Blue as a color.
The prevailing note of the sky, the one oftenest seen, is, of course, blue—a color we may not perhaps linger over because it is so common. And yet how seldom it is appreciated! Our attention is called to it in art—in a hawthorn jar as large as a sugar-bowl, made in a certain period, in a certain Oriental school. The æsthetic world is perhaps set agog by this ceramic blue. But what are its depth and purity compared to the ethereal blue! Yet the color is beautiful in the jar and infinitely more beautiful in the sky—that is beautiful in itself and merely as color. It is not necessary that it should mean anything. Line and tint do not always require significance to be beautiful. There is no tale or text or testimony to be tortured out of the blue sky. It is a splendid body of color; no more.
Sky from mountain heights.
The night sky.
You cannot always see the wonderful quality of this sky-blue from the desert valley, because it is disturbed by reflections, by sand-storms, by lower air strata. The report it makes of itself when you begin to gain altitude on a mountain’s side is quite different. At four thousand feet the blue is certainly more positive, more intense, than at sea-level; at six thousand feet it begins to darken and deepen, and it seems to fit in the saddles and notches of the mountains like a block of lapis lazuli; at eight thousand feet it has darkened still more and has a violet hue about it. The night sky at this altitude is almost weird in its purples. A deep violet fits up close to the rim of the moon, and the orb itself looks like a silver wafer pasted upon the sky.
Blackness of space.
The darkening of the sky continues as the height increases. If one could rise to, say, fifty thousand feet, he would probably see the sun only as a shining point of light, and the firmament merely as a blue-black background. The diffusion of light must decrease with the growing thinness of the atmospheric envelope. At what point it would cease and the sky become perfectly black would be difficult to say, but certainly the limit would be reached when our atmosphere practically ceased to exist. Space from necessity must be black except where the straight beams of light stream from the sun and the stars.
Bright sky-colors.
Horizon skies.
The bright sky-colors, the spectacular effects, are not to be found high up in the blue of the dome. The air in the zenith is too thin, too free from dust, to take deep colorings of red and orange. Those colors belong near the earth, along the horizons where the aërial envelope is dense. The lower strata of atmosphere are in fact responsible for the gorgeous sunsets, the tinted hazes, the Indian-summer skies, the hot September glows. These all appear in their splendor when the sun is near the horizon-line and its beams are falling through the many miles of hot, dust-laden air that lie along the surface of the earth. The air at sunset after a day of intense heat-radiation is usually so thick that only the long and strong waves of color can pass through it. The blues are almost lost, the neutral tints are missing, the greens are seen but faintly. The waves of red and yellow are the only ones that travel through the thick air with force. And these are the colors that tell us the story of the desert sunset.