EYE AS A CAMERA OBSCURA. 341 



twilight, the pupil is correspondingly dilated, and all the light which the tapetum 

 reflects finds a free passage for its escape. 



3d, In the eye of man, as well as in that of a large number 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 existing specimens, the front and the an- 

 terior sides of the eye-chamber are the darkest, so that the reflecting power is 

 greatest at the bottom of the eye.* 



4th, In the human eye, where, more even 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 yellow spot, which has a marked reflective power, and is easily 

 discerned by ophthalmoscopes. 



The results which are announced in the preceding argument 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. 



4. The eyes of vertebrate animals are only to a limited extent camerce obscurce, 

 and internally are least dark in the portions most directly exposed to the action 

 of light, and where the seat of perfect vision is placed. f 



5. The back of the iris, over which the retina does not pass, is the darkest in- 

 ternal portion of the eye in vertebrates ; and next to it, in the majority of these, 

 are the ciliary processes of the choroid, and its anterior lateral portions. 



* Hunter states that in animals where the pigment of the choroid is light in colour, " the lightest 

 part is always at the bottom of the eye, becoming darker gradually forwards, and in such it is often 

 quite black ; viz., from the termination of the retina to the pupil ; or if not black, it is there much 

 darker than anywhere else. This is generally the case in the eyes of the human subject." Catal. 

 Mus. R. C. S., London. Vol. iii., p. 133. 



f Comparative anatomists must decide to what extent these observations demand qualification 

 in reference to particular tribes of animals. The nocturnal lemurs, which have a uniformly coloured 

 dark choroid, no tapetum, and a very sensitive retina, probably exhibit intra-ocular reflection to a 

 small extent compared with other quadrupeds. A similar remark applies to birds, qualified by 

 the fact that the bottom of the eye-chamber is occupied in them by the marsupium or pecten, an organ 

 the use of which has not been ascertained, so that we cannot be certain how it modifies vision. But 

 as the researches of Kolliker and H. Muller demonstrate that the general structure of the retina is 

 the same in all vertebrates, it appears certain that, however dark and absorptive of light the choroid 

 may be in some of them, the retina in all will act as a mirror towards light incident upon it; and 



