550 THE HUMAN BODY. 



relationship. All other color sensations, as orange, suggest 

 two of the above, and may be described as mixtures of them; 

 but they themselves stand out as fundamental color sensa- 

 tions. Moreover, it follows from the above, that more than 

 two simple color sensations are never combined in a com- 

 pound color sensation. 



Since red always excludes green, and yellow blue, we may 

 call them anti-colors (the complementary colors of Young's 

 theory), and are led to suspect that in the visual organ there 

 must occur, in the production of each, processes which pre- 

 vent the simultaneous production of the other, since there is 

 no a priori reason in the nature of things why we should not 

 see red and green simultaneously, as well as red and yellow. 

 Along with our color sensations there is always some color- 

 less from the black- white series; which we recognize in speak- 

 ing of lighter and darker shades of the same color. 



Hering assumes, then, in the retina or some part of the- 

 nervous visual apparatus, three substances answering to the 

 black-white, red-green, and yellow-blue sensational series, the 

 construction of each substance being attended with one sen- 

 sation of its pair, and its destruction with the other. Thus, 

 when construction of the black-white substance exceeds de- 

 struction, we get a blackish-gray sensation; when the pro- 

 cesses are equal the neutral gray; when destruction exceeds 

 construction a light-gray, and so on. In the other color 

 series similar things would occur; when construction of red- 

 green substance exceeded destruction in any point of the 

 retina we would get, say, a red feeling; if so, then excess of 

 destruction would give green sensation. The intensity of 

 any given simple sensation would depend on the ratio of the 

 difference between the construction and destruction of the 

 corresponding substance, to the sum of all the constructions 

 and destructions of visual substances going on in that part of 

 the visual apparatus : in this way anabolic and katabolic 

 nutritive processes would be the material basis of visual sen- 

 sations. The intensity of a mixed color sensation would be 

 the sum of the intensities of its factors, and its tint and 

 shade dependent on the relative proportion of these factors. 

 When the construction and destruction of the red-green sub- 

 stance are equal no color sensation is aroused by it; and we 

 get gray, due to those simultaneously occurring changes in 

 the black- white substance which are always present, but were 



