SECRETS OF THE SALMON 



passing through the water and the plate glass. 

 The distance from the camera to the fly was short, 

 giving only a very small depth of focus, so that 

 the pictures were easily blurred. We found that 

 the light entering the surface of the water, when the 

 fly was above the surface, seemed always to give 

 a somewhat blurred image of a floating fly, and 

 I do not believe that we can get any clearer photo- 

 graphs than are shown. It is quite likely that the 

 vision of the floating fly is equally blurred to the 

 fish. When I looked with my eyes through the 

 plate, the image of the floating fly was never per- 

 fectly sharp. I want to thank Mr. Clark for his 

 able and generous assistance. The photograph 

 (Figure 12) shows the camera and tank in position. 

 In order to imderstand what was seen it will be 

 necessary to enter a little into the phenomena of 

 light and its behavior in its passage from air to 

 water. The diagrams will make the matter sim- 

 ple and explain the subsequent illustrations. 

 When a ray of light enters the surface of still 

 water from above, from any angle except directly 

 vertical, it is bent or deflected. This deflection 

 is found to take place according to a well known 

 law. The diagram (Figure 13) shows an easy way 

 of expressing it. Describe a circle with its centre 



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