PHYSIOLOGY OF COLOK VISION KELLER AND MACLEOD. 727 



seven spectral colors a seiiHation approaching that of white is pro- 

 duced ; ^ by choosing various proportions of the spectral colors this 

 white becomes tinted with all possible intermediate hues. 



From these facts we might imagine that the retina contains a 

 special kind of sensory component for each of the seven spectral 

 hues, that equal stimulation of all produces the sensation of white, 

 and that varying degrees of stimulation of certain of them, that of 

 the hues which are intermediate between those of the spectrum. 



Such an hypothesis could not, however, be of much practical value 

 in explaining the color jDhenomena with which we have to deal in 

 daily life. It had to be simplified. This was done by Thomas 

 Young and Helmholtz, who discovered that three of the spectral 

 hues, such as red, green, and violet, or certain other triads, are suffi- 

 cient, 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 ; 

 Avhen green and vio- 

 let, it is blue; and 

 when violet and red. 

 it is purple. Not only 

 this, but the xarious 

 intermediate hues can 

 readily be obtained by 



altering the propor- R' ^--- — " ~ " "b -^ V 



tions of the primaries ; p.,^,. i.-color triangle. 



thus to produce 



orange, a disk containing a larger proportion of red and a smaller 



proportion of green is used, and so on. 



To represent these fundamental facts and hold them in mind the 

 so-called color triangle has been constructed (fig. 1). At the angles 

 of this triangle are placed the primary hues, the other spectral hues 

 being distributed along its two sides at distances which are propor- 

 tional 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 

 caljed 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 wdien mixed with it on the retina in approximately equal quan- 



1 It would be pure white were it possible to obtain artificial pigments that reflected none 

 other than their own characteristic hues. 



