MIRAGE

(Fr.) An optical illusion'very common at sea, and especially in Irish latitudes, and sometimes also witne ed on land, particularly in Egypt and Persia, and on the margin of rivers and lakes, or on the sea-shore. It arises from unequal refraction in the lower strata of the at Biosphere, and causes remote objects to be seen double, as if reflected in a mirror, or to appear as if suspended in the air. When the effect is confined to apparent elevation, the English sailors call it looming; when inverted images are (fanned, the Italians give it the name of Fata Morgan a (see the term). Ships in the whale fisheries are often descried, and sometimes known, by means of the mirage, at considerable distances. Captain Scores by recognised his father's ship at the distance of more than 30 miles, and consequently when below the horizon, by its inverted image in the air, though he did not previously know that it was cruising in that part of the fishery. The mathematical theory of the phenomenon is given by Uiot, In the Mcmvires dc IInstitut for 1809. (See also Caddington's Optics; Biot's Traitt de Physique, tomeiii.; Brewster's Optics, Cabinet Cyclopedia.) [s. 773]