554 Proceedings of the British Association. 



by which, he said, the rectilinear bands in polarization were seen more 

 clearly than by other methods. 



' On certain Cases of Elliptically Polarized Light,' by Prof. Powell. — 

 At the last meeting of the Association, Prof. Lloyd gave a theoretical 

 investigation of certain results obtained by Sir D. Brewster relative to 

 thin films from which polarized light is reflected. Besides completely 

 explaining those results, Prof. Lloyd infers, that such films ought 

 to give the portions of light reflected at their two surfaces differing in 

 phase, and that the light should be consequently in general elliptically 

 polarized. The author of the present paper, before he was aware of the 

 investigation of Prof. Lloyd, had made many observations on the ellip- 

 tical polarization of light by reflection from metallic and other sur- 

 faces, — the method of observation being by the well known dislocation 

 of the polarized rings. Some of these experiments went merely to 

 prove the existence of elliptic polarization in cases where it had not 

 previously been detected, as in certain minerals and other bodies in 

 which it is seen, though of small amount. In other cases the reflecting 

 surface consisted of the thin films formed on polished metal by tarnish, 

 by heat, or by the galvanic process of Nobili. In these instances, a 

 verification was afforded of Prof. Lloyd's theory by direct observation. 

 But, further, — these films give periodic colours ; and in passing from 

 one tint to another, the ellipticity, as disclosed by the form of the 

 rings, underwent regular changes, passing from a dislocation in one 

 direction to the opposite, through points of no dislocation or of plane 

 polarization, the rings being alternately dark and bright centered. This 

 afforded a further field for the application of theory, and Mr. Airy 

 investigated a formula for the rings under these varying conditions, 

 with which the phenomena are in perfect accordance. 



Mr. Scott Russell communicated to the Section the results of experi- 

 ments recently made by him, and which he wished to present as a 

 supplement to the former Report of a Committee on Waves. \_Athenceiim, 

 Nos. 517, 565, 566, 618, 675.] On former occasions he had submitted to 

 the Section observations that were principally directed to the examina- 

 tion of one kind of wave, but his present communication referred to 

 new and beautiful phenomena of a different class. Much of the difficul- 

 ty experienced in attaining clear conceptions of the phenomena and 

 mechanism of waves is to be attributed to this circumstance, that we 

 are apt to confound with each other, under the general name of wave 

 motion, a variety of phenomena essentially different in their origin, 

 their form, and their laws. This essential diversity the author of this 

 paper had formerly endeavoured to establish, more especially in the 



