SECTION III 



THE THREE-COMPONENTS THEORY (YOUNG-HELMHOLTZ) 



CHAPTER I 



STATEMENT OF THE THEORY 



The Yoiing-Helmlioltz theory of colour sensations is based upon the 

 facts of the mixture of pure-colour stimuli (see Part I, Sec. II, Chap. iii). 

 We found that within a certain range, which includes all ordinary 

 conditions of colour- vision, and with certain well-defined exceptions 

 {v. p. 36), every conceivable light or light-mixture gives rise to a sensa- 

 tion which can be accurately matched by the sensation produced by a 

 suitable mixture of only three lights. If we choose three actual wave- 

 lengths, R, G, and F, on the colour-diagram and join them by straight 

 lines, we find that part of the curve is outside the triangle thus obtained. 

 This means that there are certain spectral colours which cannot be 

 obtained in their full spectral saturation by the mixture of R, G, and V. 

 If we choose a point on the curve outside the triangle, e.g., GBl, we can 

 by experiment obtain an equation of the following form : 



aR + ^GBl ^yG + eV. 



We thus find an expression for GBl 



^GBl = yG + eV - aR 



which represents the unmixable colour in terms of R, G, and V. 



In order to avoid negative quantities we must assume the existence 

 of colour-sensations which lie outside the colour-diagram. The fact 

 that, by previously stimulating the eye with the complementary colour, 

 we can obtain a colour-sensation which in saturation far exceeds that 

 obtained by viewing the spectral colour without such previous excitation 

 {v. p. 109), affords some evidence that this assumption is not unwarranted. 



