VISION 1053 



colour produced by the mixture of two complementary colours may 

 be equally well produced by the mixture of the corresponding 

 spectral colour with a certain quantity of ordinary white light. 

 And it is found that when two spectral colours which are not com- 

 plementary are mixed together the resultant is not white, but a 

 colour which may be matched by some spectral colour lying between 

 the two (or by purple), either without addition or plus a larger or 

 smaller 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 synthesis, of im- 

 pressions 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 dis- 

 criminated 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, pick 

 chose red, green, and blue; most commonly red, green, and violet 

 are accepted as the primary colours. Red, yellow, and blue, 

 although so long considered 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, 



