LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 75 



does not mean that the surface of the plate does not appear 

 red if observed directly. In Hess' experiment only a small 

 fraction of the light waves reaching the mirror are those 

 reflected from the plate. Inside of the tube, which appar- 

 ently was not blackened, the light waves were probably 

 reflected back and forth. The illumination of the mirror 

 is weak, and the visual angle for its surface at 4 m. distance 

 is small. Conditions are unfavorable for observing the 

 image of the red surface in the mirror, but not because red 

 rays are lacking. Hess recalls that many fish spawn at 

 20 m., that this especially brilliant variety may spawn at 

 60 m. and, because of the absorption of red at this great 

 depth, the red of their lower surfaces can have no biolog- 

 ical significance. There are few data as to the absorption 

 of light of different wave-lengths in fresh water lakes. It 

 has been determined, however, by Murry and Hjort (1912) 

 by photographic methods that in the ocean at 100 meters 

 there are many rays of all kinds, though fewest of the red. 

 From data of v. Henri et Larguier des Bancels it is deter- 

 mined that the eye is about 3,000 times as sensitive, as 

 the most rapid photographic plate. Although the red rays 

 are absorbed with increasing depth in shallow water, on 

 the other hand, spectral colors abound. This is shown by 

 v. Aufsess (1913) who placed a mirror beneath the water 

 of an Alpine lake and in it viewed the terrestrial landscape. 

 The branches of trees and all other external objects then 

 appeared fringed with spectral color, ranging from red 

 near the horizon to blue higher up. Most young fish live 

 in such water but may retire to deeper water as they grow 

 larger. Hess' evidence does not satisfactorily show that 

 no red rays reach the spawning grounds. But if none do, 

 that does not prove the absence of color sensitivity in fish. 

 If the absence of a particular stimulus in the normal en- 

 vironment of an animal were to be accepted as evidence 

 of the lack of response to it, we should be forced to hold 

 that Paramoecium is not affected by the electric current. 

 Experiment shows it to be sensitive to the electric current, 

 and experiment alone can decide the response of any animal 

 to a particular stimulus, whether that stimulus is or is 

 not normal to the environment. I have shown that the 



