158 American Fisheries Society 



1914). But since the lighting of his world gives objects 

 the appearance of flatness the fish has perhaps less use 

 for stereoscopic vision than have we. 



When the normal human eye is at rest it sees clearly 

 only objects at a distance of more than twenty feet. To 

 see nearer objects clearly the eye must be focussed. Fo- 

 cussing may be accomplished in optical apparatus in two 

 ways. In a camera we focus by moving the front or back 

 of the camera so as to alter the distance between lens and 

 ground-glass. For remote objects this distance must be 

 short; for nearer effects it must be increased. But if 

 the lenses in our cameras were flexible we might keep 

 them in focus by altering the shape of the lens to accom- 

 modate objects at different distances. For distant objects 

 the lens would have to be made flatter, for near objects 

 more spherical. In practice we cannot alter the shape 

 of a glass lens, but we focus by substituting a lens of 

 different curvature. It is by changing the shape of the 

 lens that focussing is accomplished in the eye of man 

 and other higher land animals. When the eye is at rest 

 the lens is flattened and thus adjusted for distant objects. 

 For nearer objects, through muscular action, the lens 

 becomes more spherical. An unconscious muscular effort 

 is therefore needed to see near objects clearly. To the 

 land animal distant objects are clearly visible. Their 

 rapid approach through the thin atmosphere often means 

 danger of food. It is therefore essential that the resting 

 eye of the land animal should be fitted to see them. At 

 the same time it needs to be instantaneously adjustable 

 for near objects. 



In the fish the spherical lens of the eye does not 

 change shape. When the eye is at rest it sees clearly 

 only objects at a distance of about three inches to a foot 

 (Beer, 1894). The fish then sees like a rather short- 

 sighted person; everything at a distance is blurred. To 

 see distant objects it contracts a muscle which pulls the 

 lens nearer to the retina. The action is like the focussing 

 of a photographic camera by moving the lens. It may 

 focus objects 30-40 ft. distant. So the fish with resting 



