THE SENSES 941 



quantity of ordinary white light. From all this it follows that 

 the retina may be excited by an infinite number of different 

 physical stimuli, and yet the resultant sensation may be the 

 same. This leads straight to the conclusion that somewhere 

 or other in the retino-cerebral apparatus simplification, or syn- 

 thesis, of impressions must take place ; and we have to inquire 

 what the simplest assumptions are which will explain all the 

 phenomena. Now, it is not possible, from two spectral colours 

 alone, to produce a sensation corresponding to all the others. 

 By mixing three standard spectral colours, however, in various 

 proportions, we can produce not only the sensation of white 

 light, but that of every colour of the spectrum (and of purple). 

 These statements are based on demonstrated facts obtained by 

 very numerous experiments on colour mixtures. The hypotheses 

 framed to explain the facts are to be carefully discriminated 

 from the facts themselves. 



Primary Colours. The simplest assumption we can make, 

 then, is that there are three standard sensations, and that either 

 the retina itself can respond by no more than three distinct 

 modes of excitation to the multiplex stimuli of the luminous 

 vibrations, or that complex impulses set up in the retina are 

 reduced to simplicity because the central apparatus is capable 

 of responding by only three distinct kinds of sensation. Which 

 three sensations we select as fundamental or primary is, to a 

 certain extent, arbitrary. Fick chose red, green, and blue ; 

 most commonly red, green, and violet are accepted as the 

 primary colours. Red, yellow, and blue, although so long con- 

 sidered the primary colours, from data yielded by the mixture of 

 pigments, will not do ; for no possible combination of them will 

 produce either a pure green or white light. 



The Young-Helmholtz Theory. The theory which has been 

 most widely accepted is that of Young, generally called, on account 

 of its adoption and extension by Helmholtz, the Young-Helmholtz 

 theory. Red, green, and violet are taken as the fundamental or 

 elementary colour sensations. In its more modern form it assumes 

 that in the retina, or in the retino-cerebral apparatus, there are 

 three kinds of elements (i) a substance or a component 

 chiefly affected by light of comparatively long wave-length 

 (red), to a less extent by light of medium wave-length (green), 

 and to a still less extent by the shortest visible waves (violet) ; 

 (2) a component mainly affected by medium, but also to a 

 certain extent by long and short waves ; (3) a component chiefly 

 affected by the short vibrations, less by the medium, and still 

 less by the long waves. The curves in Fig. 413 illustrate these 

 relations. 



The theory explains as follows the phenomena of colour- 



