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 Joun 
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, 
Ban, a 
the place of perfect vision, instead of being additionaily 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 
F by a concave mirror (tapetwm 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. 
3 4. The eyes of vertebrate animals are only to a limited extent camerw obscure, 
} 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. 
ba 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. 
| 
j * 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.8., London. Vol. iii., p. 133. 
“+ 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 marsupiwm 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 K6ii1ker and H. Miitter 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 
