LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 65 



Washburn and Bentley used a brighter red in non-food 

 tests to detect a possible brightness discrimination by the 

 fish. 



Reighard varied the brightness of one of the two colors 

 so that one was used both lighter and darker than the other 

 color. Following the practice current at the time and 

 finding that the fish discriminate between two similar 

 objects when one is used both brighter and less bright than 

 the other for the human eye, he infers the presence of color 

 vision. The brightness values of the color used should 

 have been determined with dark-adapted eye but since 

 in no case was the attempt made to obtain matched 

 brightness, but rather to insure a considerable difference 

 in brightness, this criticism has little force. The conclusions 

 of the writers so far considered are based on the assumption 

 that the brightness values of the colors used are approxi- 

 mately the same for the fish eye, as for the human eye, 

 light-adapted. For this assumption there was no evidence. 

 Had they conducted a long series of experiments in which 

 one of the two colors was changed in intensity by small 

 degrees, they might have found a region in which the two 

 colors matched for the fish eye. This region might have 

 made itself known by temporary failure of the fish to dis- 

 criminate. A recovery of ability to discriminate in this 

 region of matched brightness would have been evidence of 

 color vision. A persistent failure to discriminate at these 

 matched values would have been evidence of color-blindness. 



Hess (1909) held the proofs of color vision so far offered 

 to be inconclusive. When he illuminated an aquarium 

 placed in a dark room, half with blue light and half with 

 red by the use of colored glasses, young (15 cm.) Atherina 

 hepsetus gathered in the blue, even when it appeared to 

 his light-adapted eye somewhat darker than the red. When 

 the red was made sufficiently bright, the fish gathered in 

 the red instead of in the blue. By properly adjusting the 

 relative brightness of the blue and the red he succeeded 

 in getting the fish distributed evenly in both halves of the 

 aquarium. These results indicated to him that the fish 

 were responding to intensity differences rather than to 

 wave-lengths. He observed that these young fish swam 



