170 Mr. H. M. Bernard on 



Returning to the eye, the granules may be described as 

 advancing and retreating according as they are stimulated by 

 light or as the stimulus is withdrawn. It may also be that, 

 like the kindred chromatophores of the skin of animals en- 

 dowed with the chromatic function, they are susceptible to 

 nerve-stimulation, inasmuch as it is said that the granules 

 advance in a darkened eye if the companion eye is stimulated 

 by light. But this " sympathetic " advance might perhaps be 

 explained in another way. 



While the advance is apparently due to the stimulus of the 

 light, the retreat may be due to lateral pressure on the part of 

 the cuticular rods. That such a pressure exists we may perhaps 

 conclude from the fusiform shape assumed by those granules 

 which, being on the outer portions of the cells, slide up and 

 down most frequently between the rods. The pointed ends of 

 these fusiform bodies lead one at first naturally to see in them 

 the instruments for stimulating the nerves. It has been pointed 

 out that they might, for all we know, vibrate as rapidly as cilia. 

 It w r as long before I became convinced that this shape was 

 chiefly useful in enabling the granules to force their way up 

 and down rapidly between the closely packed rods, and also in 

 enabling them to form compact masses, the fresh arrivals 

 wedging themselves in between those in front. The pointed 

 ends have, I am convinced, no other function than that of 

 facilitating their alternate advancing and retreating move- 

 ments, for rapid crowding at special points and equally rapid 

 dispersal. 



The actual cause of Li y lit Sensation. — At first sight it must 

 appear that no theory can be simple which seeks to explain 

 how the eye can accurately register (as it does, say, in the 

 process of reading) several hundreds of distinct words, each 

 composed of many letters, per minute, each letter forming an 

 image which remains but the traction of a second. My 

 theory, however, claims to be, comparatively speaking, a 

 simple one. It assumes the existence of no forces or sub- 

 stances which we do not know to exist, not only in the body, 

 but even in the eye itself; and it is, moreover, applicable to 

 every eye, known for certain to be such, in the animal 

 kingdom. 



Just as the play of colour in the skins of animals endowed 

 with the chromatic function is due to the constant shifting of 

 the variously coloured granules within the chromatophores, so 



these granules really contain excretory matter which should be discharged 

 at the surface of the body, we should be justified in concluding that the 

 exposure of the skin to light must be generally beneficial to health. 



