THE SENSATION OF SIGHT. 253 



the three primary colours depend upon a perfect rectilinear 

 triangle inclosing the complete colour-system, for only in 

 that case is it possible to produce all possible tints by 

 various combinations of the three primary colours at the 

 angles. It must, however, be remembered that the 

 colour-disc only includes the entire series of colours 

 which actually occiu: in nature, while our theory has to 

 do with the analysis of our subjective sensations of colour. 

 We need then only assume that actual coloured light does 

 not produce sensations of absolutely pure colour ; that 

 red, for instance, even when completely freed from all ad- 

 mixture of white light, still does not excite those nervous 

 fibres alone which are sensitive to impressions of red, but 

 also, to a very slight degree, those which are sensitive to 

 green, and perhaps to a still smaller extent those which 

 are sensitive to violet rays. If this be so, then the sen- 

 sation which the purest red light produces in the eye is 

 still not the purest sensation of red which we can con- 

 ceive of as possible. This sensation could only be called 

 forth by a fuller, pmer, more saturated red than has ever 

 been seen in this world. 



It is possible to verify this conclusion. We are able 

 to produce artificially a sensation of the kind I have de- 

 scribed. This fact is not only important as a complete 

 answer to a possible objection to Young's theory, but is 

 also, as will readily be seen, of the greatest importance 

 for understanding the real value of our sensations of 

 colour. In order to describe the experiment I must first 

 give an account of a new series of phenomena. 



The result of nervous action is fatigue, and this will be 

 proportioned to the activity of the function performed, 

 and the time of its continuance. The blood, on the other 

 hand, which flows in through the arteries, is constantly 

 performing its function, replacing used material by fresh, 

 and thus carrying away the chemical results of func- 



