648 THE POPULAR SCIENCE MONTHLY. 



light, adapted now to our eyes; and it is that which contributes 

 much to the beauty of our landscape scenery. 



But if dust makes the sky appear clear, why is the color of the 

 sky blue ? Why does dust, of the different constituents of white 

 sunlight, reflect the blue rather than the green, yellow, and red ? 

 This fact is connected with the size of the dust particles. Only 

 the finest dust settles so slowly that it can be spread everywhere 

 by means of the air currents, and can be found constantly in all 

 strata of the atmosphere ; and special importance can be ascribed 

 only to these finest particles. The coarse parts soon fall to the 

 ground. Let us consider the fine mechanism of light, the ex- 

 tremely short ether waves which determine its existence. These 

 waves, although they are of even less than microscopic size, are 

 not all equally long. The shortest are those that give blue light, 

 while all the other colors are produced by longer waves. The 

 fine atmospheric dust contains many particles which are large 

 enough to reflect the short blue ether waves, fewer that can reflect 

 green and yellow, and still fewer large enough to reflect the long 

 red waves. The red light, therefore, goes on almost without 

 hindrance, while the blue is more liable to be diverted, and thus 

 to reach the eye. A similar phenomenon may be observed on a 

 larger scale on water which is roughened with waves of different 

 lengths, and on which pieces of wood are floating. The pieces of 

 wood stand in the same relation to the water waves as the dust 

 particles to the ether waves. The great long waves pass the 

 blocks undisturbed, only rocking up and down ; while the finer 

 ripples of the water are turned back, as if the blocks were firm walls. 



The finest dust thus appears blue. Look at the smoke that 

 rises from the glowing end of a cigar. It appears on a clear day, 

 especially in the presence of much blue light, of the most beauti- 

 ful sky blue. But that part of the smoke that is drawn through 

 the cigar, and is seen at the other end, is composed of coarser 

 particles, which are large enough to reflect the longest ether 

 waves, including all the constituents of white light. It therefore 

 appears whitish. The same difference is found between the dust 

 in the country and that in the town. There is much coarse dust 

 in large towns, when the sky over them is often gray, while 

 only the finest blue dust is carried up in the country. The dust 

 is also variously assorted at different heights above the surface 

 of the earth. The coarser dust will be found at the lower levels, 

 where it is produced. On mountains we have most of the dust 

 beneath us, while the rarefied air can sustain only the finest 

 floating particles. Hence the sky on high mountains is clear and 

 deep blue, even almost black, as if it were without dust. Only 

 when we look at the lower strata, toward the horizon, does the 

 color pass into gray. 



