The Senses and the Nervous System 69 



the soap (Diagram A). If S is the soap on the bottom of the 

 tub, the ray of light from S is bent as it comes out of the 

 water at X and goes in the direction XE to reach our eye at 

 E. Now, as we are accustomed to objects being in line with 

 the path over which the light from them reaches us, the soap 

 seems to us to be along the prolongation of the line EX, 

 and as the effect of the water is also to make it slightly 

 nearer to us, our eye sees the soap at S', on the line EXS', 

 instead of at S. The only exception is that if we look directly 

 down, as in Diagram Ai, the soap is actually on the line of 

 sight that it appears to be, for what is called the "normal" 

 ray of light, that is, the ray whose path is at right angles to 

 the surface, is not bent at all. 



It is this principle of refraction which serves as the basis 

 of operation of all lenses, whether for magnifying-glass or 

 spectacles or telescope or microscope or eye. It is a matter 

 which we shall not try to penetrate for fear of becoming 

 bogged in the laws and mathematics of optics, but over whose 

 surface we shall skim in order to get an understanding of 

 what the fish is up against. 



If the fish never tries to see outside its own element, its 

 life is simple. The swordfish passes its life in the open sea, 

 finds its food beneath the waters, and probably takes no in- 

 terest in what goes on outside. On the contrary, the basses 

 and the trouts and the salmons spend a large part of their 

 time near the shore, in shallow water, and have to look be- 

 yond the surface for both food and enemies. And when they 

 do that, the lines of vision get bent, and things are no longer 

 what they seem. Fish can see a man on the bank when, if 

 man and fish were both in the air, the man would be hidden 

 by the corner of the bank. For, if we replace our soap in a 

 tub by a fish in a stream, we have the diagram on the follow- 

 ing page. 



