Sensory Discrimination: Vision 171 



pure colors ; since, that is, they reflect to the eye light of 

 wave-lengths besides that of their predominant color, the 

 use of pure spectral light is to be preferred. The ap- 

 paratus by which such light can be used with its intensity 

 accurately controlled is very elaborate, and was devised 

 by Yerkes and Watson (831). Using this apparatus, the 

 Watsons (772) found that rats and rabbits failed to 

 distinguish between red and darkness. A similar indica- 

 tion had been previously obtained by Waugh (775) on the 

 gray mouse ; he found that red filters and pigments could 

 be distinguished from gray when the two were equal in 

 brightness to the human eye, but that the discrimination 

 tended to disappear when the red was made lighter, and 

 to improve when it was made darker. 



The dog and cat also suffer under the imputation of color- 

 blindness. Tests by Pawlow's method on the dog failed 

 to indicate that it can react to color differences as such 

 (830) . Smith (687) , it is true, working with colored papers, 

 argues in favor of the dog's color vision from the fact that 

 the dogs showed some evidence of learning to distinguish 

 the colors from all the grays used. De Voss and Ganson 

 (184) found that none of the six colored papers they used 

 could be discriminated by cats from all the shades of gray 

 in their series : each color was confused with some partic- 

 ular gray. Even the monkey is suspected of color-blind- 

 ness: Watson (768) reports as the chief result of his 

 experiments with this animal that red has little or no 

 stimulating power upon it. 



