410 HISTORY OF OPTICS. 



that whether the ray were polarized by reflection 

 from glass, or from water, or by double refraction, 

 the modification of light so produced, or the na- 

 ture of the polarization, was identical in all these 

 cases; that the alternatives of ordinary and ex- 

 traordinary refraction and non-refraction, were the 

 same, by whatever crystal they were tested, or in 

 whatever manner the polarization had been im- 

 pressed upon the light ; in short, that the property, 

 when once acquired, was independent of everything 

 except the sides or poles of the ray ; and thus, in 

 1811, .the term ; ' polarization" was introduced 4 . 



This being the state of the subject, it became 

 an obvious question, by what other means, and 

 according to what laws, this property was commu- 

 nicated. It was found that some crystals, instead 

 of giving, by double refraction, two images oppo- 

 sitely polarized, give a single polarized image. This 

 property was discovered in the agate by Sir D. 

 Brewster, and in tourmaline by M. Biot and Dr. See- 

 beck. The latter mineral became, in consequence, 

 a very convenient part of the apparatus used in 

 such observations. Various peculiarities bearing 

 upon this subject, were detected by different expe- 

 rimenters. It was in a short time discovered, that 

 light might be polarized by refraction, as well 

 as by reflection, at the surface of uncrystallized 

 bodies, as glass ; the plane of polarization being 

 perpendicular to the plane of refraction ; further, 



4 Mem. Inst. 1810. 



