CHAPTER X 

 THE SENSE OF SIGHT: COLOR VISION (Continued) 



THERE are three well-known ways in which colors may 

 be used as stimuli in experiments on animals : by the use 

 of colored papers (reflected light); by the use of a prism 

 (the spectrum which is obtained may be used as directly 

 transmitted or as reflected light) ; and by the use of light 

 filters (transmitted light). In the experiments on the color 

 vision of the dancer which have thus far been described only 

 the first of these three methods has been employed. Its 

 advantages are that it enables the experimenter to work in 

 a sunlit room, with relatively simple, cheap, and easily 

 manipulated apparatus. Its chief disadvantages are that 

 the brightness of the light can neither be regulated nor 

 measured with ease and accuracy. The use of the second 

 method, which in many respects is the most desirable of the 

 three, is impracticable for experiments which require as large 

 an illuminated region as do those with the mouse; I was 

 therefore limited to the employment of light filters in my 

 further tests of color discrimination. 



The form of filter which is most conveniently handled is 

 the colored glass, but unfortunately few glasses which are 

 monochromatic are manufactured. Almost all of our so- 

 called colored glasses transmit the light of two or more regions 

 of the spectrum. After making spectroscopic examinations 

 of all the colored glasses which were available, I decided 

 that only the ruby glass could be satisfactorily used in my 



