The Senses and the Nervous System 73 



that it is bent back and goes toward F. In other words, the 

 fish at F sees not only the object at W along the line FW, 

 but it also sees the reflection of W, upside down, along the 

 line FY. The surface outside the point X', in addition to 

 being an impenetrable ceiling, is a mirror. 



The line FX', then, marks the limit on one side, and the 

 line FX" on the other side, of light from above which can 

 reach the fish. If we now place ourselves above the water 

 and look down on it, we transfer ourselves from Diagram D 

 to Diagram E. X' and X" become points on the rim of the 

 circle, on the surface, through which the fish can see out into 

 the air. The line X'X" becomes a diameter of this circle, 

 and the center is at O, directly above the fish. Outside the 

 limits of that circle the surface is to the fish an impenetrable 

 ceiling-mirror. The circle itself is the transparent window 

 through which he sees. And the final effect of refraction is 

 that the whole visible above-water world outside the circle 

 becomes a border on the inner edge of that window. 



To understand this, consider Diagram F. The horizontal 

 lines, HX' and HHX" will become to the fish the lines FX' 

 and FX" and will bound the visible world. Anything on 

 the horizon, at H and HH, will appear to be in the pro- 

 longation of those lines. Anything above the horizon, like 

 the bank B or the man M or the tree T, will appear to 

 project inward from those lines. It is as if the surface of the 

 water were a moving-picture screen' with a circular picture 

 thrown on it. If Diagram F is a cross-section of a round 

 pool with the banks the same height all around, then what 

 the fish sees as he looks up is roughly represented by the 

 circle X'X" sketched in Diagram G. 



If you understand all this from reading it over once, well 

 and good. You are fortunate in having that kind of a mind. 

 If you do not understand, go and lie flat on your back, hold 

 the book over your head, and look steadily at Diagram G. 

 Forget all the rest, and just make believe that you are a 



