THE HUMAN BODY. 



are due to ethereal vibrations of totally different wave- 

 lengths; in other words a physical light in which there are 

 no waves of the "yellow" length may cause in us the sen- 

 sation yellow, which is only one more instance of the gen- 

 eral 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 modern view of 

 specific nerve energies (p. 191) makes it highly improbable 

 that our different color sensations 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 ex- 

 citable by its own proper stimulus. 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 number of primary color 

 sensations, and that the rest are due to combinations of these. 

 That a compound color sensation may be very different 

 from its components when these are regarded apart, is 

 clearly shown by the sensation white aroused either by 

 what we may call red and blue-green, or green and purple, 

 stimuli acting together; or of yellow due to grass-green and 

 red. To account for our various color sensations we may, 

 therefore, assume a much smaller number of primary sen- 

 sations than the total number of color sensations we expe- 

 rience; all can in fact be explained by assuming any three 

 primary color sensations which together give white, and 

 regarding all the rest as due to mixtures of these in various 

 proportions; there may be more than three, but three will 

 account for all the phenomena, black being a sensation 

 experienced when all visual stimuli are absent. This is 

 known as Young's theory of color vision, and is that at 

 present most commonly accepted. The selection of the 

 three primary sensations is somewhat arbitrary, but they 

 are usually regarded as red, green, and violet. It is 



