100 ON THE RELATION OF OPTICS TO PAINTING. 



process to which we can attach the same name, fatigue, 

 as that for the corresponding one in the muscle. Any 

 activity of our nervous system diminishes its power for 

 the time being. The muscle is tired by work, the 

 brain is tired by thinking, and by mental operations ; 

 the eye is tired by light, and the more so the more 

 powerful the light. Fatigue makes it dull and in- 

 sensitive to new impressions, so that it appreciates 

 strong ones only moderately, and weak ones not at all. 

 But now you see how different is the aim of the 

 artist when these circumstances are taken into account. 

 The eye of the traveller in the desert, who is looking 

 at the caravan, has been dulled to the last degree by the 

 dazzling sunshine ; while that of the wanderer by moon- 

 light has been raised to the extreme of sensitiveness. 

 The condition of one who is looking at a picture 

 differs from both the above cases by possessing a cer- 

 tain mean degree of sensitiveness. Accordingly, the 

 painter must endeavour to produce by his colours, on 

 the moderately sensitive eye of the spectator, the same 

 impression as that which the desert, on the one hand, 

 produces on the deadened, and the moonlight, on the 

 other hand, creates on the untired eye of its observer. 

 Hence, along with the actual luminous phenomena of 

 the outer world, the different physiological conditions 

 of the eye play a most important part in the work of 

 the artist. What he has to give is not a mere tran- 



