Sensory Discrimination : Vision 145 



The most elaborate and careful experiments that have yet 

 been made on vision in the lower animals are those of 

 Yerkes on the Japanese dancing mouse. The method con- 

 sisted in teaching the animals to associate one of two differ- 

 ently illuminated compartments with a disagreeable electric 

 shock. In the perfected experiments on brightness dis- 

 crimination, the illumination of the compartments was 

 varied in intensity by arranging a light above each. One 

 light could be kept at a constant height and the other raised 

 or lowered. Weber's Law was proved to hold for the one 

 individual tested ; the ratio of the difference in brightness to 

 the absolute brightness being about one-tenth, between the 

 limits of 5 and 80 hefners of absolute brightness. For test- 

 ing color discrimination, after a series of experiments with 

 colored papers, a somewhat similar apparatus was used, 

 the light being filtered through colored screens (Fig. 12). 

 No ability to discriminate green and blue was displayed 

 unless the two were made very different in brightness. Light 

 blue and orange, green and red, violet and red, were dis- 

 criminated even when their brightnesses were considerably 

 varied. Yet the possibility that these discriminations were 

 made on the basis of brightness rather than color differences 

 is suggested by an interesting kind of evidence. After a 

 mouse had learned to choose, for example, green rather than 

 red, it was offered a choice between light and darkness, 

 and showed a uniform preference for the former, although 

 untrained mice do not. This looks as though the green had 

 been previously chosen as the lighter of the two colors. If 

 such were the case, the brightness values of the colors for the 

 mouse must be quite different from those which they have for 

 a human being. In fact, there are reasons for thinking that 

 the red end of the spectrum is much darker to the mouse's 

 than to the human eye. Even allowing for the possibility 



