ON THE .RELATION OF OPTICS TO PAINTING. 89 



rising from the flame stands out quite dark in the 

 surrounding bright turbidity; that is to say, the air 

 rising from the flame has been quite freed from dust. 

 In the open air, besides dust and occasional smoke, we 

 must often also take into account the turbidity arising 

 from incipient aqueous deposits, where the tempera- 

 ture of moist air sinks so far that the water retained 

 in it can no longer exist as invisible vapour. Part of 

 the water settles then in the form of fine drops, as a 

 kind of the very finest aqueous dust, and forms a finer 

 or denser fog; that is to say, cloud. The turbidity 

 which forms in hot sunshine and dry air may arise, 

 partly from dust which the ascending currents of 

 warm air whirl about; and partly from the irregular 

 mixture of cold and warm layers of air of different 

 density, as is seen in the tremulous motion of the 

 lower layers of air over surfaces irradiated by the sun. 

 But science can as yet give no explanation of the 

 turbidity in the higher regions of the atmosphere 

 which produces the blue of the sky ; we do not know 

 whether it arises from suspended particles of foreign 

 substances, or whether the molecules of air themselves 

 may not act as turbid particles in the luminous ether. 



The colour of the light reflected by the opaque 

 particles mainly depends on their magnitude. When 

 a block of wood floats on water, and by a succession of 

 falling drops we produce small wave-rings near it, 



