The Senses and the Nervous System 75 



fish and that that is what is visible to you as you float effort- 

 lessly beneath the waters and cock an eye at the surface. If 

 this makes you dizzy, close your eyes for five minutes, then 

 skip to the next section. Or take a nice long walk. 



However, if you feel capable of resuming the struggle, 

 consider Diagram H. The slope of the lines FX' and FX" 

 can never change. They mark the angle at which horizontal 

 light would be bent if it entered the water, and are im- 



6 />. 70 in. >. <^Zhli in. -*- 



Diagram H 



mutably fixed by the laws of refraction. This means that the 

 angle X'FX" is unalterably fixed. It is slightly greater than 

 90 degrees. And this means that the size of the fish's circular 

 window will vary with his distance from the surface. 



For, by all the laws of God and man, if the angle X'FX'' 

 remains constant at a little over 90 degrees, the relation be- 

 tween OF and X'X" must remain constant, and X'X" must 

 always be a little more than twice as long as OF. If the fish 

 is three feet below the surface, his circular window is over 

 six feet in diameter j if he comes up to within one foot of 

 the surface his window shrinks to little more than two feet 

 in diameter (Diagram H). But, although the window ac- 

 tually shrinks, it appears to the fish to remain the same size, 

 for the apparent size of any object is governed by the angle 

 it subtends, which is why a pencil looks larger to you when 

 held just in front of your nose than at arm's length. We have 



