ON THE RELATION OF OPTICS TO PAINTING. Ill 



of ^ in the red ; or when the colour is dimmed 

 by being looked at for a long time, a difference of 

 sVto^V- 



Ked therefore acts as a colour towards whose shades 

 the eye i? relatively less sensitive than towards that of 

 blue. In agreement with this, the impression of glare, 

 as the intensity increases, is feebler in red than in 

 blue. According to an observation of Dove, if a blue 

 and a red paper be chosen which appear of equal 

 brightness under a mean degree of white light, as the 

 light is made much dimmer the blue appears brighter, 

 and as the light is much strengthened, the red. I 

 myself have found that the same differences are seen, 

 and even in a more striking manner, in the red and 

 violet spectral colours, and, when their intensity is 

 increased only moderately, by the same fraction for 

 both. 



Now the impression of white is made up of the 

 impressions which the individual spectral colours make 

 on our eye. If we increase the brightness of white, 

 the strength of the sensation for the red and yellow 

 rays will relatively be more increased than that for 

 the blue and violet. In bright white, therefore, the 

 former will produce a relatively stronger impression 

 than the latter; in dull white the blue and bluish 

 colours will have this effect. Very bright white appears 

 therefore yellowish, and dull white appears bluish. In 



