378 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



image of the bottom, even when the latter is close enough to the surface 

 to be seen reflected from it. 



Through his surface window the fish sees everything from zenith to 

 horizon in all directions. This hemispherical aerial field is not narrowed 



Fig. 129 — Visual field of a fish in the upward direction. 



a, the water surface and the aerial window as seen from beneath, b, explanation of the 

 window: rays striking the surface at an angle within the window are refracted to the eyes 

 of the fish, but rays striking outside the window from beneath are totally reflected. Within 

 an angle of 97.6° the fish sees out into an aerial hemisphere; but outside of this angle he 

 sees objects on the bottom, reflected in a silvery surface. The surface must of course be 

 completely calm. 



or widened according to the size of the window and the depth of the fish. 

 It always contains everything above the plane tangent to the water sur- 

 face at the rim of the window, but the distortion and the brightness of 

 objects within it do vary. The objects seen proportionately largest are 



