84 The Life Story of the Fish 



the lens he would not get enough refraction. Further, per- 

 haps because of this fact, his lens is not elastic like ours. It is 

 rigid and its curvature cannot be changed. The only means 

 left to the fish to accommodate for varying distances is to 

 move his lens toward or away from the retina, just as a 

 camera lens is moved toward or away from the plate. To do 

 this, he has a special muscle, the retractor lentis. By means 

 of this muscle the distance between the lens and the retina 

 can be varied. For long-range vision the lens is brought 

 nearer to the retina than for short range. 



Now, all muscles expect to be allowed to rest when not in 

 use. It is customary for them to spend their time off In a 

 relaxed condition. In the human eye, the apparatus is so de- 

 signed that when the small muscles which control the curva- 

 ture of the lens are relaxed, the lensjs at its flattest, and is, 

 therefore, set for distant vision. In the fish, when the re- 

 tractor lentis relaxes, the lens remains In the forward posi- 

 tion, and is thus set for near-by vision. And all this is very 

 proper and fitting. For air is a medium through which light 

 travels a long way, and man Is expected to have his eyes on 

 the horizon, whereas water permits light to travel only a 

 short way, and the fish Is expected to concentrate on near-by 

 details. The fish goes poking around In the Immediate fore- 

 ground with his little round lenses way out at the front 

 ends of his eyes, as far from the retina as he can get them, 

 and when he wants to look Into the distance he has to haul 

 them in by means of the retractor lentis, like toy balloons 

 on a string. A German observer goes so far as to claim that 

 he has actually been able to see this back-and-forth move- 

 ment of the lens in the large eyes of trigger-fish in aquaria. 



The surprising resemblance which we spoke of between 

 the fish's visual machinery and the human's is that in each 

 the eyeball is moved by the same set of six muscles. For the 

 fish does move its eyes, although very much less than we 



