338 DR GEORGE WILSON ON THE 



However that may be, it is sufficient for my present purpose to point to the 

 albino animals, whose eyes are totally destitute of pigment, and reflect light from 

 every point of the surface, both of the retina and the choroid, but, nevertheless, 

 exercise the faculty of sight in perfection. Their eyes, even when the iris is fully 

 contracted, remain, in virtue of the transparency of that membrane, camerce lucidw; 

 their possessors cannot render them camerce obscurce ; and yet they are excellent 

 organs of vision. 



2. If the reasoning pursued in reference to the albino eye be valid, it will serve 

 also to dispose of the difficulty experienced by some in explaining how vision is 

 compatible with the presence of a tapetum lucidum in the eyes of many animals, 

 This tapetum is equivalent to a concave mirror of polished metal, replacing the 

 pigment of the choroid over a greater or smaller part of its surface, especially at 

 the deepest or most posterior portion of the chamber of the eye, so that lying be- 

 hind the retina, it is more or less directly opposite the pupil, and receives the light 

 which enters by it. A brilliant reflecting surface of this kind is found in many of 

 the mammalia, both graminivorous and carnivorous, as the horse, the ox, the 

 sheep, the cat, the dog. It is present in the eyes of the whale, seal, and other marine 

 mammalia ; and in fishes, such as the shark, in which it is peculiarly brilliant. It 

 occurs also in certain of the mollusca, as the cuttle-fish ; in certain insects, as the 

 moths ; but never, I believe, in birds. It is most largely developed in animals 

 which are nocturnal in their habits, or live like fishes in a medium which is dimly 

 illuminated. All must be familiar with the glare of light which it throws from 

 the eye of the cat or dog, when these animals exhibit dilated pupils in twilight. 



This tapetum lucidum, has been a great stumbling block to physiologists. The 

 albino eye was set aside as abnormal ; and the reflection of light in normal eyes 

 from the retina and choroid was overlooked, or regarded as accidental, but that from 

 the tapetum could not be. Most writers, however, dismiss it with an unsatis- 

 factory and very brief comment, unable evidently to reconcile its presence with 

 the maintenance of that internal darkness of the eye, which is supposed to be 

 essential to vision. 



Thus, J. Muller, in the passage already quoted, alludes to dazzling from ex- 

 cess of light, and indistinct vision, as inevitable peculiarities of the tapetal eye. 



well's Colour-Top allowed us to test carefully, and with the result, that her sense of colour was acute, 

 precise, and normal. In short, her vision by diffuse light was optically as perfect as that of the 

 majority of mankind, and to appearance, a diminution in the sensitiveness of the retina was all that 

 was required to make vision equally perfect in direct light. 



The iris in this case was pale blue, and the pupil was not pink by diffuse daylight. It became 

 so, however, in the neighbourhood of a gas flame, and her friends were familiar with the fact that by 

 gas-light, her eyes often " flashed fire." 



We found it easy to observe this ocular luminousness at will, and the fact is important, as prov- 

 ing that the comparatively perfect vision which this Albino possessed, was exercised by eyes, within 

 which a large amount of cross-reflection of light was continually occurring. 



