88 THE VISUAL PROCESS 



It is only by coincidence that the Purkinje shift has the particular 

 extent that it has, in any given retina. The luminosity maxima of the 

 scotopic and photopic spectra might just as well happen to be farther 

 apart in wavelength, or closer together; or even, by chance, identical, 

 for they are determined by very different factors. In the one case, the 

 maximum is determined by the maximum of absorption of rhodopsin — 

 in the other case, by the peak in the resultant absorption spectrum of 

 the photochemical substances in the cones. In some animal with a rho- 

 dopsin of slightly different color, and with a slightly different color- 

 vision system, the Purkinje shift might be much greater or much less 

 than in man — or could conceivably be absent (or even might take place 

 in the opposite direction, though no such case is known.) 

 f Trichromatic Vision — The fundamental qualities of cone-mediated 

 sensations, then, are hue, saturation, and brightness. At least a part of 

 the whole process by which these qualities are established in conscious- 

 ness is essentially physiological. A part of the process is psychological. 

 It would be very nice, considering the avowed scope of this book, if we 

 could carry our treatment of cone vision just to the boundary line and 

 stop. But unfortunately there is no branch of psycho-physiology in which 

 it is more difficult to say where more-or-less 'physiological' sensation ends 

 and strictly 'psychological' perception begins. Some hues, such as red, 

 green, and violet, appear to be simple sensations. Others, like orange 

 and yellow-green, are mixtures analogous to the sour-sweetness of lemon- 

 ade or to a chord in music — trained observers can always discriminate 

 the separate elements of the complex. But then there are hues, pure 

 yellow and pure blue, which seem to be more like percepts than sensa- 

 tions, for each is the product of two simultaneously-evoked sensation 

 elements, yet cannot be analyzed into those elements. Here, the sum 

 differs from its parts in a qualitative manner — it is as though when we 

 hybridized horses with zebras, the offspring were always giraffes! 



Since the sensations of all hues and white can be aroused by appro- 

 priate mixtures of three wavelengths — primaries — chosen from the ends 

 and middle of the spectrum, normal human vision is said to be trichrom- 

 atic (tri = three) . It was an eighteenth-century French printer, LeBlond, 

 who discovered (through a misinterpretation of Newton's writings) that 

 with only seven colored inks, and black, he could print pictures contain- 

 ing the whole gamut of colors theretofore obtainable only with a legion 

 of inks. Being a very economical person, LeBlond experimented further 

 and found that he could get along with only three colored inks. Thomas 



