130 ON THE RELATION OF OPTICS TO PAINTING. 



the use of the pure colours, otherwise they distract 

 the attention, and the picture becomes glaring. It 

 is necessary, on the other hand, to avoid a onesided 

 fatigue of the eye by too prominent a colour. This is 

 effected either by introducing the prominent colour 

 to a moderate extent upon a dull, slightly coloured 

 ground, or by the juxtaposition of variously saturated 

 colours, which produce a certain equilibrium of irrita- 

 tion in the eye, and, by the contrast in their after- 

 images, strengthen and increase each other. A green 

 surface on which the green after-image of a purple one 

 falls, appears to be a far purer green than without 

 such an after-image. By fatigue towards purple, that 

 is towards red and violet, any admixture of these two 

 colours in the green is enfeebled, while this itself pro- 

 duces its full effect. In this way the sensation of A 

 green is purified from any foreign admixture. Even 

 the purest and most saturated green, which Nature 

 shows in the prismatic spectrum, may thus acquire a 

 higher degree of saturation. We find thus that the 

 other pairs of complementary colours, which we have 

 mentioned, make each other more brilliant by their 

 contrast, while colours which are very similar aro 

 detrimental to each other, and acquire a grey tint. 



These relations of the colours to each other have 

 manifestly a great influence on the degree of pleasure 

 which different combinations of colours afford. Two 



