298 WASTE-LAND WANDERINGS. 



in front of them probably has puzzled a good many- 

 people. A recent explanation, which does not, howev- 

 er, differ from that of the older ichthyologists, but is 

 couched in simple, non-technical terms, runs as follows : 



" If a line extending through the centre of the pupil 

 to the centre of the retina were the actual axis of vision, 

 then such lateral-eyed fishes as the freshwater sunfishes 

 would have separate fields of vision ; but I am convinced 

 that the true axis extends through the anterior margin 

 of the ovate pupil to the posterior side of the retina. 

 The spot of most distinct vision is this posterior portion 

 of the retina, and on this spot the images formed in both 

 eyes coincide. 



" When the fish is quietly swimming in the water, this 

 true axis is directed horizontally forward. This is true 

 whether the fish has its body horizontal or not. The 

 movement of the eyeball, to retain a horizontal direc- 

 tion of the true visual line, is a rotating one on the ap- 

 parent optical axis ; the rotation of the ball is not very 

 noticeable, except in those fishes that have a dark band 

 across the eye. One of those best marked for this pur- 

 pose is the black-banded sunfish. There are quite a num- 

 ber of species marked with distinctness enough to show 

 the rotation well, and the movement is an interesting 

 one to watch. When the body of the fish is almost per- 

 pendicular, the eye retains its natural horizontal direc- 

 tion ; this is true whether the fish is swimming oblique- 

 ly upward or downward. 



" The medium in which fresh- water fishes live gives 

 them a chance to see a great distance only in the hori- 

 zontal direction, and the proper adjustment of the eye 



