36 
professor powell’s observations on certain cases of 
served by Sir D. Brewster, including those of Mr. Airy* on the colours of thin plates 
in polarized light. But Professor Lloyd also infers the further application of the same 
principles to the case of elliptic polarization in the reflexion from polished metals, on 
the hypothesis that their superficial laminae may be regarded as thin plates, or at least 
act in an analogous manner. 
Before I was acquainted with Professor Lloyd’s theoretical investigations, I had 
pursued an experimental examination of the phenomena of elliptic polarization in the 
reflexion from various surfaces, in the course of which I was led to some cases which 
seem to have a more special bearing upon theory, particularly in connexion with the 
views j ust referred to. 
My observations were all conducted by the method of observing the modifications of 
the polarized rings under different conditions, both of surface and of incidence ; and 
were directed to ascertaining both the existence and amount of ellipticity shown by 
the dislocation of those rings, as also to the peculiar character indicated by the direc- 
tion in which the dislocation takes place ; the protrusion of the alternate quadrants 
appearing in certain cases in one direction and in others in the opposite. 
The observations are reducible to two classes: — 1st, those designed to contribute to 
the inquiry what substances possess the property of elliptic polarization ?, by examining 
the light reflected from various bodies ; of which I here notice only a few cases which 
appeared remarkable ; 2ndly, observations on certain cases of Jilms of several kinds, 
including those formed on metal by oxidation, or other action upon the metal itself, 
as well as by extraneous deposition. In these cases the ellipticity generally exists in 
different degrees, and with different characters as to direction, while in some instances 
it is destroyed or reduced to plane polarization. 
I at first noticed these effects as produced in some cases of highly polished metal 
which had become tarnished by long exposure, and in which iridescent films had 
formed on the surface without destroying the polish. And again, in trying the effect 
of heating a metal plate, while observing the rings, I found the ellipticity disappear, 
but soon perceived that the effect was due to the coloured films formed on the surface, 
and remained when the plate was cold. I was thus led to institute more exact ex- 
periments of the same kind, in which the tints were formed in regular succession, as 
well as to examine steel in different stages of tempering. 
From these cases I was naturally led to those of the films of metallic deposit pro- 
duced by the galvanic process of Mobili ; specimens of which were kindly furnished 
me by Dr. Daubeny. In these films it is well known the colours follow, at least ge- 
nerally, the orders of Newton’s scale ; the thickest film being deposited where the 
action is most intense, or where the surfaces in connexion with the two poles most 
nearly approach one another. In all these instances, then, I had a succession of films 
formed upon metal, in which the changes effected in the polarization could be traced 
in regular order. 
* Cambridge Transactions, 1832. 
