ASSOCIATION AND COLOR DISCRIMINATION 481 



Experiments with monochromatic blue and yellow filters 



Tests with blue and yellow filters were applied to two stickle- 

 backs, no. 65 and no. 66. The lights were flashed alternately 

 upon the fishes, the blue being associated with food and the 

 yellow with paper. The yellow light seemed to the human eye 

 considerably brighter than the blue, and the blue plate was much 

 more opaque than the green plate which was used in combination 

 with the yellow in the test made upon mudminnow no. 60. If 

 intensity of light was the determining factor in these discrimi- 

 nations by the fishes, the yellow-blue combination offered the 

 greatest contrast of any experiment tried and might have been 

 expected to secure the most positive results. But, strange to 

 say, the records of this experiment fail to show any clear evi- 

 dence of discrimination. Only 22.22 per cent of the total number 

 of trials proved to be correct for stickleback no. 65, and 16.12 

 per cent in the case of stickleback no. 66. It was demonstrated 

 in a later experiment that the failure was not due to the complex- 

 ity of the test since these fishes learned to discriminate red and 

 green lights (fig. 7, A, B; table 3). 



Experiments with monochromatic red and green filters 



Red and green lights were presented alternately to four 

 sticklebacks including the two which had given negative results 

 in the experiment with blue and yellow lights just described. 



The experiments on no. 65 and no. 66 were among the last 

 tests made, but an account will be given of them here so that 

 these results may be compared with the yellow-blue experiment. 

 The fishes very soon demonstrated that they detected qualitative 

 differences in the green (no. 74) and red (no. 71) lights which 

 were flashed upon them, reacting differently to each light. 

 After several trials the fishes exhibited hesitation in approach- 

 ing the forceps in red light or refused to do so altogether, while 

 in every case they snapped at the forceps in green illumination. 

 The test lasted only seventeen days for stickleback no. 66 and 

 twenty days with stickleback no. 65, since it was the purpose 

 of this experiment to observe whether they could discriminate 



