PROCEEDINGS 



OF 



THE ROYAL IRISH ACADEMY. 



1842. No. 36. 



November 14. 

 SIR Wm. R. HAMILTON, LL.D., President, in the Chair. 



The Secretary read a paper by Sir David Brewster, " on 

 the Compensations of Polarized Light, with a description of 

 a Polarimeter for measuring Degrees of Polarization." 



The author first directed attention to the difference of 

 opinion between him and most other philosophers, as to the 

 constitution of partially polarized light ; it being generally 

 supposed that such light is a mixture of common light and 

 perfectly polarized light, whilst he considers that the entire 

 quantity of light undergoes a physical change by approxima- 

 ting more or less to the condition of light completely pola- 

 rized. Upon this view he had long since explained the laws 

 of polarization discovered by himself, but he had been 

 anxious to obtain experimental evidence capable of deciding 

 between the two ideas, and in this he considers that he is 

 now successful. 



By means of experiments, — described in the paper, — the 

 author points out that when two portions of light oppositely 

 polarized compensate each other, the proportions and con- 

 ditions necessary are not those which could result from mix- 

 tures of common light with fully polarized light, and hence 

 infers that the pencils must be wholly in different physical 

 conditions. These experiments led him to the invention of 



VOL. II. 2 A 



