STATEMENT OF THE THEORY 215 



denote the function of </>, x^ 'P which represents x by R, and the correspond- 

 ing other two functions by G and V, then these quantities, R, G, V, are 

 to be denoted as components of the colour sensation, and similarly 

 any linear function of them (aR + 6G + cV) may also be thus denoted. 

 From the mathematical elaboration of these assumptions v. Helm- 

 holtz stated the Young-theory in the following form : 



" (1) In some part of the conducting nerve substance, under the 

 influence of coloured light, three different, independent, and mutually 

 unopposed elementary activities arise ; we will them call the elementary 

 stimulations. Their amount is directly proportional to the correspond- 

 ing colour-values, x, y, z of the objective light ; they correspond to the 

 R, G, V of the above description. 



" (2) All activities passing further towards the brain, as well as 

 the sensations actually entering into consciousness under the given 

 conditions of the reacting brain, are only actions of the three elementary 

 stimulations, R, G, V, and in amount are functions, cf), x^ ^> of those 

 elementary stimulations. 



" (3) Either the elementary stimulations themselves or three 

 mutually unopposed actions dependent upon them are conducted inde- 

 pendently to the central organ." 



Putting these conclusions in simpler language and in their most 

 general form, R, G, V, are any three points so situated that, when joined, 

 the triangle thus constructed completely encloses the colour-diagram 

 of the given spectrum. In this manner positive values are ensured. 

 From observations on colour-mixtures with the given spectrum we can 

 construct valency curves which represent the stimulation values of any 

 spectral light for each of the three components, R, G, V, of the resultant 

 sensation. Thus 



F = xR + yG + zV 



means that the light F is matched by a mixture of x parts of R light, 

 y parts of G light, and z parts of V light, R, G, V being the physical 

 stimuli in the mixture. If these physical stimuli act respectively upon 

 the physiological counterparts or elements of sensation, R, G, V, then 

 xR, yG, zV clearly represent the strengths with which the light F acts 

 upon the R, G, V elements, i.e., they are its R, G, V valencies or values. 

 The valency curves are therefore nothing more than the gauging 

 curves of the spectrum to which reference has already been made 

 {v. p. 39). But it has been shown that every gauging value belonging to 

 one such group of curves must always be a linear function of the three 



