from Bradley to Fresnel. 113 



afterwards taken up by David Brewster (b. 1781, d. 1868), who 

 in 1815* showed that there is complete polarization by reflexion 

 when the reflected and refracted rays satisfy the condition of 

 being at right angles to each other. 



Almost at the same time Brewster made another discovery 

 which profoundly affected the theory of double refraction. It 

 had till then been believed that double refraction is always 

 of the type occurring in Iceland spar, to which Huygens' 

 construction is applicable. Brewster now found this belief to be 

 erroneous, and showed that in a large class of crystals there are 

 two axes, instead of one, along which there is no double 

 refraction. Such crystals are called Uaxal, the simpler type to 

 which Iceland spar belongs being called uniaxal. 



The wave-theory at this time was still encumbered with 

 difficulties. Diffraction was not satisfactorily explained ; for 

 polarization no explanation of any kind was forthcoming ; the 

 Huygenian construction appeared to require two different 

 luminiferous media within doubly refracting bodies ; and the 

 universality of that construction had been impugned by 

 Brewster's discovery of biaxal crystals. 



The upholders of the emission theory, emboldened by the 

 success of Laplace's theory of double refraction, thought the 

 time ripe for their final triumph ; and as a step to this, in 

 March, 1817, they proposed Diffraction as the subject of the 

 Academy's prize for 1818. Their expectation was disappointed ; 

 and the successful memoir afforded the first of a series of 

 reverses by which, in the short space of seven years, the 

 corpuscular theory was completely overthrown. 



The author was Augustin Fresnel (b. 1788, d. 1827), the 

 son of an architect, and himself a civil engineer in the 

 Government service in Normandy. During the brief dominance 

 of Napoleon after his escape from Elba in 1815, Fresnel fell into 

 trouble for having enlisted in the small army which attempted 

 to bar the exile's return ; and it was during a period of enforced 

 idleness following on his arrest that he commenced to study 



Phil. Trans., 1815, p. 125. 

 I 



