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.