84 THE VISUAL PROCESS 



brain — probably also, as we shall see, even when it is excited monocularly 

 by monochromatic yellow spectral light. We can, if we like, make an 

 artificial distinction among the psychological primaries, between those 

 which can be easily produced by mixtures and those which cannot; but 

 even red and violet, though at the ends of the spectrum, can be produced 

 by mixtures. The spectrum really has no ends — it only seems to have, 

 due to the way in which a prism forms it. Really, it is a closed entity, for 

 red and violet are adjacent, psychologically — their mixture results in 

 purple, which lies outside the spectrum but fills the gap between red and 

 violet in a spectrum which we might imagine bent into a ring (Fig. 29b). 



Though the primaries can all be synthesized, they cannot be analyzed 

 — which is what makes them primaries. In orange one can discern both 

 the red and yellow components; in purple, the blue and red. But though 

 blue can be made by mixing green and violet, it does not look as though 

 it contained either. Yellow and violet, and red and green, are sometimes 

 called 'disappearing color pairs', since when the members of such a pair 

 are mixed, neither member can be seen in the mixture. 



The mixture of three properly chosen primaries (the most convenient 

 are red, green, and violet — and these three do have, in a certain way, 

 an edge on the other two chief primaries, yellow and blue) arouses the 

 colorless sensation of white or gray, which is also afforded by mixed 

 complementary pairs of colors such as orange and green-blue, green- 

 yellow and violet, red and blue-green, etc. In each such pair it can always 

 be noted that at least one member is not a simple color or primary; and 

 the two members, between them, always contain red, green, and violet or 

 can be matched by mixtures of them in pairs. The complement of any 

 hue can also, obviously, consist of white light minus that hue. A mixture 

 may be complemented by a pure hue, and the latter by one other pure 

 hue, by simple or complex mixtures, or by white minus the first pure hue. 



Saturation — The whole of the sensation aroused by a colored light or 

 object has aspects other than hue itself. It has brightness of course, the 

 psychological counterpart of physical intensity as with achromatic stim- 

 uli; and it has saturation. Saturation means coloredness as apart from 

 color, and quite apart from brightness. In a darkroom we could aim, 

 at the same ground-glass, a beam of pure colored light and a beam of 

 white light. The ratio of color to white in the resulting spot of light 

 would be the measure of its saturation. With more white added, the 

 saturation would go down and the brightness would go up; but instead 



