Mirage

me-razh', an optical illusion due to the difference in the density of layers of air. It is a well known fact that rays of light falling very obliquely on the surface of a medium of different density do not enter the new medium, but glance off. To an observer standing on the brink of a pool trees on the opposite bank seem to have their doubles under water. The trees that grow downward are seemingly there, because rays of light from the real trees glance from the surface of the water as from a mirror and enter the eye as though they came from the water. In the same way, if the eye of an observer were just beneath the surface of the water, and a fish were to rise toward the surface some distance away, the observer would see not only the fish, but an image of the fish in. the air.

The principle of the mirage is to be explained in the same way. In heated countries, as in deserts, the layer of air next the earth becomes exceedingly hot and is rarefied. The border line or border surface between this rarefied layer of air and the heavier air, like the surface of the water, acts like a mirror. Images of distant objects are seen apparently upside down, like the image of the trees in the water. A scientist who accompanied Napoleon's expedition into Egypt states that the French soldiers were astonished by the mirage of the desert. "Villages in the distance appeared to be built upon an island in the middle of a lake. As the observer approached, the boundary of the apparent lake retreated and the village disappeared, while another lake and another village appeared farther on." In like manner, images of objects on the surface of the earth may be seen in the clouds. At certain times the coast of France opposite Dover, ordinarily below the horizon, is seen apparently in an elevated position. The phenomenon is by no means confined to heated regions. As layers of different density produce the mirage it may appear in any latitude. On more than one occasion the writer has seen a mirage in Minnesota on a frosty winter morning. The images were not inverted. The masters of whaling vessels in Arctic waters report that they are not infrequently made aware of the presence of another ship by seeing its inverted image in the sky. Travelers in the Sahara desert, Tartary, and other regions mention the mirage more frequently because, when their need of water is the sorest, they are often tantalized by an apparent pool at no great distance. As they hasten toward it, hoping to slake their thirst, it recedes and fades away. The picture of water is so perfect that it is difficult to believe that it is only the image of a fleck of cloud in an otherwise cloudless sky. It is said that wise old camels, accustomed to the mirage, are never deceived. The Arabs of the Sahara call a mirage "the lake of the gazelles."