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Show PROFESSOR TAIT ON MIRAGE. Professor P. G. Tait gave a lecture on mirage before the members of the Edinburgh Philosophical Institution. Commencing with a reference to the phenomenon as seen in looking along a road on a summer day, and pointing out that the effect observed under those circumstances was similar to that produced by mixing two liquids of different refractive indices, such as alcohol and water, Professor Tait went on to speak of the form of mirage seen in the desert, where there were vast tracts of sand boiled by the sun, and the layer of air in contact with the sand was consequently hotter than the air above it. This, he said, occasioned a peculiar distribution of density, there was a gradual increase of density for a considerable number of feet above the surface. Light therefore moved a good deal faster in the layer near the hot sand than a few feet above it, and the rays from objects falling into that layer were bent upward, and reached the eye of the observer as if reflected from the surface of the sand. Rays coming through air in this way irresistibly conveyed the idea of a ?? surface, and that gave rise to the impression that there were pools or lakes of water on the dry sand of the desert. The phenomenon was very carefully observed by the scientific men attached to the French army in Egypt in the end of last century, and Mouge had given a most complete explanation of that form of mirage. A second form was then observed in the Arctic regions, of which a great many beautiful illustrations were given by Scoresby. The principal phenomenon here was what was called "looming," where a distant object showed an extravagant increase of vertical height, without any alteration in breadth. Distant hummocks of ice were the magnified into immense towers and pinnacles, and a ship was sometimes abnormally drawn out till it appeared twelve or fourteen times as high as it was long. The celebrated Pata Morgana of the Straits of Messius was of the same character. Rocks were seen drawn up to ten or twelve times their proper vertical height, and houses, as well as human beings and animals, appeared in the like exaggerated form. The most remarkable instance on record of this form of mirage was one observed in 1798. From Hastings forty or fifty miles from the French coast, that coast was seen quite distinctly. In ordinary weather it could not be seen at all owing to the rotundity of the earth, but on the occasion it appeared to be elevated above the horizon, so that fishermen on the Hastings shore could point out the various harbors and promontories, as if they had been only a few miles distant. A third and perhaps the most extraordinary form of mirage was that observed by ?? in 1799, in which a ship at sea showed three distinct images a lower and an upper one in the upright position and an intermediate one in which the object stood upside down. Professor Tait devoted the remainder of his lecture to an exposition of the cause of this phenomenon which he said, ?? reproduced by looking along the underside of a heated poker at an object held just before its point. Premising that light moved in straight lines only so long as it moved in a uniform medium whose refractive index gradually changed there was a correspondingly gradual change of direction, he explained, by the help of diagrams displayed by the lime light, the behavior under different circumstances of the curved rays proceeding from an object to the eye. It was pointed out that when the curve passing through the vertices of those light curves lay toward the eye, the object was seen inverted, but when it lay from the eye there was a direct image, and an arrangement out of light curves, such as would produce the three images in question, was shown to occur when a stray turn of cold air below was separated from a warm stratum above by an intermediate stratum, in which the ?? gradually decreased from the cold to the warm. The phenomenon was produced in view of the audiences by placing a flannel behind a glass vessel containing brine at the top, and between the two a stratum in which they were intermixed.-World of Science. |