134 ON THE RELATION OF OPTICS TO PAINTING. 



Before the shaded parts of blue bodies, the yellow 

 aerial light produces its effect, and imparts to the 

 blue more or less of a grey tint. The same thing 

 happens in front of red and green, though to a less 

 extent, so that, in their shadows, these colours merge 

 into yellow. This also is closely in accordance with 

 the aesthetic requirements of artistic unity of compo- 

 sition in colour. This is caused by the fact that the 

 divergent colours show a relation to the predominant 

 colour, and point to it most distinctly in their shades. 

 Where this is wanting, the various colours are hard 

 and crude ; and, since each one calls attention to itself, 

 they make a motley and disturbing impression ; and, 

 on the other hand, a cold one, for the appearance 

 of a flood of light thrown over the objects is 

 wanting. 



We have a natural type of the harmony which a 

 well-executed illumination of masses of air can produce 

 in a picture, in the light of the setting sun, which 

 throws over the poorest regions a flood of light and 

 colour, and harmoniously brightens them. The 

 natural reason for this increase of aerial illumination 

 lies in the fact, that the lower and more opaque 

 layers of air are in the direction -of the sun, and 

 therefore reflect more powerfully; while at the same 

 time the yellowish red colour of the light which 

 has passed through the atmosphere becomes more dis- 



