflF the incident light absolutely. To our pupil no 
 light from the depths of the other's e.ye can return which has not pro- 
 ceeded from it [1. e., our pupil]. And as, in general, none at all has 
 proceeded from our pupil, it sees in the darkness of the other's eye 
 merely the reflection of its own blackness; only those portions of the 
 retina become visible to it on which its own dark image is copied. 
 
 We have until now assumed that the observed eye furnishes abso- 
 lutely accurate images. When that is not the case, the propositions 
 heretofore laid down do not hold strictly true, the returning light will
 
 THE OPHTHALMOSCOPE 11 
 
 indeed proceed to the illuminating body, but it will also in part pass 
 by tiiat, and an observer who approximates himself to the line of direc- 
 tion of the incident light, will be able to perceive a part of the light 
 which is coming out. On this fact are based the methods of Gumming 
 {Medic. Chirurg. Transaotions, Vol. 29, p. 2b-i) and Briicke (</. Mailers 
 Archiv. 1»47, p. 225) for observing the illumination of human eyes. 
 From what has been said it is manifest that, in this way, the illumina- 
 tion must be the greater, the less exactly the rays of a luminous point 
 are concentrated on a point of the retina, therefore especially in faulty 
 adaptation. Besides, 1 have convinced myself that one may observe 
 a weak illumination, according to the method of E. Briicke, even in 
 eyes with good acuity and under perfect adaptation for the luminous 
 body, from which is to be concluded that, under all circumstances, a 
 small quantity of incident light is scattered laterally. The cause 
 thereof may be inexactness of the eye, incomplete transparency of its 
 refracting parts, or ditiraction at the border of the pupil. 
 
 In any case, the observer perceives, in this experiment, only a small 
 part of the returning light, and indeed precisely that which is irregu- 
 larly refracted and which cannot be used for the production of a 
 regular image. Some other method is necessary for the attainment 
 of our object, a method which makes it possible to look into the eye 
 not merely somewhat, but exactly, in the direction of the incident 
 light. The expedient for this has already been found in an accidental 
 observation by E. Briicke. v. Erlach, who wore spectacles, saw, in- 
 deed, the eyes of an acquaintance shine, when the acquaintance saw 
 reflected in the lenses of the spectacles a light which there was in the 
 room. In this way, therefore, uncovered lenses were employed as 
 illuminating mirrors, and through these very objects the observer 
 looked toward the observed eye. Precisely the same expedient we 
 shall employ for our purpose, replacing, however, the spectacle lenses 
 to advantage by well ground plane glasses. 
 
 In a darkened room, where only a single source of light, a well burn- 
 ing lamp or an opening in the window shutter for the sunlight, is 
 present, let one set a small, plane glass plate in such a way that the 
 observed eye may perceive therein the mirrored image of the light, 
 without, however, its necessarily gazing at this mirrored image directly. 
 From out the anterior surface of the lens there falls, by this arrange- 
 ment, light into the observed eye, and through the same glass at the 
 same time the observer can view the eye, without, while so doing, 
 being in the least aware of any light which is being reflected from its 
 anterior surface. One sees that, in this way, it becomes possible to 
 look into the subject's eye in precisely the same direction as that in
 
 12 THE OPHTHALMOSCOPE 
 
 which the light I'alls upou it. Under these circumstances the eye of 
 the observer in fact receives light from out the depths of the other 
 eye, and sees its pupil apparently grow luminous. 
 
 In Fig. 1, let A be the tiame, C the glass plate, D the observed, G 
 the observing eye. The light from A which falls upon the mirroring 
 plate, is by that partly reflected, and the reflected part continues, 
 according to the laws of catoptrics, as if it proceeded from the reflected 
 image of the flame at B. For the observed eye this mirrored image 
 represents the place of the luminous object, and upon its retina is 
 thrown an inverted and minified image. Moreover, the axis of this 
 eye can be turned in any direction, say toward the object H. Accord- 
 ing to the already developed rules, the refractive media of D cast the 
 image of its retina and of its retinal images again at B. For B is the 
 apparently present object for the eye D, and the rays returning from 
 D must proceed again to their place of origin. On the way from D 
 to B this light encounters once more the reflecting plate, a part is 
 reflected and returns to the real flame A, another portion passes 
 through the glass and strikes the eye of the observer G. 
 
 By this arrangement the pupil of the eye D appears to shine with a 
 red light, and indeed as a rule more strongly than I have seen it do 
 by Briicke's method. According to that method, there contributes 
 to the illumination only the little light which, in the eye, is not com- 
 pletely and regularly refracted; according to the method just de- 
 scribed, on the contrary, the entire light, with the exception of the 
 part (to be sure not an inconsiderable part) which is lost by the pas- 
 sage through the reflecting glass. Moreover, the illumination is of 
 very different strengths, when different portions of the retina receive 
 the image of the flame. "When the eye D turns in different directions, 
 the clear retinal image must always remain in the prolongation of the 
 line B D, and will therefore fall successively on various portions of 
 the background. If it falls on the place of entrance of the optic 
 nerve, then the most of the light is reflected, the pupil lights up with 
 a strong yellowish white, almost as if the flame stood behind it. The 
 retina proper, on the other hand, reflects less, and indeed red light. 
 In general, the image of the flame upon it appears the brighter, the 
 nearer; the darker, the farther, it lies from the place of entrance of 
 the optic nerve. On the contrary, the place of direct vision, the yellow 
 spot (which is struck when the observed eye D gazes directly at the 
 mirrored image of the flame at B) reflects, by way of exception, very 
 much less light than the parts which are nearest to it, and is there- 
 fore the most unfavorable spot for this experiment. 
 
 In order to fulfill the condition that the observer gaze into the eye
 
 THE OPHTHALMOSCOPE 13 
 
 exactly iu the direelion of the iueideut light, the glass plate may be 
 directed either by the observed or by the observer, if the former is 
 to do it, let him turu the plate lirst of all so that he sees therein the 
 mirrored image of the light, then again so that this image appears to 
 him exactly iu the same direction as the observing eye, that, iu other 
 words, the latter and the mirrored tiame cover each other. In this 
 way the required condition is fulfilled. At the same time occurs this 
 inconvenience, that the observed eye must look directly at the tiame, 
 the retinal image tlierefore falling precisely on the spot whence the 
 light is the least reflected. If, however, the observed eye, after it has 
 found the correct position, turns a little sidewise, in order to let the 
 light shine more brightly, then the pupil becomes displaced and the 
 correct position is disturbed. Still, one can then assist the matter by 
 gentle turning of the mirror now this way and now that. 
 
 It is better, however, to perform the experiment in another way, 
 whereby the observer holds the glass himself. One must, by this 
 method, shade the face that is being observed, and make the reflecting 
 plate so small that it is barely large enough to see through. The light 
 reflected from it then produces on the shaded face of the observed a 
 small, bright spot, which has about the form of the reflecting glass. 
 This point should be so managed by the observer that its centre falls 
 upon the observed eye, while he himself looks through the glass. In 
 this way the glass may easily be placed con-ectly, and the observed 
 eye may, without the slightest difficulty, be turned toward all sides 
 in order to cause the image of the flame to fall on different parts of 
 the retina. 
 
 Every person can, furthermore, in similar fashion, by the aid of a 
 bit of plane glass, see one of his own eyes grow luminous. He should 
 step before a mirror, set up a lamp at one side, hold the glass before 
 his right eye in such a way that he sees the flame reflected in the plate, 
 and turn the glass so that the image of the flame coincides with the 
 mirrored image of his left eye; then the left eye sees the mirrored 
 image of his right pupil grow luminous, but of course only weakly so, 
 because the retinal image falls on the outer side of the eye at a con- 
 siderable distance from the optic nerve. 
 
 Moreover, the same simple expedient permits itself to be employed 
 with advantage for illumination, in every instance when one wishes to 
 look into a dark cavity with a narrow opening, for example, the audi- 
 tory meatus, the nose, and so on. In order to view the drum mem- 
 brane, one should seat the subject of the experiment with his back 
 toward the window, preferably in sun.shine, draw the auricle a little 
 downward, and east the reflected sunlight into the auditory meatus.
 
 14 THE OPHTHALMOSCOPE 
 
 while one gazes through the glass. In this way one may very easily 
 and conveniently illuminate the tympanic membrane as strongly as he 
 wishes, and so observe it. 
 
 In order to see the pupil become luminous, any simple plate of glass 
 suliices for the mirror ; one does not need in that case to pay particular 
 attention to the intensity of the light. Should it be desired, however, 
 by means of this light to recognize distinctly the structure of the retina 
 and the character of the image of the flame, then one must endeavor 
 to make the illumination as strong as possible. That can be done in 
 two ways, namely, by a proper choice of the angle under which the 
 incident light is reflected from the mirroring plane, and by an increase 
 in the number of the reflecting plates. I will now unfold the prin- 
 ciples which have guided me in this connection during the construc- 
 tion of my instrument, and which would also serve as a basis should 
 oculists think it necessary to produce modifications of the instrument 
 for practical purposes. For those of my readers to whom the physical 
 conceptions involved are not familiar, I remark furthermore that this 
 exposition is not necessary for an understanding of the sections to 
 follow. 
 
 From every limiting surface of a glass plate, the more light is re- 
 flected the larger the angle of incidence, that is, the angle between the 
 ray and a line which stands vertical to the plate. Since, in the ease of 
 reflection from the upper surfaces of transparent bodies, the light 
 waves of different undulatory directions conduct themselves differ- 
 ently, we must think of the incident light as divided into two equal 
 parts, of which the one is polarized parallelly to the reflecting surface, 
 the other vertically thereto. The light-intensity of all the incident 
 light we will call J, therefore that of each of the two divisions men- 
 tioned V2 J> the angle of incidence (angle between the incident ray 
 and the incident-perpendicular) a, the angle of refraction (between 
 the refracted ray and the incident-perpendicular) aj, the index of 
 refraction n. If a is given, we find first of all ai by means of the 
 equation 
 
 sin. a ^ n sin. a^. 
 
 The intensity P of the light reflected from a limiting surface between 
 air and glass and polarized vertically to the plane of incidence, is, 
 according to the formula of Fresnel 
 
 J tang2 (a — aj 
 P = _. 
 
 2 tang2 (a-f 0,)
 
 THE OPHTHALMOSCOPE 15 
 
 Likewise the intensity Q of the reflected light which is polarized par- 
 
 allelly to the plane of incidence 
 
 J sin- {a — flj) 
 
 Q 
 
 2 sin* (a -|- oj 
 
 When several reflecting plates lie parallel, one behind another, and 
 the illuminating surface is sufficiently large for its mirrored images, 
 which are produced by the individual reflecting surfaces, to superim- 
 pose themselves, in greatest part, for the observed eye, then the in- 
 dividual images combine into one image of greater brightness. By 
 computation of the (luantities of light reflected to and fro between 
 the different surfaces, one is able to determine for every system of 
 parallel surfaces, how much light is, on the whole, reflected. For an 
 indefinite number n of the reflecting surfaces one finds the sum 11 of 
 the light polarized vertically upon the plane of incidence 
 
 nP 
 n = J 
 
 J-l-2 (n — 1) P 
 
 and the sum ii of that which is polarized parallelly to the plane of 
 incidence 
 
 nQ 
 
 S = J 
 
 J + 2 (n — 1) Q 
 
 As I find these formulae in no writing on physics I give their deriva- 
 tion briefly at the end of this essay. 
 
 The sum n -|- 2 gives us the entire quantity of light which is thrown 
 back from the system of reflecting surfaces and which proceeds to the 
 observed eye. We will set it down as equal to IT, so that 
 
 H = n + 5 
 
 When the width of the pupil remains unchanged, the brightness of 
 the retinal image is proportional to this quantity of light. The quan- 
 tity of light returning from the eye we may therefore set down as 
 equal to m II, where m designates a coefficient whose value is constant 
 for ditl'erent light-intensities, though dependent on the nature of the 
 place on the retina from which the light proceeds. The returning light 
 divides at the reflecting surfaces once more into a reflected and a 
 transmitted portion, only the latter arriving in the observer's eye. 
 The light which is reflected at the retina possesses, as is generally the
 
 16 THE OPHTHALMOSCOPE 
 
 case with diffuse reflected light, uo longer any polarization, conduct- 
 ing itself in this respect, therefore, like the light from the light-source 
 as it strikes upon the mirror. Inasmuch as, in addition, it fails upon 
 the plates under the same angle, proportionately as much of it is 
 reflected and transmitted as of the former [the light from the light- 
 source]. If we designate the transmitted part by X, then we have 
 the proportion 
 
 X : m H = ( J — H) : J. 
 
 From this may be computed the quantity of light X, which passes into 
 the eye of the observer. For H = and H = J ; that is, when no 
 light or all the light is reflected, X will = 0. Between these extreme 
 values of H exists a maximum of the value of X, which can be com- 
 puted according to the known rules of the differential calculus. The 
 maximum occurs when 
 
 H = i/o J. 
 Then will 
 
 X == i/i m J. 
 
 By this condition is also determined for a given number of reflecting 
 plates the angle under which the reflection must occur in order to 
 give to the observer the brightest image. Unfortunately, the equation 
 which expresses the dependence of the value H on the angle of inci- 
 dence a, cannot be solved after a; we can therefore find the proper 
 values of a only approximately by means of computational trials. 
 Besides, it is of no use to drive the exactness of this computation very 
 far, first, because the brightness for the observer is not materially 
 altered, even when the position of the glasses is not that requisite for 
 the maximum, and, secondly, because the alterations in the width of 
 the pupil pi'oduced by different intensities of the incident light can- 
 not be taken into account. 
 
 As the pupil of the observed eye becomes smaller under stronger 
 incident light, the brightness of the retinal image will not increase 
 entirely in the same proportion, when the values of H increase, as 
 they should do according to the developed formula?. It is therefore 
 more advantageous to re-establish in the instrument the values of H 
 as somewhat smaller than would be requisite for the maximum of H 
 in the foregoing computation. One reaches, for example, the value, 
 which slightly deviates from the foregoing maximum, 
 
 X = 1/5 m J 
 
 when the light is reflected from one glass plate at an angle of about 
 70°, from three at an angle of 60°, of four at 55°, and these posi- 
 tions are therefore approximately the most advantageous.
 
 THE OPHTHALMOSCOPE 17 
 
 The necessai-y brightness, therefore, eau eveu be reached with a 
 SLUgle glass plate for a mirror. The use of several plates at a smaller 
 iuciaeiiL-e-aiigle has, however, esseutial advantages if oue would attaiu 
 to distiuet images of the retina. 1' irst of all, glass plates, eveu when 
 they have well ground parallel surfaces, are not always internally of 
 so homogeneous a structure as still to yield, by an oblique view, good, 
 distinct images. Then, it is more dilUcult, by a very oblique view, to 
 give to a reticcting plate the correct position toward the observed eye, 
 and to hold the plate therein. Also, the observer, by the lateral parts 
 of his head, cuts off more easily the rays of light which should fall 
 upon the mirror; especially may this be avoided with difticulty when 
 the angles of incidence are more than 7U°. Finally, it remains to 
 be especially considered that a small quantity of the light which falls 
 into the observed eye is in fact i-eflected from its cornea and appears 
 to the observer as a washed-out light spot in the visual held. This 
 falls over the centre of the pupil, when the observed eye turns 
 straight toward the mirror, therefore when it looks directly at the 
 mirrored image of the flame ; it falls more to oue side when the ob- 
 served eye gazes in any other direction, disturbing, however, the 
 observation of the retina always more or less. It is therefore an 
 essential advantage if one can weaken the corneal reflex for the 
 observer to a considerable degree. Now, in fact, that image appears 
 much weaker when 4 plates reflect at 56°, than when 3 reflect at 60° 
 or one at 70°, while the retinal image, as already mentioned, holds 
 to just about the same illumination. That is to say : the apparent 
 brightness of the corneal reflex is not proportional to that of the 
 retinal image, because the light which falls into the observed eye, and 
 which is partly or wholly polarized by reflection, is depolarized by 
 the diffuse reflection at the retina — something which does not occur 
 from the specular reflection at the cornea. If the cornea, of the 
 (|uantity of liglit A which falls upon it, reflects the portion fxA, then 
 the quantity of light which, in our experiments, passes from the 
 cornea into the eye of the observer, equals, according to the same 
 principles and the same designation as before. 
 
 ^ 
 
 n [J — 2 n] -f /Lt S [ J — 2 2] 
 
 Computation gives the result already stated. It is therefore from 
 every point of view more advantageous to attain the necessary bright- 
 ness by increasing the miniber of the plates, while they reflect the 
 light at the polarization angle of 56°, than by increasing the angle
 
 18 THE OPHTHALMOSCOPE 
 
 of incidence, indeed the corneal reflex could be made to disappear 
 entirely by increasing very much the number of the plates. 
 
 1 have assumed, in the foregoing explanations, that the flame of a 
 good oil-lamp with a double draught is employed as the light-source. 
 When the experiment is properly conducted, the light of such a lamp 
 is not so strongly reflected as very much to dazzle or fatigue the 
 lateral parts of the retina of the observed eye. One can therefore 
 easily continue the experiments as long as one likes. Only when the 
 eye looks directly at the mirrored image of the flame, will this degree 
 of brightness be found not long endurable. If one has at his disposal 
 a more intense light, for example sunlight, which falls into a dark 
 room through an opening in the window-shutter, then one can see the 
 picture of the retina much brighter, if one, after proper weakening 
 of the light, causes it to reflect from a mirroriug-plate as vertically 
 as possible, than when this takes place obliquely. The quantity of 
 light which one may permit to enter into the eye is limited particu- 
 larly by the sensitiveness of the lattei". If, now, one has at his dis- 
 posal excessively strong light, which by every kind of reflection, if 
 it is not at the same time adequately weakened in another way, ex- 
 ceeds this limit, then the observer sees the retinal image, which 
 has reached the limit of endurable intensity, at its brightest when as 
 little as possible is lost at the second reflection. That is, however, 
 the case when the light is thrown back from a plate almost vertically. 
 
 I have not had opportunity to institute such an investigation by 
 means of sunlight; I do not believe, however, that, by that method, 
 any considerable advantages are to be secured, because, in the case 
 of vertical reflection, the apparent brightness of the disturbing cor- 
 neal reflexes increases at a much higher rate than that of the retinal 
 image. 
 
 There was expressed to me a number of times the supposition (at 
 first blush a very plausible one) that, by a convex lens which should 
 concentrate toward the observed eye all the light which falls upon it, 
 the quantity of light falling into the eye and therefore also the bright- 
 ness of the retinal image, could be considerably increased. I will 
 therefore here direct attention to the fact that, in this way, not the 
 brightness but only the size of the retinal image is increased. When 
 we bring the eye to the point of union of the light-rays, which have 
 passed through a lens, then the entire surface of the lens appears to 
 us luminous with that light-intensity which belongs to the luminous 
 point. Instead of the smaller retinal image of the luminous point, 
 there forms itself for us therefore a larger one with the same in- 
 tensity, that of the lens-surface. Moreover, by no complicated ar-
 
 THE OPHTHALMOSCOPE 19 
 
 rangement of lenses can the biMghtness be increased. In order to 
 perceive tliis, we need only to remind ourselves of this fact from 
 the theory of telescopes, that through no telescope or similar ar- 
 rangement of lenses can an object of appreciable diameter appear 
 brighter than with the naked eye. As, now, the inhabitant of the 
 seeing eye subjectively perceives the surface no brighter through the 
 lenses, so can, objectively, the image in his eye by the use of no sort 
 of lenses be brighter than without them. For to an objectively 
 brighter retinal image there must always correspond a stronger sub- 
 jective light-perception. 
 
 2. Production of a Distinct Image of the Retina. 
 
 We now come to investigate how, by means of the light which, 
 returning from the retina of the observed eye, falls into the eye of 
 the observer, we may be able to receive distinct images of the retina 
 itself, and of the picture of the light-source cast upon it. For this 
 purpose let us take again our Fig. 1. According to the explanations 
 just made, the ocular media will so refract the rays returning from 
 points of the retina of the eye D, that they come together outside the 
 eye and indeed in the corresponding points of the image B. The 
 image which the ocular media cast of the retina and of the retinal 
 image of the flame, coincides therefore in size and position with the 
 first reflected image of the flame. An observer who (reckoning out- 
 ward from the mirror) stands on the other side of B and at the 
 distance of distinct vision from B, would therefore in fact be able to 
 see that image of retinal objects distinctly. His visual field, how- 
 ever, limited by the pupil of the observed eye, would, at the com- 
 paratively considerable distance of the two eyes from one another, be 
 so small that it would be impossible to combine the viewed details 
 into a complete picture. 
 
 The regard which we must pay to the enlargement of the visual 
 field, makes it much more necessary to approximate the two eyes as 
 closely to each other as possil)le. Then, however, the image B falls 
 in general behind the back of the observer, and can not be plainly 
 seen by him. If, for example, in Fig. 1, the observing eye is at G, 
 then it receives the light rays which proceed out of the eye D and 
 which come together at the points of B. Now a normal eye can in- 
 deed unite upon its retina parallel rays, as these move from infinity, 
 and divergent, as these come from nearer points, but not convergent 
 rays. The simplest way to assist in this matter, and to make the 
 convergent bundles of rays divergent, is a concave lens, which is
 
 20 THE OPHTHALMOSCOPE 
 
 inserted between the mirror and the eye of the beholder, as in Fig. 
 1 at F. 
 
 According to the known laws of refraction in concave lenses, the 
 convergent rays which strike upon F will, after their exit from the 
 lens, either be less convergent (when, that is to say, the focal distance 
 is greater than FB) or they become parallel (when the focal distance 
 equals FB) or, finally, divergent, as if they came from points of an 
 image E behiud the obsei-ved eye (when the focal distance is smaller 
 than BF). In the latter ease the concave lens acts precisely as it does 
 in opera glasses, where it likewise converts the inverted imperfect 
 image, which the objective lens should cast at its focus, and which 
 lies on the side of the observer, into one which stands upright and 
 which appears to the observer to be on the other side of the glasses. 
 In our case, likewise, the ocular media form the objective glass of a 
 microscope, which is constructed on the principle of a Gallileonian 
 telescope, while the concave lens represents the ocular. 
 
 If the accommodation distance of the two eyes DB and GE are 
 given, and in addition the mutual distances of the eyes and the con- 
 cave lens are settled according to the principles above set forth, that 
 is, made as small as the mirror permits, then is the focal distance 
 which is to be given to the concave lens to be determined according 
 to the known laws of refraction in lenses. This is found to equal 
 
 EF BF 
 
 EB 
 or: 
 
 (EG — GF) (BD — DF) 
 
 EG + BD — DG 
 
 The greater are the accommodation distances EG and BD, the 
 greater must also be the focal distance of F. The observer will, 
 therefore, if one of the two eyes is short-sighted, employ stronger con- 
 cave lenses, but, if one eye is far-sighted, weaker ones, than for two 
 normal eyes. When the observing and the observed eye exchange 
 their roles, without altering the condition of their accommodation, 
 there will generally become necessary a glass of a different focal dis- 
 tance, and, indeed, as GF < DF, a weaker one, when the more short- 
 sighted eye observes, than when it is observed. Still, a closer con- 
 sideration of the foregoing formula shows that this difference is 
 extremely slight in the case of not too short-sighted eyes, so that, in 
 the case of such, the same glass can serve for mutual observation.
 
 THE OPHTHALMOSCOPE 21 
 
 The magnification is determined according to the known laws of 
 optics in this way, that the image E, viewed from the center of the 
 lens F, must appear under the same visual angle as B, its imaginary 
 object. Since the eye G, the lens F and the eye D stand as closely 
 together as possible, then will B appear from F only a little larger 
 than from D. The eye G therefore sees the retinal image of the flame 
 magnified, and indeed just as large, or, considered exactly, a trifle 
 larger, than the eye D sees the original flame. The parts of the 
 retina on which the image of the flame falls, appear likewise in the 
 image E again, magnified of course in the same proportion as that. 
 
 According to what has just been said, the proportion of this en- 
 largement is equal to that of the retinal image to its object. Let us 
 take as the distance of the decussation-point of the refracted rays 
 from the retina, according to Volkmann's measurings, 4 lines, for the 
 distance of the object from the eye the normal visual distance of 8 
 inches, then the magnification is found to be 24 times. 
 
 We have compared the ocular media in our experiment with the 
 objective of a microscope, the concave glass with the ocular. Now, in 
 place of the latter, one should be able to produce a combination of 
 two convex lenses, which stand at a distance from one another of 
 less than the sum of their focal distances, as is the ease in the ordi- 
 nary compound microscope. The first of the lenses would, like the 
 collective glass of this instrument, unite the weakly converging light- 
 rays which proceed from the observed eye, more promptly to an 
 image, which, situated between the lens and its focal distance, would 
 exhibit the flame-image upright, the retina inverted. This image 
 could then be seen magnified by the second convex lens. I have 
 debated the results of such a combination, according to the known 
 laws of optical instruments, with respect to magnification, illumina- 
 tion, visual field, etc. As the computation showed that in this way 
 no essential advantages were to be secured, as compared with the 
 simple concave lenses, it will here suffice to adduce those results very 
 briefly. It is hereby presupposed that the first lens, so far as the 
 mirror permits, is approximated to the observed eye, and that the 
 observing eye lies close to the second lens. 
 
 First of all, as to the illumination, the maximum thereof is directly 
 attained by a concave lens for the middle of the visual field. If the 
 same thing is to occur by two convex lenses, then these must be so 
 chosen and arranged that no other enlargement takes place than by 
 the concave lens, that is, in such a way that the magnified retinal 
 image of the flame appears to the observing eye under the same
 
 22 THE OPHTHALMOSCOPE 
 
 visual angle as the mirrored image of the flame does to the eye that 
 is being observed. 
 
 If this enlargement is to occur, the image from the first lens must 
 fall, as in the ordinai-y ocular tubes of the compound microscope, in 
 the middle between both lenses. In the ease of a weaker magnifica- 
 tion, it is possible to cause a larger portion of the visual field to 
 appear in the maximum of brightness; in the case of stronger, on 
 the contrary, that can no longer occur even in the middle. As ad- 
 vantageous, therefore, as even a stronger magnification might be, still 
 such a one is not practicable, because the illumination would thereby 
 suffer too much, and a living eye would not well endure for a longer 
 time without dazzling the incidence of still stronger light than that 
 reflected from a good lamp. Then, too, is the fact that the living e_ve 
 cannot be thoroughly fastened, as would be necessary for the fixation 
 of individual parts of the image in the case of stronger magnification. 
 Next to be considered is the visual field. The part of the retina 
 which one can survey is always the smaller the farther one removes 
 oneself from the observed eye; the larger the nearer one comes to it. 
 The limit of approximation is, however, set in this way : that the 
 obliquely placed mirror-plates have to be inserted between the eye 
 and the glass-lenses. 
 
 In order to compare by means of computation the effects of various 
 lenses, we must therefore accept as equallj- great the distance of the 
 concave glass and that of the first convex glass from the observed 
 eye. If then at the same time the condition is observed, that the 
 brightness in the middle of the visual field should reach its maximum, 
 then are found definite focal distances of the convex lenses for every 
 given distance from the eye, which make the visual field at its largest. 
 If one choose the focal distances of both the convex lenses in accord- 
 ance with these determinations, then it further appears that when 
 the distance of the lens from the ej'e is smaller than the focal distance 
 which one may give to the objective of a telescope from the aperture 
 of the pupil without prejudicing the distinctness of the image, there- 
 fore in the case of achromatic lenses smaller than perhaps the ten- 
 fold pupillary diameter, the concave lens, if larger, the convex lenses 
 can give a larger visual field.* Now, in the case of the closest possible 
 approximation of the lenses to the obsei*\'ed eye, the distance between 
 both will of course, on account of the mirror being placed in the 
 
 * The senteripe, in the original, is hopelessly obscure; it is, therefore, also 
 obscure in the translation. The reader should recall the fact that Helmhnltz. at 
 the time when he wrote the " Beschreibwrg," was not yet master of a literary 
 stvic, and T liave deemed it far the fairer way not to force into the sentence a 
 meaning of my own. — (T. H. S.)
 
 THE OPHTHALMOSCOPE 23 
 
 interval between the lenses, remain in general somewhat larger than 
 the tenfold pupillary diametei", and one would therefore be able to 
 secure by means of two convex lenses a slight advantage for the visual 
 field. Inasmuch, however, as the lenses, in order to yield this ad- 
 vantage, must have focal distances of 36 to -10 lines, it may become 
 very diHicult to receive an image of the same distinctness as by a 
 concave lens which may have a focal distance of 8 to 10 inches. I, 
 at least, have not been successful in this matter, by the combination 
 of such convex lenses as stood at my disposal. Moreover, it transpii-ed, 
 in the experiments with such lenses, that the correct location of the 
 instrument for the perception of the retinal image is both found 
 and kept with much greater difficulty. With a simple concave lens 
 it is, to wit, not necessary that the axis of the lens be directed exactly 
 upon the observed eye, if only the mirror casts light into it. This con- 
 dition, however, must be observed in the case of two convex lenses. 
 
 Consec]ueutly it appears to be more advantageous to retain the 
 simple concave lens as ocular, while one almost everywhere else in 
 optics replaces it to decided advantage by convex lenses. A decided 
 advantage of the latter occurs, to be sure, even in our case, which 
 would make their emploj'ment desirable, to wit, the advantage that, 
 by an altered distance of the lenses from each other, one can adjust 
 the apparatus to all visual distances of the observed and the observ- 
 ing eye, while, for this purpose, one must exchange the concave lens 
 for another. If one could completely make fast the head of the 
 observed person and tlie instrument, convex lenses would in conse- 
 quence be more convenient ; without such arrangements, however, all 
 their other advantages are outweighed by the disadvantage of the 
 difficult placing of the instrument. I have therefore myself always 
 employed only a simple concave lens. 
 
 3. Description of the Opiitii.vlmoscope. 
 
 In order to institute observations of the kind described, it is con- 
 venient to unite tlie mirror-plates and the concave lens by means of 
 a suitable frame. I propose for such a combination the name Atigen- 
 spiegel, by analogy with similar instruments. The instrument is 
 viewed in Fig. 2 from in front, in Fig. 3 exhibited in horizontal 
 cross-section. The reflecting plates hh are fastened, by means of the 
 brass piece gg, to the circular plate aa, at an angle which is equal to 
 tiie chosen angle of incidence of the light rays — in the figure, 56°. 
 The brass piece gg forms with the glass plates a hollow, right-angu- 
 larly triangular prism. In Fig. 3 one sees into the inner cavity there-
 
 24 THE OPHTHALMOSCOPE 
 
 of, and lias before him one of the riglit-angularly triangular basal sur- 
 faces. Of the three quadi'augular lateral surfaces of the prism, that 
 which corresponds to the hypotheuuse of the basal surtace, is formed 
 by the glass plates, that which corresponds to the longer cathetus 
 stands free, that corresponding to the shorter cathetus lies on the disc 
 aa, and carries a cylindrical process p, which, by means of a corre- 
 sponding circular opening in the plate aa, so clasps through, that it 
 holds the prism fast against the plate, but permits a turuiug on its 
 axis. The glass plates are held against the prismatic brass piece by 
 the frames kkkk, wliose over-reaching lateral edges are secured to the 
 brass piece gg by the screws 11. The disc aa rests on the cylinder bbec 
 without being permanently fastened to it. In the border of aa, namely, 
 there are cut four openings of the form f, to which openings there 
 correspond four screws ee with cylindrical heads and thin necks, 
 inserted into the border of the cylindrical ring bb. in Fig. 2 are 
 shown only two of these screws, in order to let the holes f be seen. 
 The heads of the screws allow of their shoving through the broad 
 circular portions of the openings, and if then the disc aa is turned 
 about its center, the necks of the screws pass into the smaller, slit- 
 shaped part of the same opening, while their heads lap over and 
 fasten the disc to the ring bb. In that way it is possible to remove 
 the disc very easily and quickly from the setting of the concave lens, 
 and to exchange the lens for another. The concave lens nu lies be- 
 tween the plate aa and the floor of the cylindrical piece dd, which is 
 screwed into bbcc and can be set back by screwing round, when it 
 becomes necessary to lay two lenses one upon the other for very short- 
 sighted eyes. The whole is fastened to the handle m. For a normal- 
 eyed observer, the numbers 6 to 12 of the ordinary concave spectacle 
 lenses, are suiScient for the adjustment to all adaptational conditions 
 of the eyes to be investigated. For the viewing of other normal eyes, 
 I generally employed Nr. 10. For very short-sighted eyes, two lenses 
 should be superimposed. 
 
 As to the reflecting plates, those of ordinary mirror-glass are not 
 appropriate, because their two surfaces are as a rule not sufficient!}' 
 parallel to cause the images which they cast of the lamp-flame to 
 coincide in the way that they should. The glasses must therefore for 
 our use be especially ground, in order to receive parallel surfaces, 
 though this condition need not be fulfilled with such exactness as in 
 the case of the plane-parallel glasses which one employs in the finer 
 measuring instruments. 
 
 A good blackening of the non-reflecting surfaces is essential. Since, 
 of the bright light which falls upon the instrument, only a propor-
 
 THE OPHTHALMOSCOPE 25 
 
 tionately small part returns from the retina of the observed eye, all 
 the remaiuiug remnants of the light, which might perhaps get into 
 the eye of the observer, must be done away with. First of all, the 
 inner surface of the ocular piece dd must be blackened, and the ob- 
 server must place his eye as closely iuto it as possible, iu order to cut 
 off all the light which could fall from the flame upon this surface. 
 Secondly, the outer surface of the disc aa and of the prismatic mirror- 
 frame kkkk must be blackened, iu order that blank metal surfaces, 
 which are turned toward the observed eye, may not produce disturb- 
 ing corneal reflexes. Thirdly, however, the inner surface of the 
 mirror-frame is to be blackened with especial care. The light of the 
 flame which falls on the reflecting plate, passes in greater part through, 
 and strikes the plate gg. All that is not here absorbed, returns to the 
 mirror, is reflected from this in the same direction to the observing 
 eye, in which the weak light from the retina of the observed eye 
 arrives, and mingles with the image of this membrane. I have found, 
 in this matter, the general methods of procedure of mechanics for 
 blackening brass-pieces to be inadequate, and the framework of the 
 mirror must be tapestried internally with black velvet, which ab- 
 sorbs the light more completely.* 
 
 *Tho subsequent history of the ophthalmoscope "down to a time within the 
 memory of men still living," is, very lirielly, as follows; Ruete, in 1852, invented 
 the "indirect method" (D. Av{ienspie<jcl u. d. Optometer, Giittingen, 1852). He 
 employed a concave perforated mirror, held at a considerable distance from the 
 ol served eye, and letwcen the mirror and the eye one (sometimes two) spherical 
 convex lenses. Ilelmholtz, also, had made use of convex lenses, but these he had 
 placed behind the mirror, finding them there, of course, of very little value (see 
 herein). 
 
 Helmholtz, next, explained most thoroughly (Vierordt's Archiv, 1852, p. 827) 
 the method which Ruete had invented. Tn the very same paper Helmholtz described 
 what he called "the simplest method," by which an eye could be examined by 
 means of only a candle, a screen and a convex spherical lens. He also mentioned 
 (still in the same most momoraljle article) the so-called Rekoss discs — i. e., two 
 rotatory discs, each containing four concave lenses inserted not far from the 
 perijiheries of the discs. One of the discs held lenses from fi in. to 9 in. focus, 
 the other those from 10 to 13 in. The Rekoss disc, or discs, with numerous modi- 
 fications, is, as all are aware, in use at the present day. Rekoss was not an oidithal- 
 mologist, but an instrument maker of Konigsl.erg (where Helmholtz at the time 
 was living). 
 
 Coccius, in 185.3, invented an instrument which found much favor for years. 
 It consisteil of a lens, set in a frame, in front of a plane mirror. The distance 
 between the mirror anii the lens could bo altered very considerably. 
 
 Kduard Jaeger, in 1854, produced a combination of the Helmlioltz and the 
 Ruete instrument — that is to say, the plates of silvered glass in the Helmholtz 
 instrument were made replaceable by a concave silvered mirror, such mirror to be 
 u^ed for the indirect method. To this affair of Jaeger's, Strawbridge, of Phila- 
 delphia, in 1871, added three interchangeable Rekoss discs. 
 
 The ophthalmoscope of I.iebreich is too familiar to all to require the slightest 
 description. So, almost, is that of Loring, with its single disc and double row of 
 lenses, the disc being movable up and down for the jnirposo of bringing into 
 action either the one row or the other. Wadsworth, of Boston, invented the
 
 26 THE OPHTHALMOSCOPE 
 
 When one desires to use the instrument, he sets the person to be 
 examined in a dark room and next the corner of a table on which, at a 
 level with the eye and sidewise from the face, stands a well-burning, 
 double-draught lamp. It is convenient to set upon the table, at a 
 fitting visual distance, some not too bright object, whereon one can 
 point out to the observed eye certain points for lixation, for example 
 a blackboard divided into squares, each of which is designated by a 
 number, while one causes the eye to fix various points one after an- 
 other. The image of the flame falls ever on different parts of the 
 retina, which the observer, therefore, may investigate one after an- 
 other in any order desired. Between the flame and the observed eye 
 an opaque screen must be erected, in order to shade the eye, so that 
 directly incident flame-light may not produce a very disturbing cor- 
 neal reflex and a narrowing of the pupil. Still, the border of the 
 shadow must pass very close before the observed eye, in order that 
 the ophthalmoscope, which must itself remain in the light, may be 
 carried toward that eye as closely as possible. The observer seats 
 himself before the observed, brings the ophthalmoscope, without at 
 first looking through it, into about the right position, when its reflect- 
 ing surface casts a bright light upon the face. When one has so turned 
 the mirror that the middle of its light falls upon the eye, and the 
 axis of the instrument is directed precisely into it, one looks through. 
 A person then has, as a rule, at once before him the bright image of 
 the flame, or finds it after more or less moving about. Moreover, one 
 can also, looking through the instrument, discern, to a certain extent, 
 the eye and the clear light which must fall upon it, even if indis- 
 tinctly and as if they were faded, and also, in that manner, with the 
 help of these [the eye and the light upon it] discover the correct posi- 
 tion. If, though the pupil appears luminous, one cannot see the various 
 parts of the retina distinctly, then one must insert another concave 
 lens. An observer who has accustomed himself to alter at will the 
 adaptation of his eye, easily discovers whether he sees more plainly 
 by a far-sighted or a near-sighted adaptation, and whether, accord- 
 ingly, he must choose more or less strongly curved lenses. Moreover, 
 many persons make the matter difficult, especially those who are not 
 accustomed to looking through optical instruments, and short-sighted 
 persons who see through them with difficulty, insomuch as they invol- 
 untarily adapt the eye for great nearness, because they think of the 
 
 "mirror obliniiolv set," which enables the nbserrer today to look straight through 
 the lenses instead of at an an<Tle. Both the Loring and the Wadsworth instru- 
 ments are especially valuable for refraction purposes. 
 
 The electric light ophthalmoscopes are in the hands of every practising oph- 
 thalmologist at the present day, and require no description.
 
 THE OPHTHALMOSCOPE 27 
 
 object to be seen as beiug very close. In that way the eyes of the ob- 
 server are greatly fatigued, and readily begin to be injected and to 
 water. It is necessary here, as in the case of all optical instruments 
 possessing an alterable adaptation, to adjust the eye for the distance, 
 and then to adjust the instrument to the eye. 
 
 After a little practice it is not difficult to find the right lens and 
 the correct position of the instrument. Also, one can easily, on his 
 own eye, show these matters to anyone who has never yet seen them, in 
 order first to render him familiar with the appearance of what he is 
 going to look for. In that way it will be made much easier for him 
 to discover independently the very same things in the eyes of others. 
 Let the instructor, for this purpose, first of all discover the particular 
 lens through wliich he can see the student's retina plainly, and then 
 let him place this in the ophthalmoscope; then through the same glass 
 the student can see distinctly into the eye of the teacher, if neither of 
 the two is very short-sighted. In the latter case (as explained al- 
 ready) the more short-sighted person needs a somewhat weaker glass 
 when he observes than when he is observed. Let the instructor, then, 
 bring one of his own eyes into the position which has been described 
 as that for the eye to be observed, and let him so hold the ophthal- 
 moscope before him that he may be able, at the same time, to look 
 through its central openings and glimpse the mirrored image of the 
 flame in the mirror, hand over to the student the instrument in this 
 position, and let him look through it. The student will then see in 
 the eye the image of the flame. In order to teach him to recognize 
 the appearance of the parts of the retina, let the teacher throw the 
 image of the flame on the place of entrance of the optic nerve, beeaiise 
 in that place the largest and most recognizable vascular trunks exhibit 
 themselves. Let him, for this purpose, turn the eye gradually more 
 and more to the inner side of the mirrored image of the flame, until this 
 suddenly becomes smaller to him, or disappears. That happens, as 
 is known, when the image falls upon the place of entrance of the optic 
 nerve. Besides, most persons more easily succeed in seeing and recog- 
 nizing the imacre of the flame than the tiny parts of the retina in the 
 bright ground thereof. 
 
 4. Viewing the Retina and the Image op the Flame. 
 
 Should one desire to investigate the retina completely, then it is 
 convenient, as already mentioned, to set up a blackboard covered 
 with numbers as a visual [loint for the eye to be investigated. As 
 soon as this eye fixes one of the numbers, looking past the mirror a
 
 28 THE OPHTHALMOSCOPE 
 
 little to the inward side thereof, the observer will almost always recog- 
 nize in the visual held one or two of the larger vessels. He causes the 
 eye to turn to one of the near-lying figures, and netices whether he is 
 brought nearer to the origin or to the branching of the vessels. While, 
 in this way, he traces the vessels in the direction of their larger trunks, 
 he comes at length to the place of entrance of the optic nerve. This 
 distinguishes itself from the rest of the eye-ground by its white color, 
 for it is not covered with pigment and a hne vascular network, but 
 here the white cross-section of the nerve lies wholly free, at the very 
 most shot through by tiny, isolated vessels. Mostly to the inner 
 side, near by, the arteries and veins of the retina press forward from 
 the depths. At times one sees a portion of the vessel still hiding in 
 the substance of the nerve, and understands that, in the living, this 
 substance is decidedly transparent. One distinguishes the two kinds 
 of vessels from each other by the brighter color of the blood and the 
 double contours of the walls in the arteries and in their first ramifica- 
 tions. I have not been able to recognize pulsations with certainty. 
 The first main branches of the vessels border the optic nerve at its 
 inner side, in order to spread out later, above and below, across the 
 retinal field. The appearance of the sharply pencilled red vessels on 
 the clear white ground is of surprising elegance. Somewhat farther 
 to the inner side, close by the nerve, I have always remarked a small, 
 sickle-shaped stripe of shadow, which appears to take its origin from 
 a fold of the retina. 
 
 In the other parts, the ground of the eye looks reddish, and in- 
 deed first of all round about the optic nerve of a somewhat 
 clear, light-red, the darker, on the contrary, the farther you 
 pass from that place. One sees here larger and smaller branch- 
 ing blood-red vessels, which stand out plainly from the back-ground. 
 The ground itself appears to be not entirely homogeneous, but indis- 
 tinctly reddish. This would seem to arise from the fact that the close 
 capillary net is too fine, too weakly illuminated and too transparent 
 to be distinguished plainly from the underlying weakly, light-gray 
 substance of the retina. That the ground looks brighter in the vicin- 
 ity of the optic nerve is no doubt owing to the fact that the retina 
 here, on account of the superimposed fibres of the optic nerve, is 
 thicker, while, toward its periphery, it becomes continually thinner. 
 ]\Ioreover, the place of direct vision (the yellow spot) is essentially 
 distinguished in appearance from the parts which lie immediately 
 aboiit it. In order to got this point before oneself, one causes the 
 eye which is being observed to look directly at the mirrored image of 
 the flame. The retina then appears much darker, grayish-yellow 
 
 I 
 i
 
 THE OPHTHALMOSCOPE 29 
 
 without intermixture of I'cd; and one sees no traces of capillary ves- 
 sels. Then, too, one is greatly annoyed while gazing on the yellow 
 spot, by the tiny image from the cornea, which obtrudes itself pre- 
 cisely in the center of the visual field, while, during the observation 
 of the lateral portions of the retina, it lies to one side. 
 
 After deciding what, in the healthy eye, can be made out concern- 
 ing the nature of the retina, I have no doubt that one will be able 
 to recognize all such disease conditions as permit of recognition by 
 the sense of sight in other transparent parts — for example, the cornea. 
 Increased repletion of the vessels and vascular varicosities must prove 
 easy to make out. Exudates into the substance of the retina, or be- 
 tween that structure and the pigment membrane, must yield them- 
 selves to observation, very much as affections of the cornea do, by 
 their brightness against a dark ground. If they lie in part before 
 the retina, they will then enclose its vessels in a veil. I here recall 
 that, according to Briicke, the recent retina is just about as trans- 
 parent as the other ocular media, and that, apart from its vessels, it 
 is only visible in our experiments because it is strongly illuminated 
 on the deep-black ground of the pigment membrane. Fibrinous 
 exudates, which are nearly always less transparent than the ocular 
 media, must also for that reason, when they lie in the fundus of the 
 eye, considerably strengthen the reflex. Then too I believe thnt opaci- 
 ties of the vitreous body will be much more easily and certainly recog- 
 nizable, partly by the illumination of a reflecting glass-plate, partly 
 by the ophthalmoscope. One will even be able to determine with ease, 
 from the indistinctness of the image of the flame and of the retinal 
 vessels, the degree of the opacity. If, in the case of such an opacity, 
 scintillating particles have detached themselves, then too a person will 
 be able to take note of these. In brief, I believe that I may hold the 
 expectation not to be exaggerated, that all the alterations of the 
 vitreous body and of the retina which, until now, have been found in 
 cadavers, will also permit of recognition in the living eye — a possi- 
 bility which appears to promise the most remarkable advances for the 
 hitherto undeveloped pathology of this structure.* 
 
 Finally, it is of interest, for certain physiological purposes, to in- 
 vestigate the accuracy with which the eye forms images. It is best to 
 employ for this purpose a thread, which one draws along horizon- 
 
 •PrnbaMy the most sicmifioant sentence ever ■pennP'I hv an orhtlislmolo'T'st. 
 How crIo''i''iislv the tre^t man's nrorhecv has been fnlflllpfl is Vnnwn nnt merely 
 to B^ef'ialists and cenera] prartitionerp, but even, in ponip rle'rree. to first venr 
 ine'1i''.il st"'1eTit» nnd the efli'i'aterl portion of the laitv. In fnft. therp are i'lst 
 twn kinrlq of ophthnlmoloary, that whii'h came before ari'l thit which followed after 
 Helmholtz's " Beschreibung eines Augenspiegds." — (T. H. S.)
 
 30 THE OPHTHALMOSCOPE 
 
 tally before the flame. Its image remains single, while vertical threads 
 are manifolded by the manifold reflections. 
 
 First of all one gets an opportunity to convince oneself, by the 
 appearance of the image, that the difllerent adaptations of the eye 
 really depend upon alterations in the refractive media. One should 
 cause to be fixed an object which is just about as far removed from the 
 observed eye as the thread is from the flame. The observer then sees 
 the elements of the retina and the image of the thread distinctly at the 
 same time. Should the thread be carried nearer to or farther from 
 the eye, then it becomes indistinct in the retinal image, or entirely 
 disappears, while the parts of the retina remain sharp. One perceives 
 from this that the retinal images of objects which stand at various 
 distances from the eye, are in fact not equally distinct. Then again, 
 one should so place the thread that it appears distinct in the retinal 
 image at the same time with the vessels, and should cause the observed 
 eye to fix a point which is either much farther or much nearer than 
 that upon which it was formerly directed. Immediately one sees 
 the retina and the image of the flame become gradually indistinct. 
 
 It should incidentally be remarked that, on the white surface of 
 the optic nerve no image is cast, even when the image appears abso- 
 lutely sharp on the immediately surrounding portions of the retina. 
 Inasmuch as the observer, in the case of a person over whose optic 
 nerve cross-section little vessels run, sees these quite as plainly as 
 those of the retina adjacent, therefore that indistinctness of the image 
 of the flame cannot proceed from the passage of the end of the optic 
 nerve out of the level of the retina. I believe rather that one must 
 regard the transparent condition of the optic nerve mass as the real 
 cause. 
 
 Moreover, one is able, whenever it becomes necessary, to convince 
 oneself readily in an objective manner of the presence and the degree 
 of the short- or far-sightedness of the observed eye. Let the observer 
 first investigate a normal eye, which he causes to fix objects at vari- 
 ous distances, and notice what concave lenses he is obliged to use in 
 the various stages of adaptation of the eye. In the investigation of 
 any other eye, he then learns from the number of the concave glass 
 through which he saw the retina distinctly the corresponding adapta- 
 tional distance of the observed eye. The observer is, by this method, 
 entirely independent of the assertions of the other person, for he 
 himself sees, as it were with that other's eye, at least by means of its 
 refractive media. In this way, for example, I was able to convince 
 myself in a completely amaurotic eye, that that eye was simultane- 
 ously in a high degree short-sighted. In that way was decided in 
 
 i
 
 THE OPHTHALMOSCOPE 31 
 
 this case a question of great importance for the anamnesis, whether, 
 that is, certain earlier dituculties of sight recounted by the patient, 
 should be referred to shortsightedness or to comiiiencing amblyopia. 
 
 An important physiological conclusion thrust itself upon me in 
 these investigations, 'the iree-lying cross-section of the optic nerve is 
 apparently so transparent that the light which falls upon it must 
 penetrate deeply into the mass of the nbres, inasmuch as, now and 
 then, one sees the bendings of the central artery and vein shimmering 
 forward through the substance of the nerve. When the little image of 
 the Mame falls on the place of entrance of the nerve, then all its nbres, 
 or at least a very large part of them, are struck by more or less in- 
 tense light, and yet, obviously, they perceive no light. If they did 
 perceive it, then that entire portion of the visual field which cor- 
 responds to them would have to appear illuminated. Not only, how- 
 ever, is that not the case, but there is even less light perceived than 
 when the image falls upon some other portion of the retina. We must 
 from this conclude that the fibres of the optic nerve are incapable 
 of being affected by objective light (ethereal vibrations), while, never- 
 theless, they perceive every other kind of irritation as subjective light. 
 This is a paradox, which, of course, has its ground in the ambiguity 
 of the word "light," and is far removed from being an actual con- 
 tradiction. The vibrations of the ether which we call light, produce, 
 like every other mechanical or electrical irritation, when they strike 
 the retina, the sensation which we call light. But from this, that the 
 retina, protected from pressure and electrical currents and exposed to 
 the action of ethereal vibrations, is much ofteuer struck and excited 
 by the former than by the latter, it by no means follows that light 
 must be regarded as an especially adequate irritant for the retina 
 and the elements of the optic nerve and as standing in contrast to all 
 the other kinds of irritation. There are no difSculties in supposing 
 that all the irritations which are able to affect the optic nerve system 
 produce sensations of light, that, however, the ethereal vibrations are 
 able to act only on the retina. A similar state of affairs is found in the 
 case of the nerves of touch, with respect to heat and cold. Here too 
 the peripheral expansions behave differently from the trunks. For 
 the latter, slight variations in temperature are no irritant at all, as 
 it appears, and the greater variations, which are able to irritate, pro- 
 duce no temperatural sensations. Besides, one is able to conclude 
 still further that, in the retina, not the fibres, which spread out in a 
 radiating manner from the optic nerve, but the spherical elements, 
 are sensitive to light. "Were it the former, then must light which 
 strikes on any place in the retina be perceived by all those fibres which
 
 32 THE OPHTHALMOSCOPE 
 
 in part end in this place, and in part pass across it on their way to- 
 ward the retinal periphery. There would therefore extend, in the 
 visual field, from every illuminated point, a bright shine toward the 
 borders of the field, which is not the case. We maj' accordingly fur- 
 ther conclude that even the continuations of the optic nerve fibres in 
 the retina are insensitive to light. There remain only the ganglionic 
 bodies and the nuclear-like structures of the retina, in which the 
 ethereal oscillations are able to act as an irritant. 
 
 Appendix. 
 
 Derivation of the formula for the quantitij of light which is re- 
 flected from several glass plates. 
 
 Whether this formula is correct for n reflecting surfaces is shown 
 by the fact that it is also correct for (n + 1). As it also proves right 
 for n = 1 and n = 2, it must do the same for any desired value of n. 
 
 Let the quantity of light which at the given angle of incidence is 
 thrown back by a reflecting surface, when the quantity 1 passes off of 
 light polarized vertically against the plane of incidence, be p, that 
 thrown back by n such surfaces P,„), that thrown back by (n-f-1), 
 P(nti). It is demonstrable that if 
 
 np 
 
 P.n,= 1) 
 
 l+(n-lp) 
 then also that equation is correct which arises from this by the sub- 
 stitution of n -|- 1 for n : 
 
 (n + l)p 
 
 1 +np 
 
 For the sake of a better designation, let us assume that the system 
 of n reflecting plates lies horizontal and that light falls on it from 
 above. Let the (n -|- 1) th surface be added to the system below. The 
 quantity of light which passes downward from the lowermost nth 
 surface of the compound system to the (n + 1) th surface let us call 
 X; that which, reflected from the (n + 1) th surface, mounts to the 
 system of the n surfaces, y. The quantity x is composed partly of the 
 portion of the incident light which has passed through the system 
 of n surfaces, partly of the portion of y which is reflected from this 
 system. Therefore is 
 
 x = l-P,„,+yP,„, 3)
 
 THE OPHTHALMOSCOPE 33 
 
 The quantitj' y originates from that pai"t of x which is reflected from 
 the (u + l)th surface. It is therefore 
 
 y = xp 4) 
 
 Tlie quantity P(„+i) which passes upward from the uppermost sur- 
 face, proceeds in part from that portion of the incident light which 
 is reflected from the system of n surfaces, partly' from that portion 
 of y which passes through this system. It is therefore 
 
 P(n,l, = P,n, + y (1— P(n,) 5) 
 
 If one eliminates x and y from equations 3, 4 and 5, one gets 
 
 P[l-P(n,]- 
 
 P(Dti) = "(n) H 6) 
 
 1 — PP(0> 
 
 If we place in this equation 6 the value of P(n, from equation 1, we 
 get in fact, after the necessary reductions, equation 2, whose cor- 
 rectness was to be proved. 
 For one reflecting surface is 
 
 P<i)=P 
 Equation one (to be tested) gives the same value. 
 
 For two reflecting surfaces we get the value P,,, without employ- 
 ing equations 1 or 2, if, in the derivation of equation 6, we suppose 
 that n = l and (n)P^p. Equation 6 then becomes 
 
 P(l-P)- 
 
 P<=)=P + 
 
 l-p2 
 
 2p 
 
 1 + P 
 Equation 1 gives the same value. 
 
 As the latter accordingly is correct for n ^ 1 and for n = 2, then 
 it follows from the proof adduced, that it is correct also for n = 3, and 
 if it is correct for n ^ 3, that it also is correct for n = 4, and so on 
 to infinit}'. 
 
 In a precisely similar way the matter proceeds in the case of light 
 polarized parallel to the surface of incidence. 
 
 If we assume the quaiititv of incident light to be equal to Y-t J, and 
 2P, 
 
 that p = and designate that which we have here called P with 
 
 J 
 II, we get the formula in question.
 
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