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, John 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, f 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. Helmholtz referred to his instru- 

 ment chiefly as a retina-speculum, and as such it was described by Dr Sanders, 

 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. Ruete 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' 1 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 John Hunter, F.R.S., vol. iv., 

 p. 278. 



f Catalogue of Museum, R. C. S., Lond., vol. iii., p. 133. 



