1 68 The Animal Mind 



(3) Each light was in turn exposed alone, one passage being 

 left dark. 



Indications of the presence of darkness adaptation in 

 the chick appeared from the facts that light adapted chicks 

 chose red and yellow rather than green, while for dark 

 adapted chicks the preference was reversed. 



Rouse observed that differently colored lights had on 

 the average different effects in quickening the rate of 

 breathing in the pigeon; the strongest effect being pro- 

 duced by blue, the weakest by red (648). 



44. The Problem of Visual Qualities: Mammals 



The earlier experiments on the visual discriminations of 

 mammals, like those with other animals, failed to reckon 

 adequately with the brightness error, the possibility that 

 discriminations between colors are made as a color-blind 

 human being would make them, the colors being seen as 

 different shades of gray (138, 140). The first method, 

 as we' have seen, which suggested itself as a means of 

 eliminating this error was that of showing that an animal 

 could, or could not, distinguish a color from the gray 

 which a light-adapted human being would see in its 

 place. Such a gray can be determined by the so-called 

 " flicker method." If a disk be made of a colored and 

 a gray paper, when it is rotated a little too slowly to 

 give a smooth mixture, the peculiar appearance of " flick- 

 ering" will be observed if the color and the gray are not of 

 equal brightness, but will disappear when a gray equally 

 bright with the color is selected. The determination of 

 this equivalent, however, has really no bearing on the 

 problem of color vision in animals. If they are color-blind, 

 their difficulty would more probably lie in distinguishing 



