348 



THEORY OF VISION. 



[1855. 



To SCO, therefore, into the deeper chambers of a living eye, we 

 must arrange matters so that we can look along the straight 

 line of the reflected rays, without intercepting the light from 

 which they originally came. 



" The experiment is thus made : — ' In a dark room, with a 

 single flame at the side of the experimenters, and on a level 

 with their eyes, the person whose eye is to be observed holds a 

 piece of glass (a microscope glass slip), so as to catch the image 

 of the flame on it; he then, by inclining the glass, brings the 

 image of the flame opposite the pupil of the observer's eye ; 

 the latter will then see the pupil of the observed eye luminous, 



of a reddish-yellow bright colour A person may also 



see one of his own pupils luminous : standing before a looking- 

 glass, and seeing the image of the flame in the reflector with 

 his right eye, let him bring this image opposite the pupil of 

 the left eye in the looking-glass ; the left eye will then per- 

 ceive the right pupil in the mirror luminous.' " 



Various German physiologists have invented instruments 

 called ophthalmoscopes, for the purpose of viewing the reflected 

 light from the retina. It appears, however, that the choroid 

 as well as the retina is a reflector of light, and thus deprives 

 the eye of the character of a camera ol3scura. Yet, notwith- 

 standing these facts, which cannot fail to have been for some 

 time noticed by physiologists. Dr. Wilson is not aware that any 

 material alteration in the current theory of vision has been pro- 

 posed by any writer. With respect to Albinos he observes : — ■ 



" It cannot then, I think, be questioned, that in those ani- 

 mals which exhibit the full development of long hereditary 

 albinism, the sensitiveness of the retina to light has undergone 

 a permanent abatement, whilst the iris has probably altered 

 also in thickness and contractility. I venture to predict, that 

 if ever an albino race of men shall be observed or developed, 

 they will prove, after the lapse of a generation or two from 

 their founders, to have eyes as serviceable as those of the ma- 

 jority of mankind. 



" It is sufficient for my present purpose to point to the albino 

 animals, whose eyes are totally destitute of pigment, and reflect 

 light from every point of the surface, both of the retina and 

 the choroid, but, nevertheless, exercise the faculty of sight in 

 perfection. Their eyes, even when the iris is fully contracted, 

 remain, in virtue of the transparency of that membrane, 

 camerx lucidie; their possessors cannot render them camerx 

 ohscuras ; -and yet they are excellent organs of vision. 



" If the reasoning pursued in reference to the albino eye be 

 valid, it will serve also to dispose of the diffieirlty experienced 

 by some in explaining how vision is compatible with the pre- 

 sence of a tapetiim lucidum in the eyes of many animals. 

 This tapetum is equivalent to a concave mirror of polished 

 metal, replacing the pigment of the choroid over a greater or 

 smaller part of its surface, especially at the deepest or most 

 posterior portion of the chamber of the eye, so that lying be- 

 hind the retina, it is more or less directly opposite the pupil, 

 and receives the light which enters by it. A brilliant reflect- 

 ing surface of this kind is found in many of the mammalia, 

 both graminivorous and carnivorous, as the horse, the ox, the 

 sheep, the cat, the dog. It is present in the eyes of the whale, 

 seal, and other marine mammalia ; and in fishes, such as the 

 shark, in which it is peculiarly brilliant. It occurs also in 

 certain of the mollusca, as the cuttle-fish ; in certain insects, 

 as the moths ; but never, I believe, in birds. It is most largely 

 developed in animals which are nocturnal in their habits, or 

 live like fishes in a medium which is dimly illuminated. All 

 must be familiar with the glare of light which it throws from 



the eye of the cat or dog, when these animals exhibit dilated 

 pupils in twilight. 



" This tapctvm Ivcidum, has been a groat stumbling block to 

 physiologists. The albino eye was set aside as abnormal ; and 

 the reflection of light in normal eyes from the retina and 

 choroid was overlooked, or regarded as accidental, but that 

 from the tapetum could not be. Most writers, however, dis- 

 miss it with an unsatisfactory and very brief comment, unable 

 evidently to reconcile its presence with the maintenance of 

 that internal darkness of the eye, which is supposed to be 

 essential to vision." 



In presenting a resume of the whole cjuestion. Dr. Wilson 

 enumerates many curious facts relating to vision and its ob- 

 ject, the following extracts furnish as condensed a view of this 

 extremely interesting subject as we feel warranted in giving : 



It thus appears that the laws of luminous reflection do not 

 necessitate imperfect vision, as 'applied to the fact, that the 

 retina and choroid return much of the light which reaches 

 them, for : — 



Is;. In the normal and also in the albino vision of all ani- 

 mals, man included, ^he amount of direct retinal and choroidal 

 reflection is necessarily coincident with the width or degree of 

 dilatation of the pupil ; the larger the pencil of light entering 

 the pupil, the larger the pencil leaving it, so that in every case 

 the reflected rays are thrown out of the eye and do not disturb 

 vision : further, — 



2nd. In those animals provided with fapeta lurida, such as 

 the cat, the dog, or the ox, which are only partially nocturnal 

 in their habits, the tapetum is so placed that in bright light it 

 is not opposite to the contracted pupil, or is so only to a small 

 degree. When, however, the choroidal mirror is called into 

 action in twilight, the pupil is correspondingly dilated, and all 

 the light which the tapetum reflects finds a free passage for its 

 escape. 



od. In the eye of man, as well as in that of a large num- 

 ber of other animals, the background of retina and choroid on 

 which the image is depicted, is not the darkest portion of the 

 ocular screen, nor even so dark as those parts of the inner 

 walls of the eye on which objects are never figured. On the 

 other hand, as John Hunter has shown, and illustrated by ex- 

 isting specimens, the front and the anterior sides of the eye- 

 chamber are the darkest, so that the reflecting power is greatest 

 at the bottom of the eye. 



itli. In the human eye, where, more than in those of the 

 lower animals, it has been contended that the conditions of a 

 camera obscura must be realised, the place of perfect vision, 

 instead of being additionally darkened, is occupied by the well- 

 known ydlmo sjMt, which has a marked reflective power, and 

 is easily discerned by ophthalmoscopes. 



The results which are announced in the preceding argu- 

 ment may be summed up as follows : — 



1. The total absence of pigment from the choroid, the 

 ciliary processes and the iris, is compatible (especially where 

 this condition is hereditary) with perfect vision. 



2. The replacement of the pigment of the choroid lining 

 the bottom of the eye by a concave mirror (^tapetum lucidum) 

 powerfully reflecting light, characterizes animals whose vision 

 is very acute. 



3. The non-tapetal or mirrorless eye of man, and of many 

 animals, differs only in degree from the tapetal or mirrored eye 

 of others ; for the retina and choroid act as a tapetum, and 

 reflect light in the same way. 



