TRICHROMATIC VISION 93 



Yellow and blue thus appear unique in some respect. We shall see 

 other aspects of their peculiarity shortly. It is important to note here 

 only the fact that hue can be influenced by intensity. Apparently when 

 the visual mechanism is being overworked, either its peripheral analytic 

 or its central synthetic portion breaks down. We can change the hues 

 that 'go with' particular wavelengths in still another way : by fatiguing 

 the reception of a part of the spectrum we can make white light appear 

 to consist only of the remainder of the spectrum, as in the production of 

 'complementary after-images'. More important, we can fatigue the syn- 

 thetic mechanism itself, for if we stare for a time at a light which repre- 

 sents white-minus-red, and then look into a spectroscope, we will see not 

 only the red where it 'belongs', but will see nearly the whole spectrum 

 as red; and where there is no red (at the short-wave end) there is only 

 darkness.* In the same way, green or violet can be made to spread out 

 and fill almost the entirety of the spectrum, but yellow and blue cannot 

 be made to do so. No better confirmation of our choice of red, green 

 and violet as primary stimuli could be desired. 



This phenomenon shows beyond question that whatever the three 

 somethings may be which comprise the color-vision mechanism, each one 

 of them has some responsiveness for practically all visible wavelengths. 

 The results of fatiguing with colors show also that if each one of the 

 somethings could be isolated and made to act all alone, its action would 

 be to arouse a sensation of its appropriate primary hue, no matter what 

 wavelength of light happened to activate it. Most of the 160-odd sep- 

 arate qualities we can experience, then, must be due to the instigation, 

 by single wavelengths, of combined actions of the three processes, no one 

 of which alone could give us more than a single, primary, hue sensation. 



A rough idea of these combined actions is given by Figure 32. Each of 

 the three colored curves represents the spectrum of responsiveness of one 

 of the three central processes which synthesize our hue qualities, and the 

 color of the line indicates the quality it arouses when allowed to act 

 singly. When the redness and green-ness processes are equally active, 

 the quality 'yellowness' results. When the green-ness and violet-ness 



*The Ericksons have recently reported experiments which suggest that all 'fatiguing" for 

 color may be central, rather than upon a peripheral exhaustion-of-photochemicals basis. 

 Their hypnotized subjects 'saw' the proper complementary after-image colors after having 

 had hallucinatory initial color-stimuli suggested to them; and these were persons who, in 

 the waking state, did not know that there is such a thing as an after-image — let alone, 

 that it should be experted to be complementary to the stimulus! 



