LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 69 



the white Hght, no such behavior was observed, and with 

 different intensities of white light no avoidance of a definite 

 moderate brightness appeared. Bauer concluded that 

 when light-adapted, the fish have color vision, a conclusion 

 reached both from his observations that the fish avoided 

 red, and from the fact that he was unable so to equate the 

 brightnesses of red and blue that the light-adapted fish 

 responded to them similarly. When the same fish were 

 dark-adapted, they entered the red and orange lighted 

 portions which they had avoided when light-adapted. He 

 regarded this as evidence that when dark-adapted, they 

 might be color blind. He presented evidence for a " Pur- 

 kinje phenomenon." 



Hess (1911), stimulated by the work of Bauer, tested 

 the relative sensitivity of fish to light when dark-adapted 

 and when light-adapted. He used carp between 8 and 9 

 mm. long. He put the fish in their containers in the sun 

 to get them light-adapted and then rapidly moved the 

 container into the dark room, in which the tests of both 

 light- and dark-adapted fish were made. He noted the 

 strength of the stimulus necessary to call forth a photo- 

 tactic response in light-adapted fish and compared that 

 with the strength of stimulus for fish which had been for 

 some time undisturbed in the dark. He found their sen- 

 sitivity increased more than 1,000 times by 15 minutes 

 dark-adaptation. By varying the intensities of the light 

 used he, like Bauer, was able to make dark-adapted fish 

 distribute themselves uniformly in the red and blue halves 

 of an aquarium illuminated by filtered light. When these 

 fish were light-adapted, it was necessary to increase the 

 intensity of the blue illumination six to eight fold, in order 

 again to secure uniformity of distribution. The relative 

 brightness of red and blue for the fish had been changed 

 by light adaptation, but whether light- or dark-adapted, 

 they showed no preference for red or blue. 



One factor seems to have affected unfavorably the de- 

 terminations of Hess concerning the relative sensitivity 

 of fish after dark-adaptation. It appears that for certain, 

 experiments with light-adapted fish the aquarium contain- 

 ing the fish was removed from a lighted window to a dark 



