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EYE AS A CAMERA OBSCURA. 333 
On either view, we must qualify the statement that the eye is a camera ob- 
scura; but before attempting such qualification, it is necessary to consider the 
reflection of light from the choroid behind the retina. As the latter membrane 
during life is quite transparent, and is traversed throughout its entire thickness 
by many of the rays which fall upon it, these must, in part, be reflected from the 
choroid which receives them, unless we impute to this membrane a power of 
absolute absorption, so far as light is concerned. Something little short of such 
a power is habitually attributed to the choroid, and not unnaturally. The dark- 
ness of the pupil, even in the lightest normal eye, is by most persons referred to 
the dark background against which we are supposed to see it, as its cause. The 
pink pupil of the Albino is in the same way connected with the crimson choroid 
at the bottom of his eye; and these views are supposed to be justified by the 
appearances in the dead organ, where a black or brown pigment is found coating 
the one choroid, and absent from the other. It has thus been generally inferred. 
that the dark choroid of the perfect eye does not sensibly reflect light, whatever 
may be the case with the retina,—a conclusion certainly not warranted by dis- 
section; for, on the one hand, the choroid after death appears darker and less 
reflective of light than during life, in consequence of the bloodvessels of which it 
so largely consists becoming emptied of blood; and, on the other, Joan Hunter 
has long ago permanently illustrated in his great museum, that the pigment of 
the choroid “in the human species is,” to use his own words, “ of all the differ- 
ent shades between black and almost white,” * and, as he also states, is generally 
lightest in colour at the bottom of the eye,}+ where, of course, its position best 
enables it to act as a reflector. Excluding the white choroid, which belongs only 
to the abnormal albino, there is plainly room in the other pale tints for much 
reflection. 
But it is needless to accumulate arguments in proof that the choroid must 
reflect light, seeing that by means of the ophthalmoscope, every one may satisfy 
himself that this membrane certainly does. Hrtmuowrz referred to his instru- 
_ ment chiefly as a retina-speculum, and as such it was described by Dr Sanpers, 
writing for practical medical men, to whom the retina is much more an object of 
interest, as liable to disease, than the choroid. The latter membrane, however, 
was plainly within the reach of the speculum; and later observers have carefully 
depicted and described its appearance under their ophthalmoscopes. Rurve and 
_ luminous sensation. It must, however, in part be reflected from those anterior layers before reaching 
_ the deepest one. The internal or anterior surface of the bacillary layer is further described, as 
smooth and brilliant, the ‘ bacilli and coni” appearing like the polished surfaces of crystals, so that 
they reflect light powerfully, and the greater number of the rays which are returned from the retina, 
_ have probably been reflected from its deeper layers, whilst a portion has been thrown back from the 
_ anterior layers, without contributing to the perception of light. ‘The only point, however, which I 
am much concerned to urge is, that the retina, as a whole, reflects much of the light which reaches it. 
* Observations on certain parts of the Animal Economy. By Joun Hunter, F.R.S., vol. iv., 
p. 278. 
ft Catalogue of Museum, R. C. S., Lond., vol. iti., p. 133: 
