Optical Illusion. Ships floating in the air is an optical illusion, which may be thus explained : - On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which appeared as in the subjoined illustration, the top-mast being the only part of it seen above the horizon. An inverted image of it was seen at B, immediately above the real ship at A, and an erect image at C, both of them being complete and well defined. The sea was distinctly seen between them, as at V W. As the ship rose to the horizon, the image, C, gradually disappeared; and while this was going on, the image B descended, but the mainmost of B did not meet the mainmast of A. The two images, B C, were perfectly visible when the whole ship was actually below the horizon. These singular appearances, which have often given rise to superstitious legends, may be imitated artificially. Thus, if we take a long mass of hot iron, and, look along the upper surface of it at an object not too distant, we shall see not only the object itself, but also an inverted image of it below ; the second image being caused by the refraction of the rays of light passing through the stratum of hot air, as is the case of the mirage. (See Optical Effects, p. 115).

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