86 THE VISUAL PROCESS 



SO many mixed pairs of colors produce sensations which are not analyz- 

 able blends of their qualities, but entirely new qualities. 



Apart from the kind of unsaturation which may be produced syn- 

 thetically so to speak, by mixing into a color some whiteness from a 

 separate source entirely outside the color, there is a type of unsaturation 

 which is inherent in the colored light itself, even in a spectral light of 

 whatever purity. It is as though the monochromatic spectral beam con- 

 tained some white light which we could not remove. This kind of un- 

 saturation is due to the fact that the visual mechanism for the perception 

 of white is set in operation to some extent by any one wavelength — to a 

 greater extent by some than by others. If we look at a solar spectrum, 

 the yellow region (about A,580m[i,) looks brightest to us, and also looks 

 the least richly colored. We can separate this pallidity of yellow from 

 its high brightness, by turning to a spectrum in which each wavelength 

 represents the same amount of energy. In such a spectrum, the yellow- 

 green region (around A<557m[x) is now the brightest; but the yellow still 

 seems the least colored color, the richness of the chromas increasing from 

 it toward both ends of the spectrum. 



This kind of unsaturation, or low chroma, is particularly important 

 physiologically and psychologically. It greatly influences the results of 

 color-mixtures, for the saturation of mixtures is always low. If for exam- 

 ple we mix red and green to make yellow, the yellow we obtain is of 

 low saturation as compared even with spectral yellow, and to spectral 

 yellow we must add some white light to make a perfect match with the 

 red-green mixture. The more complex a mixture, the lower the satura- 

 tion, for we are approaching the result of mixing all wavelengths — which 

 is, of course, white itself, with the chroma-content at zero. 



The degree of saturation of a spectral light can be ascertained by 

 determining how much of it, added to white, will give that white a hint 

 of chroma. By such means, red and particularly violet are revealed as 

 highly-saturated wavelengths, yellow and green as being of low chroma. 

 We therefore say that the 'white valence' of yellow is high, by which we 

 mean that we can add yellow to another color without altering the hue 

 much more than if we had added the same amount of white. Red or 

 blue, added bit by bit to another color, have more prompt effects upon its 

 appearance — they have a low white valence, cannot take the place of 

 very much white in mixtures. 



Recalling that unsaturation is usually accomplished by actual objective 

 admixture with white, we can now see that when the degrees of unsatura- 



