412 HISTORY OF OPTICS. 



D. Brewster stated 6 as the law, which in all cases 

 determines this angle, that "the index of refrac- 

 tion is the tangent of the angle of polarization." 

 It follows from this, that the polarization takes 

 place when the reflected and refracted rays are at 

 right angles to each other. This simple and elegant 

 rule has been fully confirmed by all subsequent 

 observations, as by those of MM. Biot and See- 

 beck ; and must be considered one of the happiest 

 and most important discoveries of the laws of phe- 

 nomena in Optics. 



The rule for polarization by one reflection being 

 thus discovered, tentative formulae were proposed 

 by Sir D. Brewster and M. Biot, for the cases in 

 which several reflections or refractions take place. 

 Fresnel also, in 1817 and 1818, traced the effect 

 of reflection in modifying the direction of polar- 

 ization, which Malus had done inaccurately in 

 1810. But the complexity of the subject made 

 all such attempts extremely precarious, till the 

 theory of the phenomena was understood, a period 

 which now comes under notice. The laws which 

 we have spoken of were important materials for 

 the establishment of the theory ; but in the mean 

 time, its progress at first had been more forwarded 

 by some other classes of facts, of a different kind, 

 and of a longer standing notoriety, to which we 

 must now turn our attention. 



6 Phil, Trans. 1815. 



