36 ROBERT M. YERKES AND JOHN B. WATSON 



For green the same is true. A monochromatic green can not 

 be obtained with any of the glasses, or combinations thereof, 

 M^hich we have examined. 



For blue a glass may be obtained which transmits no red, 

 but blue can not be obtained without some violet. 



For violet there is no satisfactory glass or combination of 

 glasses. 



From these statements it is evident that among the colored 

 glasses at present manufactured, it is extremely difficult to 

 find satisfactory filters for use in the study of color vision. 

 With commercial glasses it is easy to obtain two mutually 

 exclusive chromatic stimuli (red and blue- violet for example), 

 and it is possible to divide the spectrum into three mutually 

 exclusive parts (red, yellow-green, blue-violet), but the search 

 for the proper glasses, and combinations thereof, for the latter 

 purjDOse is likely to prove discouraging and scarcely worth 

 while, in view of the possibility now to be mentioned of obtain- 

 ing good gelatine filters. 



The ruby glass of Grosse we find perfectly satisfactory as a 

 source of red light, and could equally good glasses be obtained 

 for yellow, green, blue, and violet, our qualitative experiments 

 on color vision w^ould be greatly facilitated. 



On the whole, it seems desirable that some one should attempt 

 to discover formulae for a set of colored glasses which shall 

 be as satisfactory as the ruby. The task is a difficult one, and, 

 although we have considered undertaking it, we have not found 

 time nor opportunity to do so. 



Gelatines. — Color filters consisting of pigmented films of 

 gelatine yield color stimuli by selective absorption and trans- 

 mission. There are, so far as we know, no gelatines on the 

 market which, when used singly, give monochromatic stimuli, 

 but by combining certain of the films now available it is possible 

 to obtain fairly satisfactory filters for red, green, yellow, and 

 blue. There is no apparent reason why a standard set of gelatine' 

 filters which should yield respectively red, yellow, green, blue, 

 and violet light should not be manufactured. They should be 

 prepared with extreme care, according to definite formulae, by 

 a reliable scientific firm, and the experimenter would need to 

 be on his guard against fading, for one of the chief defects of 

 gelatine films is their extreme liability to fade. 



