ON THE RELATION OF OPTICS TO PAINTING. 87 



or hilly land only present slightly moving figures, we 

 require light which is almost in the direction of the 

 surface itself, for only such a one gives shadows. This 

 is one of the reasons which makes illumination by the 

 rising or the setting sun so effective. The forms of 

 the landscape become more distinct. To this must 

 also be added the influence of colour, and of aerial 

 light, which we shall subsequently discuss. 



Direct illumination from the sun, or from a flame, 

 makes the shadows sharply defined, and hard. Illu- 

 mination from a very wide luminous surface, such as 

 a cloudy sky, makes them confused, or destroys them 

 altogether. Between these two extremes there are 

 transitions ; illumination by a portion of the sky, 

 defined by a window, or -by trees, &c., allows the 

 shadows to be more or less prominent according to 

 the nature of the object. You must have seen of 

 what importance this is to photographers, who have to 

 modify their light by all manner of screens and 

 curtains in order to obtain well-modelled portraits. 



Of more importance for the representation of 

 depth than the elements hitherto enumerated, and 

 which are more or less of local and accidental signific- 

 ance, is what is called aerial perspective. By this we 

 understand the optical action of the light, which the 

 illuminated masses of air, between the observer and 

 distant objects, give. This arises from a fine opacity 



