70 The Life Story of the Fish 



Diagram B 



Remember that it is now the light traveling from man 

 toward fish which we wish to consider, for we are investi- 

 gating what the fish sees, not what the man sees. The light 

 travels along the path MX, just missing the top of the 

 bank at B. If it were not for the refractive power of the 

 water, it would continue on in the direction XY, passing 

 over fish's back, and fish would not see man. But actually it 

 gets bent at X, takes the new direction XF, and reaches 

 fish's eye at F. Fish sees man, although the straight line from 

 man to fish, MF, is cut off by the bank of the stream at B. 

 The fish sees around a corner. 



This is one basis for the argument which has sometimes 

 been advanced that a fish can see a man before the man can 

 see the fish. This overlooks the fact that the principle of re- 

 fraction is reversible. The line of sight is equally bent going 

 in both directions, and man sees around the same corner that 

 fish does. 



But that is not the end of the matter. Consider the diagram 

 on the opposite page. 



Here we again have both fish and man able to see each 

 other around the corner of the bank B, because the line of 

 sight is bent at X. But note the result. To man, fish appears 

 to be along the prolongation of the line MX, at F'. To fish, 

 man appears to be poised precariously at M', peering down 



