STUDIES IN SPECIAL SENSE PHYSIOLOGY 405 



The conception is merely that three independent physiological 

 processes exist, each of which is defined by an equation connecting 

 it with the three independent stimulus values which, as we have 

 seen, determine the effect of any given stimulus. 



This statement represents the fundamental part of the hypo- 

 thesis, first sketched by Thomas Young, and elaborated by 

 Helmholtz. It is important to distinguish the essentials of the 

 theory from its subsidiary parts. 



The effect produced by any chromatic stimulus is supposed to 

 depend upon the resultant changes set up in three independent 

 " substances." nothing whatever being postulated with respect to 

 these substances except that the magnitude of change in each is a 

 function of, i.e. depends on, the proportions of three independently 

 variable stimuli in terms of which the given stimulus can be 

 expressed. Conversely, any given stimulus value is a function 

 of the independent activities of three visual " substances." The 

 nature of the substances, also the exact relation between them and 

 the stimuli, are left open. For convenience of illustration, Young 

 and Helmholtz assumed that the activity of each substance was 

 associated with a single colour sensation, and chose red, green, and 

 violet as " elementary sensations " from the present standpoint. 

 " Substance " A, when stimulated, was supposed to give rise to the 

 sensation of red ; B, under similar conditions, to that of green ; and 

 C to that of violet ; in this way the well-known " valency curves " 

 of the text -books were obtained. The advantage of this method 

 is that the theory seems more definite, but the disadvantage is 

 entailed that if the illustration proves irreconcilable with facts of 

 experiment, the reader omits to notice that what is found wanting 

 is merely an illustration, not the basal theory. 



It is especially important to remember that the colour equations 

 discussed in Section II. do not pretend to describe any direct 

 relationship between the hypothetical " substances " and stimulus 

 magnitudes. 



In precisely the same way, for the sake of illustration, Helm- 

 holtz suggested the existence somewhere in the retino-cerebral 

 apparatus of three sets of fibres, each corresponding to one of the 

 hypothetical substances. 



This suggestion was less happy because its utility was not so 

 great as, and the chance of misunderstanding greater than, in the 

 previous case. The red, green, and violet fibres have reigned long 



