112 ON THE RELATION OF OPTICS TO PAINTING. 



our ordinary way of looking at the objects about us, 

 we are not so readily conscious of this ; for the direct 

 comparison of colours of very different shade is diffi- 

 cult, and we are accustomed to see in this alteration in 

 the white the result of different illumination of one and 

 the same white object, so that in judging pigment- 

 colours we have learnt to eliminate the influence of 

 brightness. 



If however to the painter is put the problem of imi- 

 tating, with faint colours, white irradiated by the sun, 

 he can attain a high degree of resemblance ; for by an 

 admixture of yellow in his white he makes this colour 

 preponderate just as it would preponderate in actual 

 bright light, owing to the impression on the nerves. 

 It is the same impression as that produced if we look 

 at a clouded landscape through a yellow glass, and 

 thereby give it the appearance of a sunny light. The 

 artist will, on the contrary, give a bluish tint to moon- 

 light, that is, a faint white ; for the colours on the 

 picture must, as we have seen, be far brighter than 

 the colour to be represented. In moonshine scarcely 

 any other colour can be recognised than blue ; the 

 blue starry sky or blue colours may still appear 

 distinctly coloured, while yellow and red can only be 

 seen as obscurations of the general bluish white or 



I will again remind you that these changes of 



