THE MIXTURE OF PURE-COLOUR STIMULI 37 



laws of colour mixing, this construction is valid in all possible cases, 

 i.e. that the situation of the mixed colour coincides with that of the 

 mass centre of two equivalent masses (1) when the two constituents 

 can be mixed from the three chosen colours ; (2) when one can and the 

 other cannot so be mixed ; (3) when neither can be so mixed." v. Helm- 

 holtz 1 has supplied the mathematical proof. 



The diagram will vary in form according to the source of light and 

 according to the choice of units and fixed points. It is best represented 

 in such a form as in Fig. 7. 



Green 



/ 



Yellow 



Orange 



'Violet 

 Purple 



Fig. 7. Diagram of a colour table. 



The position of the pure white sensation is obtained by dividing the 

 line joining any two complementary colours according to the relative 

 amounts of those colours required to produce white when mixed to- 

 gether. The deviation of the curve from a straight line beyond 540 //,/A 

 indicates the unsaturated nature of the mixtures. 



Since, for example, mixtures of green and violet are less saturated 

 than spectral cyan-blue, the curve must deviate further from the white 

 point than the straight line joining green and violet. From experi- 

 mental results it is found that the curvature is sharpest in the green. 



These facts are quite independent of any theory of colour vision 

 whatever, and their importance is absolutely fundamental. " The 

 colour table merely expresses in systematic manner directly observed 

 physiological equivalents of objectively different lights ' : (v. Kries). 

 The facts show that the totality of physiological activities comprises 

 far fewer elements than the objective stimuli. The varieties of stimuli 

 of all possible mixtures derived from a source of constant intensity can 

 be reproduced on a co-ordinate system as points in a plane. 



If we introduce variations of intensity the law can be stated in general 

 terms : " The entire physiological valency of every conceivable light and 



1 Helmholtz, 3rd ed., n. p. 112. 



