342 DR GEORGE WILSON ON THE 



From those premises the conclusion is deducible that in vertebrates much 

 light is reflected from the bottom of the eye-chamber during the exercise of vision 

 without disturbing it ; but that little is reflected again, so as to return to the 

 bottom of the eye, in consequence partly of its absorption by the pigment of the 

 anterior portions of the choroid, partly of its escape through the pupil. 



It may seem to some that this reasoning proves too much, for why is there in 

 man and many other animals a pigment at the bottom of the eye, if reflection 

 from the membrane there is so free to take place ? To this I reply that the pig- 

 ment, which is never altogether inoperative, comes into special action when the 

 eye is exposed to very bright light, and saves the retina from the paralyzing in- 

 fluence of intensely luminous rays. Vision, however, cannot be continuously 

 exercised under such exposure, even where the light is not excessively brilliant, 

 in consequence of the instinctive closure of the eyelids, and the abundant secre- 

 tion of tears which then take place. The pigment at the bottom of the eye is 

 thus, I apprehend, a safeguard against sudden exposure to intense light ; but 

 during continuous vision under an illumination which does not dazzle the eye, its 

 action is secondary as an absorber of light, and it always acts as a reflector. 



Hitherto I have been arguing almost solely for the negative conclusion, that 

 the vertebrate, and especially the human eye, is not the kind of darkened cham- 

 ber which it has been supposed to be. It is impossible, however, to regard the 

 deep intra- ocular reflection which so certainly occurs in most animals, as an inci- 

 dental or useless phenomenon. That it has a direct and beneficial influence over 

 vision I cannot doubt, and I proceed briefly to indicate where the proof of this 

 is to be found. 



Intra-ocular reflection, as a normal phenomenon, is at a maximum in the 

 tapetal or mirrored eye of the lower animals. It is desirable, accordingly, to 

 study it first as occurring in them ; nor can a better example of a mirrored eye 

 be found than that presented by the shark. In it the tapetum lucidum occupies 

 the whole of the bottom, and one-half or more of the lateral surface of the cho- 

 roid, which is covered by pigment only in front. The iris, as in other fishes, is 

 incontractile, so that the diameter of the pupil never varies; and the tapetum, 

 which is colourless and very brilliant, is thus always in action as a reflector. The 

 shark, moreover, swims near the surface of the sea, where the amount of light is 

 considerable, and the acuteness of its vision is proverbial. I have selected it, 

 rather than a mammal, with eyelids and a contractile iris, because in the shark 

 luminous reflection never ceases unless in absolute darkness ; and when light is 

 shining occurs the more, the brighter the light is. Its eye thus is always in the 



the very curious observations of the latter author on the eyes of the cephalopoda (Quart. Journal of 

 Micr. Science, July 1853, p. 269), show that the retina of these invertebrates will act in the same 

 way, a remark which, mutatis mutandis, may be applied to every creature, whatever its rank in the 

 animal scale, which has shining or so-called phosphorescent eyes. A most interesting field is open 

 to naturalists in the examination, by means of the ophthalmoscope, of the eyes of living animals of 

 all grades. 



