On double images caused by atmospherical refraction

The remarkable instances of double and triple images of the same object produced by aerial refraction near the horizon, lately communicated to the Society by Mr. Huddart, Prof. Vince, and Mr. Dalby, have given rise to the present paper, in which the author attempts to explain these phenomena on theoretical principles, and to illustrate his conclusions by artificial experiments. Admitting the inference given by Professor Vince, that these appearances arise from certainunusual variations of increasing density in the lower strata of the atmosphere, our author undertakes, 1st, to investigate the successive variations of increasing or decreasing density to which fluids in general are liable, and the laws of the refractions occasioned by them; 2dly, to illustrate and confirm the truth of this theory by experiments with fluids of known densities; and lastly, to ascertain, by trial upon the air itself, the causes and extent of those variations of its refractive density on which the inversions of objects and other circumstances observed in the above phenomena seem to depend.