LAWS OF POLARIZATION. 75 



and will be brightest when the crystal marks E and "NY. The first 01 

 these images is polarized in the plane NS passing through the ray, 

 and the second in the plane EW, perpendicular to the other. And 

 these rays are oppositely polarized. It was further found 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 nature 

 of the polarization, \vas identical in all these cases ; that the alterna- 

 tives of ordinary and extraordinary refraction and non-refraction, were 

 the same, by whatever crystal they were tested, or in whatever manner 

 the polarization had been impressed 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 "polariza- 

 tion " 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 

 communicated. It was found that some crystals, instead of giving, 

 by double refraction, two images oppositely 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. Seebeck. 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 experimenters. 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 plame of refrac- 

 tion ; further, that when a portion of a ray of light was polarized by 

 reflection, a corresponding portion was polarized by transmission, the 

 planes of the two polarizations being at right angles to each other. It 

 was found also that the polarization which was incomplete with a 

 single plate, either by reflection or refraction, might be made more and 

 more complete by increasing the number of plates. 



Among an accumulation of phenomena like this, it is our business 

 to inquire what general laws were discovered. To make such dis- 

 coveries without possessing the general theory of the facts, required 

 no ordinary sagacity and good fortune. Yet several laws were detected 

 at this stage of the subject. Malus, in 1811, obtained the important 

 generalization that, whenever we obtain, by any means, a polarized 

 ray of light, we produce also another ray, polarized in a contrary 



Mem. Inst. 1810. 



