56 



PIGMENTS ON THE LOWER SIDES OF FLAT FISHES. 



the darkened area of the paper corresponds with remarkable exactness 

 to the pigmented area of the fish, and, most remarkable of all, there 

 is a small darkened patch corresponding to the isolated pigmented 

 patch on the lateral line of the lish. In this particular photographic 

 print the outer region of the bromide paper, beyond the edges of 

 the fish, is lighter than the outer part of the area covered by the 

 fish. This is simply due to the over-exposure of the uncovered 

 area of the paper, causing a partial reversal of the photographic 

 effect. It must be concluded that the rays of light which reach the 

 small depression on the lateral line of the fish pass parallel to the 

 surface of the fish, and therefore do not fall upon it ; but when they 

 reach the edges of the depression they are slightly refracted, and so 



Fig. 3. — Reproduction of impression produced by exposing a sheet of bromide 

 photographic paper, with a Flounder lying upon it, to the action of diffuse light, 

 and subse<iuently developing. 



fall upon the surface of the skin in the depression, and give rise to 

 pigmentation. It is possible that this explanation may be corrected by 

 physicists, who know more of the properties of light-rays than I do ; 

 but the proof in the photograph that light does reach the depression, 

 and produce an effect there which is absent from the surrounding area, 

 is quite sufficient for my argument. 



It will probably be admitted that what applies to one of the fish 

 applies to all. In any case, sufficient examination was made to show 

 that the differences in the amount of pigmentation corresponded to 

 differences in the shape of the lower surfaces of the fish. The lower 

 side of the sole was flat, and in contact everywhere except at the base 

 of the tail. When the fish were alive their caudal fins were usually 



