THE SENSES 823 



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 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 





FIG. 316. CURVES OF EXCITABILITY OF PRIMARY SENSATIONS FROM 



OBSERVATIONS ON COLOUR MIXTURES (KONIG). 

 The numbers give wave-lengths of the spectrum in millionths of a millimetre. 



the sensation of white light, but that of every colour of the 

 spectrum. 



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 appa- 

 ratus is capable of responding by only three distinct kinds 

 of sensation. Which three sensations we select as funda- 

 mental or primary is, to a certain extent, arbitrary. Pick 

 chooses red, green, and blue ; most commonly red, green, 

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



