214 COLOUR VISION 



The assumption is minimised by describing a triangle around the 

 diagram which will only just succeed in including every part of the 

 diagram. Such a triangle is seen in Figs. 8 and 9 (pp. 39, 40). We 

 can then obtain a universal equation for any spectral colour, F, viz. 



F = zR + yG + zV 



in terms of the three theoretical sensations, R, G, and V, in which 

 x, y, z are all real positive quantities. 



The objective quantities of light in this equation come into con- 

 sideration here only as sensation-stimuli, and as such have a physically 

 measurable value. If we make a further assumption, viz., that the 

 physiological processes which accompany sensations have a definite 

 quantitative relationship to the physical stimuli, we may say that 

 x, y, and z are respectively the red, green, and violet values of the light 

 F in terms of the fundamental colours R, G, and V 1 . 



It also follows that in the sensory process there must be three 

 corresponding kinds of activity, which coexist side by side without 

 interfering with each other, and upon which every variety of colour 

 sensation depends. 



Let us suppose that there is some method whereby we can determine 

 three measurable quantities, (/>, %, tfi representing three physiological 

 processes, which taken together completely define the character of the 

 visual sensation. We should then be able in every case to find by 

 observation the relationship between c/>, x, ifj and the values x, y, z of the 

 incident light. We should then have <, x, ift represented as three 

 functions of x, y, z and conversely x, y, z as three functions of (f>, x, <A- 

 That is, 



</>=/! (a, y, z), x=A ( x > y> z ), t f t =fs fa, y, z) and 



x =fi (</>> x> <A)> y=/2 / (^x0). 3=/3'(0x>0)- 



Since no two different groups of values for x, y, z give the same sensation, 

 i.e., the same values of <, x> *{j, therefore x, y, z can be expressed solely 

 by (j>, x, <A- These values of the x, y, z functions of </>, ^, ifj (i^;fi,fz>fa f ) 

 are therefore quantities which depend only upon the character of the 

 sensation, and moreover possess a certain individuality, since each can 

 be aroused, exist and again disappear in the nervous apparatus inde- 

 pendently of the other two and unaffected by them. This independent 

 existence is, however, exactly what we are in search of when we speak 

 of elements, ingredients, or components of the sensation. If we therefore 



1 See v. Helmholtz, 2nd ed. p. 341. 



