MIRAGE, mT-rlith', the name given to cer tain illusory appearances due to the bending of rays of light m the atmosphere. The earliest attempt to explain the mirage seems to be that of Monge, who accompanied Bonaparte's Egyp tian expedition; he thus describes what was ob served by the French soldiers: ((The villages seen in the distance appeared to be built upon an island in the midst of a lake. As the ob server approached them the boundary of the apparent water retreated, and on nearing the village it disappeared, to recommence for the next village"; he attributed the phenomenon to the hot sand of the desert keeping the lower layers of the atmosphere at a less density than the upper ones; the rays of light from the lower parts of the sky and objects in the distance arrive at the surface separating the less dense layer of air from those above, and are there subjected to total reflection; the eye sees the sky in the direction of the received rays, and this gives rise to the idea of a lake.
It is often assumed that rays of light pass through the atmosphere in straight lines; this is approximately true for short distances, but astronomers and surveyors have to correct their observations for refraction. By the laws of optics it is easy to see why a ray passing obliquely through the atmosphere, when this is arranged in horizontal layers of equal density (those of greater density being lowest), should bend, and that a vertical ray should not bend; hut optics does not tell us why a horizontal ray is much more refracted than an oblique one.
The explanation (first given by Dr. James Thomson) is easy on the undulatory theory of light. The wave front of a horizontal ray of light is at right angles to the ray, and is a vertical plane; now light is less rapidly prop agated in the lower layers of air, hence the lower part of the wave front is retarded, and when the light has proceeded some distance its wave front is no longer vertical, and the ray has bent downward (the ray is always sup posed to be normal to the wave front). Thus, in the atmosphere in its normal state the path of a ray of light is always slightly concave downward. Professor Everett thus explains the appearance of obelisks and spires," cities with many buildings, forests of naked trees and great basaltic precipices sometimes assumed by irregularities in cakes and fields of ice. It sometimes happens that several inverted images of an object are seen in the same sky; these may be accounted for by assuming that there are several layers of air, in each of which there is a rapid variation (an increase upward) of the index of refraction. Mirages are not un common in California, Nevada and Alaska.