the manner in which, according to this theory, an object viewed through a medium of various densities, producing or rather represented by a curve of contrary flexure, and at the same time a contiguous stratum of uniform density, will exhibit three different images; the one through the uniform medium, in its proper place; the other through the convex part, somewhat higher, but inverted; and the third refracted by the concave part, still higher, erect, but somewhat smaller.
Grounded upon these principles, Dr. Wollaston proceeded next to the set of experiments which are the subject of his second section. The first and most explanatory of these experiments was made with a square phial, about one-third filled with clear syrup, and the other third with pure water, the two liquids forming by degrees, at the plane of contact, a. thin stratum of decreasing density from the syrup upwards. Here the effect was obviously conformable to the theory,—an object viewed through these media being represented to the eye, erect and in its proper place when seen through the syrup of uniform density; higher and inverted behind the adjacent variable medium: and still higher and erect behind the upper part of the variable stratum. This effect of varied density was repeated by filling the remainder of the phial with spirits of wine; when at the plane of contact between the spirit and the water, another variable stratum was gradually produced, which exhibited the same phaenomenon as in the former instance.—The next experiment proves that a difference of temperature between adjacent strata of the same fluid will produce the same effect.—And a third experiment, which may be considered as a corollary, showed that the air round a heated body (a red-hot poker for instance,) will assume the same varied densities, and exhibit precisely the same appearances.
Under the third head the author observes, that though three images have as yet been rarely seen in the atmosphere, yet this circumstance does by no means invalidate the above theory; since its appearing so seldom may be well accounted for by the less rarefaction produced by the heat of the sun, than by a, red-hot iron, or the artificial means above used. Over water, the evenness of the surface, he says, is favourable to the production of such appearances.
Some observations are, lastly, added concerning Mr. Huddart's opinion, that the peculiar state of the atmosphere which produced the appearances he witnessed may have been occasioned merely by the evaporation at the surface of the sea condensing the lower strata. of the atmosphere. Dr. Wollaston does not altogether accede to this opinion; but he does not absolutely deny that the cold produced by this evaporation may in some instances occasion a density that may enter as one of the data in the theory above laid down, though other causes, such as the effects of the heat of the sun, currents of air, &c. he thinks must co—operate. To the density, however, produced by mere evaporation, he acknowledges may be ascribed the uncommon elevation of the coast of France, lately observed at Hastings by Mr. Latham; and some of the appearances described by Professor
