THE SENSATION OF SIGHT. 223 



analysis of our subjective sensations of colour. We need then 

 only assume that actual coloured light does not produce sensa- 

 tions of absolutely pure colour; that red, for instance, even 

 when completely freed from all admixture 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 sensation which the purest red light produces in the 

 eye is still not the purest sensation of red which we can conceive 

 of as possible. This sensation could only be called forth by a 

 fuller, purer, 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 described. 

 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 func- 

 tion, replacing used material by fresh, and thus carrying away 

 the chemical results of functional activity; that is to say, 

 removing the source of fatigue. 



The process of fatigue as the result of nervous action, takes 

 place in the eye as well as other organs. When the entire 

 retina becomes tired, as when we spend some time in the open 

 air in brilliant sunshine, it becomes insensible to weaker light, 

 so that if we pass immediately into a dimly lighted room we see 

 nothing at first ; we are blinded, as we call it, by the , previous 

 brightness. After a time the eye recovers itself, and at last we 

 are able to see, and even to read, by the same dim light which 

 at first appeared complete darkness. 



