Mirage. The density of the air generally diminishes with the height; rays of light proceeding obliquely upwards from an object then become more and more nearly horizontal, but generally pass away into space. Assume the density to diminish with the height with unusual rapidity, as when the air is cooler the nearer it is to the earth; the obliquely ascending rays may become quite horizontal and then bend down towards the earth, reaching it at a distant point. The observer at that point sees distant objects at an unusual elevation, or sees above the true horizon erect images of objects which may or may not be beyond the horizon. This is what the sailors generally call looming, and it causes us sometimes to see distant coasts with unusual distinctness, or to see from a mountain top a double horizon, such as is regularly seen in the autumn mornings from the Colorado foot-hills across the prairies. If the layer of air near the earth, say 50 or 100 feet thick, be uniformly dense, as in the cold air over a frozen sea, and a warmer stratum lie above it in which the density rapidly diminishes, so that the rays are brought back to the earth as above, we find, on tracing the path of the rays reaching the observer from the top and the bottom of the distant object respectively, that these rays have crossed one another in the hot stratum; the observer therefore seems to see the object suspended in the air, magnified and upside down; and this may happen while the observer sees the object itself by direct vision through the lower air. An intermediate stratum between a cold ground-stratum and a warm upper stratum gives rise to more than one image, inverted or erect, or both, according to positions. In the mirage of the Sahara and other arid deserts the conditions are reversed; the air is hottest nearest the hot sand; skylight rays descending become bent upwards; the eye receives an impression resembling that produced by the reflection of skylight from water; the illusion is rendered more perfect by the flickering due to convection currents, which causes an appearance like a breeze playing over the illusory water.
The phenomena of mirage are frequently very strange and complicated, the images being often much distorted and magnified, and in some instances occurring at a considerable distance from the object, as in the case of a tower or church seen over the sea, or a vessel over dry land, &c. Looming is very frequently observed at sea, and a most remarkable case of this sort occurred on the 26th of July 1798, at Hastings. From this place the French coast is 50 miles distant; yet from the seaside the whole coast of France 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 Captain Scoresby, while cruising off the coast of Greenland in 1822, discovered the 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 30 miles distant, was seen as if suspended in the air upside down. Beside such phenomena as these, the celebrated Fata Morgana (q.v.) of the Straits of Messina sinks into insignificance. The Spectre of the Brocken is a magnified shadow of persons, &c., on the summit of the mountain, seen at sundown and sunrise thrown on mist banks on the side of the mountain opposite to the sun, with or without rainbow colours. This is rather a glory than a mirage proper (see HALOS). Its varieties are indeed numberless, and we refer those who wish for further information to Brewster's Optics, to Biot's Traité de Physique, and for the mathematical theory of the mirage to Tait on Mirage, Trans. Roy. Soc. Edin., 1881. See also REFLEXION and REFRACTION.