92 ON THE RELATION OF OPTICS TO PAINTING. 



These colourations are moreover not peculiar to 

 the air, but occur in all cases in which a transparent 

 substance is made turbid by the admixture of another 

 transparent substance. We see it, as we have ob- 

 served, in diluted milk, and in water to which a few 

 drops of eau de Cologne have been added, whereby the 

 ethereal oils and resins dissolved by the latter, sepa- 

 rate out and produce the turbidity. Excessively fine 

 blue clouds, bluer even than the air, may be produced, 

 as Tyndall has observed, when the sun's light is 

 allowed to exert its decomposing action on the vapours 

 of certain carbon compounds. Groethe called attention 

 to the universality of this phenomenon, and endea- 

 voured to base upon it his theory of colour. 



By aerial perspective we understand the artistic 

 representation of aerial turbidity ; for the greater or 

 less predominance of the aerial colour above the colour 

 of the objects, shows their varying distance very 

 definitely ; and landscapes more especially acquire the 

 appearance of depth. According to the weather, the 

 turbidity of the air may be greater or less, more white 

 or more blue. Very clear air, as sometimes met with 

 after continued rain, makes the distant mountains 

 appear small and near ; whereas, when the air contains 

 more vapour, they appear large and distant. 



This latter is decidedly better for the landscape 

 painter, and the high transparent landscapes of moun- 



