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X. On the Elliptic Polarization of Light by Reflexion from Metallic Surfaces. 

 By the Rev. Baden Powell, M.A,, F.R.S., F.G.S., F.R.A.S., Savilian Professor 

 of Geometry in the University of Oxford. 



Received April 24,— Read June 19, 1845. 



In a former paper, inserted in the Philosophical Transactions, 1843, Part I., I detailed 

 observations on some phenomena of elliptic polarization by reflexion from certain 

 metallic surfaces ; but with reference only to one class of comparative results. From 

 these I have been led to pursue the subject into other relations besides those at first 

 contemplated ; but, from various causes, have only been able at this interval to submit 

 the results to the Royal Society as a sequel to my former observations. 



The changes in the degree of ellipticity, investigated in my former paper, corre- 

 spond to certain changes in the thickness of metaWic Jilms, If we now consider the 

 case of reflexion from a simple polished metallic surface, and admit that in this case 

 it may be supposed to take place by the penetration of the ray to a certain minute 

 depth, or to some action of a thin transparent lamina of the metal, then, in like 

 manner, — dependent on the law of metallic retardation, — the effect would vary with 

 a difference in the effective thickness of the lamina, produced by changing the incli- 

 nation of the incident ray ; and that this is the case in general is well known, viz. 

 that as the incidence is increased, the ellipticity increases up to a maximum, which 

 occurs for most metals at an incidence between 70° and 80°, beyond which it decreases 

 up to 90°. 



The original researches of Sir D. Brewster*, to which we are indebted for the 

 first investigation of these phenomena, afford a striking instance of the legitimate 

 process of inductive inquiry in its first stage, in their total exemption from all refer- 

 ence to any physical theory. In these researches, besides the change in ellipticity 

 with the incidence, there is also included the change in the virtual plane of polariza- 

 tion by metallic reflexion, which, though conjectured to be the same as that inves- 

 tigated by Fresnel for transparent substances -f-, was only examined in detail at 

 incidences at or near that for the maximum ellipticity, to which the author's object 

 immediately restricted him. But for a series of metals constant arcs were accurately 

 determined^, which are the azimuths of polarization of the ray, restored to plane 

 polarization after two reflexions from metal plates at the incidence for the maximum ; 

 while the ellipse, from which the appellation of " elliptic polarization" was derived, 

 is a purely empirical representation of the varying arcs of incidence, considered as 



* Philosophical Transactions, 1830, Part 11. f Ibid. p. 292. % Ibid. p. 294. 



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