XIRAGE', a phenomenon extremely common in certain localities, and as simple in its origin as astonishing in its effects. Under it are classed the appearance of distant objects as double, or as if suspended in the air, erect or inverted, etc. One cause of mirage is a diminution of the density of the air near the surface of the earth, produced by the trans mission of beat from the earth, or in some other way; the denser stratum being thus placed above, instead of, as is usually the ease, below the rarer. Now, rays of light from a distant object, situated in the denser medium (i. e., a little above the earth's level), coining in a direction nearly parallel to the earth's surface, meet the rarer medium at a very obtuse angle, and (see REFRACTION) instead of passing into it, are reflected back to the dense medium. the common surface of the two media acting as a mirror. Suppose, then, a spectator to be situated on an eminence, and looking at an object situated like himself in the denser stratum of air, he will see the object by means of directly transmit ted rays; but besides this, rays from the object will be reflected from the upper surface of the rarer stratum of air beneath to his eye. The image produced by the reflected rays will appear inverted, and below the real object, just as an image reflected in water appears when observed from a distance. If the object is a cloud or portion of sky, it will appear by the reflected rays as lying on the surface of the earth, and bearing a strong resemblance ton sheet of water; alsoos1he reflecting surface is irregular, and constantly varies its position, owing to the constant communication of heat to time upper stratum, the reflected image will be constantly varying, and will present the appearance of a water surface ruffled by the wind. This form of mirage, which even experienced travelers have found to be completely deceptive, is of common occurrence in the aria deserts of louver Egypt. Persia, Tartary, etc.
In particular states of the atmosphere, reflection of a portion only of the rays takes place at the surface of the dense medium, and thus double images are formed, one by reflection, and the other by refraction—the first inverted, and the second erect. The
phenomena of mirage are frequently much more strange and complicated, the images being often much distorted and Magnified, amid in sonmeinstancesoccutling at a consider able distance from the object, as in the case of a tower or church seen over the sea, or a vessel over dry land, etc. The particular form of mirage known as looming is very fre quently observed at sea, and consists in an excessive apparent elevation of the object. A mast remarkable case of this sort occurred on July 26, 1798, at Hastings. From this place the French coast is fifty miles distant; yet, from the sea-side the whole coast of Prance from Calais to near Dieppe was distinctly visible, and continued so for three hours. In the Arctic regions it is no uncommon occurrence for whale-fishers to discover the proximity of other ships by means of their images seen elevated in the air, though the ships themselves may be below the horizon. Generally, when the ship is above the horizon, only one image, and that inverted, is found; but when it is wholly or in great part below the horizon, double images, one erect and the other inverted, are frequently seen. The faithfulness and distinctness of these images at times may be imagined from the fact, that capt. Scoresby, while cruising off the coast of Greenland in 1822, discovered thi3 propinquity of his father's ship from its inverted image in the sky. Another remarkable instance of mirage occurred in May, 1854, when, from the deck of H. M. screw-steamer Archer, then cruising off Oesel, in the Baltic, the whole English fleet of nineteen sail, then nearly thirty miles distant, was seen as if suspended in the air upside down. Besides such phenomena as these, the celebrated Rita Maryann (q.v.) of the straits of Messina sinks into insignificance. The Specter of the Erocken, in Hanover, is another celebrated instance of mirage. Its varie ties arc indeed numberless, and we refer those who wish for further information to Brewster's Optics, Riot's Traits do Physique, and for the mathematical theory of the mirage to the works of Biot, Monge, and Wollaston. See also REFLECTION and