ON THE RELATION OF OPTICS TO PAINTING. 113 



colour would not be necessary if the artist had at his 

 disposal colours of the same brightness, or the same 

 faintness, as are actually shown by the bodies irradiated 

 by the sun or by the moon. 



The change of colour, like the scale of shade, pre- 

 viously discussed, is a subjective action which the 

 artist must represent objectively on his canvas, since 

 moderately bright colours cannot produce them. 



We observe something quite similar in regard to 

 the phenomena of Contrast. By this term we under- 

 stand cases in which the colour or brightness of a 

 surface appears changed by the proximity of a mass of 

 another colour or shade, and, in such a manner, that 

 the original colour appears darker by the proximity of 

 a brighter shade, and brighter by that of a darker 

 shade ; while by a colour of a different kind it tends 

 towards the complementary tint. 



The phenomena of contrast are very various, and 

 depend on different causes. One class, ChevreuUs simul- 

 taneous Contrast, is independent of the motions of the 

 eyes, and occurs with surfaces where there are very 

 slight differences in colour and shade. This contrast 

 appears both on the picture and in actual objects, and 

 is well known to painters. Their mixtures of colours 

 on the palette often appear quite different to what 

 they are on the picture. The changes of colour which 

 are here met with are often very striking ; I will not, 

 H. I 



