Reighard. — Senses and Learning in Fishes 151 



hazy. Objects at a distance of fifteen or twenty feet ap- 

 pear as though seen through a thin fog, while objects at 

 a greater distance are less distinct and finally invisible. 

 Into a bluish opalescent haze retreating fish vanish and 

 from it emerge those that approach. The opalesence is 

 due to numerous small floating organisms and inorganic 

 particles which interfere with the light and reflect it 

 back and forth. As a result of these reflections and of 

 those which take place from the bottom and from the 

 lower face of the water's surface the under-water light 

 is more diffused than that in air. It penetrates into cran- 

 nies and beneath objects with the result that shadows 

 are much less in evidence than in air and all objects 

 therefore appear less in relief. They have somewhat the 

 flatness of those in shadow pictures. These objects may 

 appear distorted when seen through layers of different 

 density owing to differences of temperature. They doubt- 

 less lack somewhat of the color they have in air for the 

 reason that water tends to obstruct the passage of red 

 light-rays. Red objects at some depth must therefore 

 appear less red than at the surface and objects of other 

 colors are somewhat altered in appearance. 



But the water affects the visual world of the fish in 

 another respect more important than any of those enum- 

 erated. While we see no sharp upper limit to our atmos- 

 phere, the fish has always above him the sharply defined 

 surface of the water. Through this he sees the outer 

 world and in it he sees reflected a large part of the un- 

 der-water world. Figure I shows a cross section through 

 a pond and from it may be learned what a fish would 

 see with its eye located at F. On the right is represented 

 a graduated rod the divisions of which are lettered be- 

 low water and numbered in air. From F, the fish sees, 

 in the angle I, the bottom of the pond between H on the 

 right and point numbered 10 at the left. In the angle 

 II he sees the rest of the bottom and the submerged part 

 of the staff below A. In the angle III, the fish sees a part 

 of the water surface and the submerged part of the rod 

 between 9 and A. All these things are seen directly and, 



