256 THE POPULAR SCIENCE MONTHLY. 



out into "bold relief the circlet of flaming mountain peaks, it is 

 like a gorgeous transformation scene. Stranger still, when the 

 sun sinks below the horizon, and a dull ashen gray has possessed 

 the western heavens, what occasions the hectic flush on the east- 

 ern horizon ? Gradually the clouds are tinged with light red 

 from the eastern horizon all over the zenith ; whence comes the 

 coloring ? 



It is a strange coincidence that these remarkably fine sun- 

 sets have been since the tremendous eruptions at Krakatoa, in 

 the Straits of Sunda. Along with the lava eruption there was 

 ejected an enormous quantity of fine dust. The decks of vessels, 

 hundreds of miles away, were covered with it. Mr. Verbreek 

 computed that no less than 70,000 cubic yards of dust actually fell 

 round the volcano. This will give an idea of the enormous quan- 

 tity of dust still floating in the atmosphere, and drifting all over 

 the world. In the upper atmosphere, too, there must always be 

 dust, for without the dust no clouds could be formed to shield us 

 from the sun's scorching rays ; and of cosmic dust there must be 

 a considerable quantity in the air, produced by the waste from 

 the millions of meteors that daily fall into it. Mr. Aitken has 

 ably shown that the brilliancy and variety of the coloring are 

 due to the suspended dust in the atmosphere. 



Observers of the gorgeous sunsets and afterglows have been 

 most particularly struck with the immense wealth of the various 

 shades and tints of red. Now, if the glowing colors are due to the 

 presence of dust in the air, there must be somewhere a display of 

 the colors complementary to the reds, because the dust acts by a 

 selective dispersion of the colors. The small dust-particles arrest 

 the direct course of the rays of light and reflect them in all direc- 

 tions ; but they principally reflect the rays of the violet end of 

 the spectrum, while the red rays pass on almost unchecked. 

 Overhead deep blue reigns in awe-inspiring glory. As the sun 

 passes below the horizon, and the lower stratum of air, with its 

 larger particles of dust which reflect light, ceases to be illumi- 

 nated, the depth and fullness of the blue most intensely increase. 

 This effect is produced by the very fine particles of dust in the 

 sky overhead being unable to scatter any colors unless those 

 of short wave-lengths at the violet end .of the spectrum. Thus 

 we see, above, blue in its intensity without any of the red colors. 

 When, however, the observer brings his eyes down in any direction 

 except the west, he will see the blue mellowing into blue-green, 

 green, and then rose color. And some of the most beautiful and 

 delicate rose tints are formed by the air cooling and depositing 

 its moisture on the particles of dust, increasing the size of the 

 particles till they are sufficiently large to stop and spread the red 

 rays, when the sky glows with a strange aurora-like light. 



