LAWS OF POLARIZATION. 411 



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 polariza- 

 tions being at right angles to each other. It was 

 found also that the polarization which was incom- 

 plete 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 discoveries with- 

 out possessing the general theory of the facts, re- 

 quired 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 direction ; thus 

 when reflection gives a polarized ray, the com- 

 panion-ray is refracted polarized oppositely, along 

 with a quantity also of unpolarized light. And we 

 must particularly notice Sir D. Brewster's ride for 

 the polarizing angle of different bodies. 



Malus 5 had said that the angle of reflection 

 from transparent bodies, which most completely 

 polarizes the reflected ray, does not follow any dis- 

 coverable rule with regard to the order of refrac- 

 tive or dispersive powers of the substances. Yet 

 the rule was in reality very simple. In 1815, Sir 



9 Mem. Inst. 1810. 



