328 DR GEORGE WILSON ON THE 



merse, there is a further necessity for a black lining to the living camera, or eye- 

 chamber; for were that lining wanting, and the internal surface of the choroid 

 highly reflective, the same ray of light might return many times across the cham- 

 ber of the eye, producing multiple images of single objects on the retina, exhaust- 

 ing its sensibility, and confusing vision. 



(4.) That this, moreover, is no hypothetical case, is held to be demonstrated 

 by the experience of the Albinoes of our race, in whom the pigment of the choroid 

 is wanting, and who see with pain and effort, unless by dim light. 



The views stated above have till very recently been universally held by phy- 

 siologists and natural philosophers; in proof of which it may suffice to quote the 

 following passage from the Elements of Physiology of Professor Muller, who has 

 written very fully on the theory of vision, and is one of the highest living autho- 

 rities among physiologists on this subject. 



" The interior of the eye, namely, the posterior surface of the iris and ciliary 

 processes, and the inner surface of the choroid, immediately external to the retina 

 itself, is coated with black pigment, which has the same effect as the black colour 

 given to the inner surface of the walls of optical instruments. It absorbs any 

 rays of light which may be reflected within the eye, and prevents their being 

 thrown again upon the retina, so as to interfere with the distinctness of the 

 images there formed. This is the use of the pigment on the posterior surface of 

 the iris and ciliary processes. But the coating of the outer surface of the retina 

 by the pigment of the choroid is also important in the same respect ; for the re- 

 tina is very transparent, and if the surface behind it were not of a dark colour, 

 but capable of reflecting the light, the luminous rays which had already acted on 

 the retina would be reflected back again through it, and would fall upon other 

 parts of the same membrane, the consequence of which would be, not merely 

 dazzling from the excessive action of light, but also indistinctness of the images. 

 Animals in which the choroid is destitute of pigment, and human Albinoes suffer 

 in this way ; they are dazzled by daylight, and see best by twilight."* 



Thus far, then, there does not appear to be room for two opinions concerning 

 the internal darkness of the human eye being a condition of perfect sight. But 

 recent discoveries require us to look at the theory of vision from an opposite 

 point of view. It is now beyond question, that even in the darkest human eye, 

 there is reflection through or across its chamber, from the surface of the retina, 

 as well as from that of the choroid ; and the observation is a very old one, that in a 

 large number of animals, a part, and sometimes the whole of the retinal surface is 

 covered, or replacedf by a reflector rivalling in brilliancy a sheet of polished silver. 



* Muller's Elements of Physiology, vol. ii., p. 1133. 1842. Translated by Balt. 



f I do not intend by the words " covered" or " replaced," to imply any opinion on the anatomical 

 relation of the tapetum lucidum to the other structures of the eye. In an optical point of view, it is 

 the substitution of a highly reflective, for a partially absorptive surface. 



