i8o NATURAL SCIENCE. March. 1897. 



cells, which are themselves directly stimulated by the movements of 

 granules within or in contact with them, would naturally appear at 

 those spots which are most frequently and brilHantly illuminated. It 

 would be to such parts that the pigment would be most drawn by the 

 light. That is not all. The very excess of pigmented matter brought 

 to the surface by the light would supply the material for a dioptric 

 apparatus. (See further Ann. Mag, Nat. Hist., Feb. 7th, i8g6, and 

 Quart. Journ. Micro. Sci., Dec, 1896.) 



Armed with this clue to the understanding of light-sensations 

 supplied us by the pigment, we turn to those cases in which no 

 pigmented matter can be detected, e.g., in the eyes of albinos and in 

 the semi-transparent siphons of Psammobia, both of which are stimu- 

 lated by light. The clue in no way fails us. The essential point 

 involved in it was the movement of granules. While it is possible 

 that deeply-pigmented granules move more actively under the action 

 of light than unpigmented granules, there is no reason whatever to 

 suppose that it is only the pigmented granules which are affected by 

 the stimulus. 



As a matter of fact, pigmented granules are of all shades of 

 colour, from deep blackish brown to light reddish and yellowish 

 browns. We have only to assume, then, that in the cases where no 

 pigment can be detected, granules are nevertheless present, which are 

 set in motion by the light, or, in the case of the shadow-sensation, 

 brought to a standstill by the sudden withdrawal of the light, and 

 that the epithelial cells stimulated by this motion or sudden cessation 

 of motion in their turn stimulate the associated nerve-cells or fibrils. 

 I may, indeed, be allowed to state in advance that, among many 

 other retinas which I am now investigating, that of the albino rabbit 

 shows clearly that this supposition is partly, if not entirely, correct. 

 Not only is coarsely granular matter present in the chromatophoral 

 cells in such eyes, but, further, this matter is frequently found massed 

 high up between the retinal rods, just like the pigment granules in 

 normal eyes. While I have no distinct proof that this matter finds 

 its way between the rods as a direct effect of the action of light, it 

 would be hazardous to deny the probability of this having been 

 the case. 



I think, then, it is abundantly clear that the pigment, in its 

 broadest sense, meaning pigmented granules, cannot be dismissed in 

 the way it usually is, as unessential to light-sensation, merely because 

 in certain abnormal eyes there is no colouring matter. On the 

 contrary, it seems to me that if, instead of turning away from the 

 functions of pigment as too problematical to be worth considering, as 

 Dr. Nagel has done in his little book, he had concentrated his atten- 

 tion upon them, his interesting facts and able discussion of them 

 might have been carried a step or two further. 



H. M. Bernard. 

 Streatham, S.W. 



