Sensory Discrimination: Vision 169 



between a color and its brightness equivalent for the color- 

 blind or dark-adapted human eye ; and quite possibly the 

 brightness which they see instead of color may be unlike 

 the brightness value of that color to either the light-adapted 

 or the dark-adapted human eye. 



Kinnaman's (401) color tests on monkeys, from which 

 he concluded that they possess color vision, employed only 

 the older methods of getting rid of the brightness error : 

 the monkeys, which had learned to identify a vessel covered 

 with a particular colored paper as containing food, were 

 shown to be unequal to the discrimination between gray 

 papers whose brightnesses were to the human eye the same 

 as those of the colors. It was also shown that a colored 

 glass could be picked out many times from among three 

 others covered with gray paper of the same brightness as 

 the color, to human vision. In Cole's (134) demonstration 

 that raccoons can distinguish colors, the colors used were 

 equated in brightness for the human eye by the flicker 

 method. 



The experiments of Yerkes on the dancing mouse (820) 

 brought into clear relief the danger of trying to eliminate 

 the brightness error by the use of grays equal in brightness 

 with the colors to the human eye. His method consisted 

 in teaching the animals to associate one of two differently 

 illuminated compartments with an electric shock. The 

 intensity of the illuminations could be regulated by varying 

 the distance of the lights from them. When only white 

 lights were used, Weber's Law was found to hold for the 

 one mouse tested : the animal could distinguish a differ- 

 ence in the brightness of the compartments amounting to 

 about one-tenth of their absolute brightness, within cer- 

 tain limits of absolute brightnesses. Light blue and 

 orange, green and red, violet and red, were discriminated 



