454 THE POPULAR SCIENCE MONTHLY 



kind of sensory component for each of the seven spectral hues, that 

 equal stimulation of all produces the sensation of white and that vary- 

 ing degrees of stimulation of certain of them, that of the hues which are 

 intermediate hetween those of the spectrum. 



Such a hypothesis could not however be of much practical value in 

 explaining the color phenomena with which we have to deal in daily life. 

 It had to be simplified. This was done by Thomas Young and Helm- 

 holtz, who discovered that three of the spectral hues, such as red, green 

 and violet, or certain other triads, are sufficient, when mixed on the 

 retina, to produce the same sensations as those which are produced by 

 the seven spectral hues. These are known as primary colors; when 

 equal quantities of each are used a sensation of white (or gray) results; 

 when only red and green, the sensation is yellow ; when green and violet, 

 it is blue ; and when violet and red, it is purple. Not only this, but the 

 various intermediate hues can readily be obtained by altering the pro- 

 portions of the primaries; thus, to produce orange a disc containing a 

 larger proportion of red and a smaller proportion of green is used, 

 and so on. 



R c ~" p 



Fig. 1. Coloe Triangle. 



To represent these fundamental facts and hold them in mind the 

 so-called color triangle has been constructed. At the angles of this 

 triangle are placed the primary hues, the other spectral hues being dis- 

 tributed along its two sides at distances which are proportional to their 

 wave-lengths and the purples along its base, which, since these hues are 

 absent from the spectrum, is represented by a broken line. 



But white light can be produced in still another way, namely by 

 retinal synthesis of certain pairs of hues which on this account are 

 called complementary. Thus red and greenish-blue, yellow and blue, 

 orange and blue-violet are complementary. We may express this all- 

 important fact by stating that for every spectral hue there is another 

 which when mixed with it on the retina in approximately equal quanti- 

 ties produces the sensation of white. When other than equal proportions 

 of complementary hues are chosen, colors are produced which are of hues 

 intermediate between those of the complementaries and which are mixed 

 with varying degrees of white. They are incompletely saturated colors. 

 These facts may be satisfactorily represented by finding a point, called 



