One may ask why it is, if light from an object, say a star, is bent from its course and separated into rays of various colors upon entering the earth's atmosphere, that we do not see the object drawn out into a band of spectral colors. It is because the angular separation of the various colors is so slight under ordinary circumstances that light from one point is blended with light from a neighboring point of complementary color to produce white light again. Under certain circumstances, however, beautiful color effects may be seen in the earth's atmosphere as a result of the refraction of sunlight. The blue color of the sky and its brightness is caused by the scattering of the rays of shortest wave-length, the violet and blue rays, by the oxygen and nitrogen in the upper atmosphere. The molecules of these gases interfere with the passage of these rays, powerfully scattering and dispersing them, and thus increasing the length of their path through the air and diffusing their color and brightness in the upper atmosphere, while permitting rays of longer wave-length, the red and orange, to pass on practically undisturbed. When an object in the heavens lies close to the horizon, the rays of light from it have to travel a longer path through the atmosphere than when the object is overhead, and that too through the densest part of the atmosphere, which lies close to the earth's surface, and in which are floating many dust particles and impurities from the earth's surface. All these particles, as well as the increased density of the atmosphere, interfere with the free passage of the rays, especially of shorter wave-lengths. The violet and blue rays are sifted out and scattered in their long journey through the lower strata of air, far more than when they come to us from an object high in the sky. Even the red and yellow rays are more or less scattered and bent aside--diffracted--by these comparatively large particles near the surface. The reddish color of the sun, moon and even of the stars and planets, when seen near the horizon, as well as the beautiful sunset tints, in which reds and pinks and yellows predominate, are due to the fact that the rays of longer wave-length are more successful in penetrating the dense, dust-laden layers of the lower atmosphere. It is to be free of the dust and impurities as well as the unsteadiness of the lower atmosphere, that observatories are built at high altitudes whenever possible.