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Mirage

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the French name of an optical illusion common in the East, and directly referred to by Isaiah (שָׁרָב, sharab', "parched ground," Isaiah 35:7; "heat," Isaiah 49:10), and perhaps indirectly by Jeremiah (Jeremiah 15:18, "waters that fail;" literally, that cannot be trusted). It is still known by the name of serib, the Arabic equivalent of the above Heb. term.. This phenomenon is as simple in its origin as it is 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. The cause of mirage is a diminution of the density of the air near the surface of the earth, produced by the transmission of heat from the earth, or in some other way; the denser stratum being thus placed above, instead of, as is usually the case, 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), coming in a direction nearly parallel to the earth's surface, meet the rarer medium at a very obtuse angle, and, 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 transmitted rays; but, besides this, rays from the object will be reflected from the upper surface of the rarer stratum of air beneath to his eve. (See fig. 1.) 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 to a sheet of water. (See fig. 2.) This form of mirage, which is most common in sandy, desert countries, is an illusive appearance of pools and lakes of water, in places where water is most needed and least likely to occur. This phenomenon offers so perfect a delusion in all its circumstances that the most forewarned and experienced travellers are deceived by it, as are even the natives of the deserts, when not sufficiently acquainted with the locality in which it appears to be aware that no water actually exists. No one can imagine, without actual experience, the delight and eager expectation, followed by the most intense and bitter disappointment, which the appearance of the serab often occasions to travelling parties, particularly when the supply of water which they are obliged to carry with them upon their camels is nearly or quite exhausted. (See fig. 3.)

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