72 THE WONDERFUL CENTURY. CHAP. . 



extreme lightness, causing them to float permanently in 

 the atmosphere. The dust being thus got rid of, and 

 pure air being entirely transparent, there is nothing in 

 the cylinder to reflect the light which is sent through its 

 centre in a beam of parallel rays, so that none of it strikes 

 against the sides; hence the inside of the cylinder appears 

 absolutely dark. But when all the larger dust particles 

 are wholly or partially burnt, so that only the very 

 smallest fragments remain, a blue light appears, because 

 these are so minute as to reflect chiefly the more refran- 

 gible rays, which are of shorter wave-length those at 

 the blue end of the spectrum, which are thus scattered in 

 all directions, while the red and yellow rays pass straight 

 on as before. 



We have seen that the air near the earth's surface is 

 full of rather coarse particles which reflect all the rays, 

 and which therefore produce no one color. But higher 

 up the particles necessarily become smaller and smaller, 

 since the comparatively rare atmosphere will only sup- 

 port the very smallest and lightest. These exist 

 throughout a great thickness of air, perhaps from one 

 mile to ten miles high or even more, and blue or violet 

 rays being reflected from the innumerable particles in 

 this great mass of air, which is nearly uniform in all 

 parts of the world as regards the presence of minute 

 dust particles, produces the constant and nearly uniform 

 tint we call sky-blue. A certain amount of white or 

 yellow light is no doubt reflected from the coarser dust 

 in the lower atmosphere, and slightly dilutes the blue 

 and renders it not quite so deep and pure as it otherwise 

 would be. This is shown by the increasing depth of the 

 sky-color when seen from the tops of lofty mountains,. 



