Page:Proceedings of the Royal Society of London Vol 4.djvu/463

Rh the dislocation of those rings, and to determine its peculiar cha- racter, as indicated by the direction in which the dislocation takes place ; the protrusion of the alternate quadrants appearing, in cer- tain cases, in one direction, and in others in the opposite. These observations are reducible to two classes ; first, those designed to contribute to the inquiry, what substances possess the property of elliptic polarization, by examining the light reflected from various bodies ; and second, those made on certain cases of films of several kinds, including those formed on metals by oxidation or other action upon the metal itself, as well as by extraneous deposition. The author found the general result, in all these cases, to be, that from any one tint to another, through each entire order of tints, the form of the rings in the reflected light undergoes certain regular changes; passing from a dislocation in one direction to that in the opposite, through an intermediate point of no dislocation, or of plane polar- ization ; and thus exhibiting a dark and a bright centred system alternately, as long as the order of tints are preserved pure. These changes in the form of the rings, he observes, are precisely those expressed by successive modifications of Mr. Airy's formula, corre- sponding to the increments in the retardation which belong to the periodical colours of the films.

The remaining portion of the paper is occupied by a description of the apparatus and mode of conducting the experiments ; and of the observations made on mica, on decomposed glass, plumbago, daguerreotype, and other metallic plates, and on the coloured films produced on steel and on copper by the action of heat, and of voltaic electricity. The author gives, in conclusion, an analytical inves- tigation of Mr. Airy's general formula.

2. " Variation of the Magnetic Needle as observed at Washing- ton City, D. C, from 3^ SO"* July 24th to 3^ July 25th, 1840, in- clusive (Gottingen mean time)," by Lieut. Gillies, of the United States Service. Communicated by Samuel Hunter Christie, Esq., Sec. R.S.

Sir JOHN WILLIAM LUBBOCK, Bart., V.P. and Treasurer, in the Chair, succeeded by the MARQUIS OF NORTHAMPTON, the President, in the Chair.

John Benjamin Heath, Esq., James MacCullagh, Esq., and George Owen Rees, M.D., were balloted for and duly elected Fellows of the Society.

A paper was read, entitled " Experimental Researches in Electri- city :" Eighteenth Series ; by Michael Faraday, Esq., D.C.L., F.R.S. Section 25. On the Electricity evolved by the Friction of Water and Steam against other bodies.