76 HISTORY OF OP1ICS. 



direction ; thus when reflection gives a polarized ray, the ccmpanion- 

 ray is refracted polarized oppositely, along with a quantity of unpolar- 

 ized light. And we must particularly notice Sir D. Urewster's rule 

 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 

 discoverable rule with regard to the order of refractive or dispersive 

 powers of the substances. Yet the rule was in reality very simple. 

 In 1815, Sir D. Brewster stated* as the law, which in all cases deter- 

 mines this angle, that " the index of refraction 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 Seebeck ; and 

 must be considered one of the happiest and most important discoveries 

 of the laws of phenomena in Optics. 



The rule for polarization by one reflection being thus discovered, 

 tentative formulas 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 polarization, 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. 



CHAPTER VII. 



DISCOVERY OF THE LAWS OF THE COLOURS OF THIN PLATES. 



facts which we have now to consider are remarkable, inasmuch 

 J- as the colours are produced merely by the smallness of dimensions 

 of the bodies employed. The light is not analysed by any peculiar 



5 JA'/w. List. 1810. 6 Phil. Trans. 1815 



