98 THE VISUAL PROCESS 



The dichromate, in distinguishing most natural colors, must fall back 

 upon saturation- and brightness-differences. The former are much the 

 more important to him. Longwave colors look alike in hue to him, but 

 very different in saturation. It is widely supposed, even by some expert 

 psychologists, that a dichromate motorist tells red traffic signals from 

 green ones on a basis of brightness, and is helpless to do so when bad 

 weather dims them both. This is not the case. The brightness of the red 

 and green lights could be varied up or down, or the red light made much 

 brighter than the green (the reverse is usually true) without inverting 

 his identifications; for the two lights would still retain their very different 

 saturations. 



For a long time, Daltonism was thought to be due to a literal absence 

 of one of the three sets of receptors, or photochemical substances, or 

 cerebral perceptual processes, of the Young-Helmholtz scheme of things. 

 It was the physiologist Fick who showed, many years ago, that this could 

 not be the explanation; but the lack-of-one-process theory is still taught 

 far and wide. To adjust Figure 32 to represent dichromatic vision in ac- 

 cordance with Fick's contributions, none of the colored curves should 

 be removed. It is only necessary to suppose that the spectrum of respon- 

 siveness of one of the three 'somethings' has shifted into coincidence 

 with that of one of the other two. 



To be specific, let us suppose that the redness curve is altered so that 

 it superimposes upon the green-ness curve, and see what should inevit- 

 ably result in the vision of the individual. Firstly, the spectrum would 

 be shortened at the red end even in bright light. Secondly, redness and 

 green-ness would always be contributed equally to the sensation evoked 

 by all wavelengths from 650m[i to 476m[X. So, in this whole great spec- 

 tral region the individual could see only yellow with varying degrees of 

 saturation and brightness. He would have to learn to call the highly- 

 saturated wavelengths red, and to call the less saturated ones yellow or 

 green. Thirdly, from 7.476m[A on to the ultra-violet, only violetness 

 could be experienced, with saturation increasing as wavelength decreased. 

 But his spectrum would contain something besides yellow and violet; 

 for (fourthly) at X476m\i all three processes would be in action to the 

 same degree : white would result at this 'neutral point' in his spectrum. 

 Fifthly and lastly, purple would not exist for him, for since redness and 

 green-ness were inextricably tied together as yellowness in the long- 

 wave part of the spectrum, the mixture of any wavelengths there, even 

 those seen by the normal as red, with any of the wavelengths seen by 



