377 



•:-,7:v,- 



XV. — On the Compensations of Polarized Light, with the Description of a 

 Polarimeter, for measuring Degrees of Polarization. By Sir David 

 Brewster, K. H., D. C. L., F. R. S., M. R. I. A., and V. P. R. S. Ed. 



Read November 14, 1842. 



In four papers, printed in the Philosophical Transactions for 1830, I have en- 

 deavoured to determine the general laws of the polarization of light, when reflected 

 from or refracted by the first and second surfaces of bodies, or when suffering 

 total or metallic reflexion. In opposition to the opinions of the most distinguished 

 philosophers, I was led to the conclusion — that when ligh^ was reflected at any 

 angle of incidence between 0° and 90° (excepting at the angle of complete pola- 

 rization), or was refracted at these angles, it did not consist, as they maintained, of 

 two portions, one of which was completely polarized, and the other completely 

 unpolarized or common light ; but that every portion of it had the same physical 

 property, namely, that of having approximated more or less to the state of complete 

 polarization. This general result, which enables us to compute all the phenomena 

 of polarization by reflection and refraction, has, in so far as I know, never been 

 called in question ; but as the investigation was conducted on the supposition, 

 that a pencil, composed of two pencils, polarized -{- 45° and — 45" to the plane of 

 reflexion, was equivalent to a pencil of common light, it became important to have 

 the general result confirmed by experiments made with common light itself; and 

 though the inquiries, the results of which I am now about to explain, had not 

 this object in view, yet it will be satisfactory to find in them a complete demon- 

 stration of my former views.* 



In considering the condition of partially polarized light, it has always appeared 

 to me probable that some method would be found of distinguishing it from a 



• Philosophical Transactions, 1830, pp. 69, 133, 145, 287. 

 VOL. XIX. 3 c 



