LIGHT, AIR, AND COLOR 



83 



the small dust particles of the higher air re- 

 gion, great quantities of these rays are caught, 

 refracted, and practically held in check in the 

 upper strata of the atmosphere. We see them 

 massed together overhead and call them the 

 "blue sky." After many millions of these 

 blue rays have been eliminated from the sun- 

 light the remaining rays come down to earth 

 as a white or yellow or at times reddish light, 

 dependent upon the density of the lower atmos- 

 phere. 



Now it seems that an atmosphere laden with 

 moisture particles obstructs the passage earth- 

 ward of the blue rays, less perhaps than an 

 atmosphere laden with dust. In consequence, 

 when they are thus allowed to come down into 

 the lower atmosphere in company with the 

 other rays, their vast number serves to dom- 

 inate the others, and to produce a cool tone of 

 color over all. So it is that in moist countries 

 like Scotland you will find the sky cold-blue 

 and the air tinged gray, pale-blue, or at twi- 

 light in the mountain valleys, a chilly purple. 

 A dust-laden atmosphere seems to act just the 

 reverse of this. It obstructs all the rays in 

 proportion to its density, but it stops the blue 

 rays first, holds them in the upper air, while 



Refracted 

 rays. 



Cold colors, 



how 



produced. 



