544 THE HUMAN BODY. 



may be inclined to ridicule the statement that yellow and 

 blue when mixed give white. When, however, we mix the 

 pigments we do not combine the sensations of the same name, 

 which is the matter in question. Blue paint is blue because it 

 absorbs all the rays of the sunlight except the blue and some 

 of the green; yellow is yellow because it absorbs all but the 

 yellow and some of the green, and when blue and yellow are 

 mixed the blue absorbs all the distinctive part of the yellow 

 and the yellow does the same for the blue; and so only the 

 green is left over to reflect light to the eye, and the mixture 

 has that color. Grass-green has no complementary color in 

 in the solar spectrum; but with purple, which is made by 

 mixing red and blue, it gives white. Several other colors 

 taken three together, give also the sensation of white. If 

 then we call the light-rays which arouse in us the sensation 

 red, , those giving us the sensation orange #, yellow c, and 

 so on, we find that we get the sensation white with a, b, c, d, 

 e, f, and g all together; or with b and e, or with c and/*, or 

 with #, d, and e ; our sensation white has no determinate re- 

 lation to ethereal oscillations of a given period, and the same 

 is true for several other colors; yellow feeling, for example, 

 may be excited by ethereal vibrations of one given wave- 

 length (spectral yellow), or by a light containing only such 

 waves as taken separately cause the sensations red and grass- 

 green; in other words a physical light in which there are no 

 waves of the " yellow " length may cause in us the sensation 

 yellow, which is only one more instance of the general fact 

 that our sensations, as such, give us no direct information as 

 to the nature of external forces; they are but signs which we 

 have to interpret. The doctrine of specific nerve energies 

 makes it highly improbable that our different color sensa- 

 tions can all be due to different modes of excitation of exactly 

 the same nerve-fibres; a fibre which when excited alone gives 

 us the sensation red will always give us that feeling when 

 so excited. The simplest method of explaining our color 

 sensations would therefore be to assume that for each there 

 exists in the retina a set of nerve-fibres with appropriate 

 terminal organs, each excitable by its own proper stimu- 

 lus. But we can distinguish so innumerable and so finely 

 graded colors, that, on such a supposition, there must be an 

 almost infinite number of different end organs in the retina, 

 and it is more reasonable to suppose that there are a limited 



