178 NATURAL SCIENCE. March, 



render it somewhat doubtful. The pigment in the eye, for instance, 

 is very granular, and granules, especially if they are, comparatively 

 speaking, large, are not the best materials for a light-proof sheath. 

 And again, the more perfect the dioptric apparatus, the less necessity 

 is there for this ensheathing of the nerve endings, inasmuch as the 

 rays are already arranged in definite small-angled pencils, whose 

 directions practically coincide with the long axes of the nerve endings. 

 Yet we do not find that pigment is less developed in the highest eyes. 

 Indeed, intense pigment is found even in the eyes of mammals whose 

 skins are pure white in every other part of the body. 



We have then, it appears to me, to look elsewhere for the chief 

 function of the pigment, the universal presence of which in normal 

 eyes makes it highly probable that that function is one of supreme 

 importance in the physiology of sight. 



Let us see first what are some of the recognised effects of light on 

 the skin. One well-known effect is that which is called sunburning. 

 This browning of the skin is due to the movement under the influence 

 of light of pigmented granules between and into the cells of the skin. 

 In the case of the Vertebrata a certain amount of pigmented matter is 

 found in the cutis, and this seems to travel outwards towards the light. 

 Whether the light helps in any way to produce more pigment, perhaps 

 by increasing the metabolism, is unknown. Certain it is that pigment 

 frequently appears where specially active metabolism incidental to 

 growth is going on. One might mention as examples the case of the 

 regrowing of the lizard's tail and the rapid multiplication of cells in 

 certain cancerous growths ; in both these cases, pigmented matter is 

 very abundantly produced. 



Our special point here, however, is that this pigmented matter 

 invades the epidermis under the action of light. It forces its way 

 between the innermost cylindrical cells and eventually reaches the 

 outermost cells where, excepting when very abundant, it disappears, 

 becoming incorporated, with loss of colour, in the dying horny cells 

 which form the outermost protection of our body. So active is this 

 process, indeed, that if the light contains a large proportion of rays 

 from the violet end of the spectrum, the sunburning may be very 

 pronounced ; excessive horn-cells are produced and the outermost 

 peel off. 



Here, then, we have a very distinct effect, involving the 

 movement of granules between and within the epithelial cells, taking 

 place in the skin under the influence of light. It is surely not difficult 

 to believe that this process might, if required, come under the 

 cognizance of the ordinary sensory cells of the skin. Judging from 

 what we now know of the origin of gustatory and auditory sensory 

 areas out of epithelial cells, which indirectly stimulate the nerve fibres 

 in contact with them, there is surely no insuperable obstacle to our 

 believing that other epithelial cells, stimulated by the movement of 

 pigmented granules set in motion by the action of light either within 



