ON THE RELATION OF OPTICS TO PAINTING. 91 



with the light from objects behind; and moreover 

 it belongs to the coarser turbidity of the lower regions 

 of the atmosphere, so that it is whiter. In hot coun- 

 tries, and with dry air, the aerial turbidity is also finer 

 m the lower regions of the air, and therefore the blue 

 in front of distant terrestrial objects is more like 

 that of the sky. The clearness and the pure colours 

 of Italian landscapes depend mainly on this fact. On 

 high mountains, particularly in the morning, the 

 aerial turbidity is often so slight that the colours of 

 the most distant objects can scarcely be distinguished 

 from those of the nearest. The sky may then appear 

 almost bluish-black. 



Conversely, the denser turbidity consists mainly of 

 coarser particles, and is therefore whitish. As a rule, 

 this is the case in the lower layers of air, and in states 

 of weather in which the aqueous vapour in the air is 

 near its point of condensation. 



On the other hand, the light which reaches the 

 eye of the observer after having passed through a long 

 layer of air, has been robbed of part of its violet and 

 blue by scattered reflections ; it therefore appears yel- 

 lowish to reddish-yellow or red, the former when the 

 turbidity is fine, the latter when it is coarse. Thus 

 the sun and the moon at their rising and setting, and 

 also distant brightly illuminated mountain-tops, espe- 

 cially siiow-mountains, appear coloured. 



