144 The Animal Mind 



tendency to avoid any particular color, they indicated a pref- 

 erence for green and blue. This result it was attempted to 

 verify by pneumographic tests, and a greater quickening of 

 breathing was recorded under green and blue lights than 

 under other light stimuli (371). 



Raccoons have been trained to discriminate cards of dif- 

 ferent colors and brightnesses in the following pairs : black- 

 white, black-yellow, black-red, black-blue, black-green, 

 blue-yellow, red-green. The two last-named discriminations 

 proved decidedly more difficult, and one of the four raccoons 

 tested never learned to distinguish red from green or blue 

 from yellow (82). 



In none of the above described experiments, however, is 

 the brightness error eliminated. Kinnaman's color tests on 

 monkeys did make an attempt in this direction. The monkeys 

 were tested with glass tumblers covered with papers of differ- 

 ent colors, and when it had been shown that they were able 

 to identify a vessel of a particular color as associated with 

 food, the possibility that their discriminations might have 

 been based on brightness rather than color was investigated in 

 the following way. First, the animals' power of distinguishing 

 different shades of gray was tested, and it was found that 

 they could barely detect a difference considerably greater 

 than that between the ''brightness values" of the colors 

 used ; that is, the grays that a color-blind human being would 

 have seen in place of the colors. Secondly, this result was 

 confirmed by covering the glasses with gray papers varying 

 in brightness somewhat more than did the colors used, and 

 finding that the monkeys distinguished these grays decidedly 

 less well than the colors. Thirdly, it was proved that a 

 colored glass could be picked out correctly many times from 

 among three others covered with gray paper of the same 

 brightness as the color (221). 



