WHAT THE FISHES SEE 



the crystalline lens and retina, the one a biconvex lens 

 producing an image, which is projected exactly upon 

 the other at one point, the central cavity or fovea, 

 where the principal visual elements, the cone-shaped 

 cells, are most numerous. These elements are rod- 

 like cells and cone cells, so called because of their 

 form; the grouping of them is more compact in the 

 fovea itself where the cones are almost exclusive. A 

 layer of pigment, situated at the point of contact of 

 these rods and cones, arrests the luminous rays, 

 absorbs them, and enables them to make their im- 

 pressions. The diversity of land vertebrates, as 

 regards keenness of sight, depends, for the greater 

 part, on the number of these sensitive elements and 

 the way they are apportioned ; a retina well fitted with 

 them assures good sight. 



But fishes' eyes are made differently. We certainly 

 find in them a crystalline lens and a retina, enveloping 

 membranes and transparent humours, but they are 

 arranged in a different way and consequently produce* 

 different results, like optical instruments with identical 

 mounts but different combinations of glass surfaces. 

 The crystalline lens is globular. In the living animal, 

 it looks like a translucent sphere; when the fish is 

 cooked it is this which forms the hard white ball 

 which is removed from the cavity of the orbit, its 

 substance having been coagulated by the heat of 

 cooking. The retina is often supplied with an abundant 

 pigment which permits of the absorption of the slightest 

 ray of light; but the sensitive receiving elements are 

 few in number and are often scattered, seldom collected 

 in sufficiently large numbers, except in certain more 

 favoured spots. When we construct an optical model 

 based on such an arrangement and compare it with 

 that of land vertebrates, we find that there is con- 

 siderable inferiority in the working. The spherical 

 crystalline lens, situated in front of the retina, can only 

 project confused images upon it. Further, the retina 

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