TRICHROMATIC VISION 95 



processes are equal, the resulting sensation is 'blue'. When all three are 

 equally activated (which of course cannot be brought about by any one 

 wavelength) 'white' results. 



At any one wavelength the ordinate, or height of the curves, has a 

 heavy portion where it lies below all three curves. This represents equal 

 amounts of activity of all three processes, and so represents the white 

 valence, or unsaturating whiteness-component, of the sensation aroused 

 by that wavelength. It needs of course to be given triple weight in any 

 estimation of the relative whiteness- and chroma-contents of the various 

 color sensations — their degrees of saturation. Above the triple line, the 

 remainder of the ordinate represents chroma. The part of it which lies 

 under two curves, taken twice, represents equal joint action of the pro- 

 cesses represented by the two uppermost curves. At ?.582m(X for example, 

 the two uppermost curves cross and these processes are therefore equally 

 aroused, yielding the compound sensation of yellow, diluted by a great 

 deal of whiteness indicated by the heavy part of the ordinate lying 

 below all three curves. Near the ends of the spectrum all of the ordinate 

 represents chroma, which is another way of saying that these wave- 

 lengths are seen with complete saturation. 



The unique character of yellow is now readily comprehensible from 

 the graph. It results from the equal action of two processes which singly 

 would yield respectively redness and green-ness, neither of which can 

 be seen in yellow. Blue has a similar mode of origin — it is the unpre- 

 dictable giraffe progeny of the horse of green and the zebra of violet. 

 All of the sensation-qualities of mixed character except yellow and blue 

 owe themselves to simpler blendings of sensation-components which, as 

 with purple and orange, can still be discerned in the blend. The very 

 names we use for mixed colors — bluish-red, reddish-yellow, and so forth 

 — emphasize the simple character of their mixtures. On the other hand, 

 no one would ever call yellow 'reddish-green', or blue 'greenish-violet' — 

 and yet, in their genesis, that is what they are. 



Let us consider just one of these mixed colors whose whole is merely 

 the sum of its parts: orange. It will serve to exemplify the manner in 

 which all such mixed colors are registered. At wavelength 600m [i in 

 Figure 32, it will be seen that the double portion of the ordinate below 

 the curve of the green-process is only half as tall as the part between the 

 green and the red curves. But this part which is under the green curve 

 is under the red curve as well, and hence is to be 'taken twice'. More- 

 over, it represents equal contributions of redness and green-ness to the 



