EPOCH OF YOUNG AND FRESNEL. 461 



and the result at which he arrived was, that the 

 reflection of light through a rhomb of glass of a 

 certain form (since called Fresnel's rhomb), would 

 produce a polarization of a kind altogether different 

 from those which his theory had previously con- 

 sidered, namely, that kind which we have spoken of 

 as circular polarization. The complete confirma- 

 tion of this curious and unexpected result by trial, 

 is another of the extraordinary triumphs which have 

 distinguished the history of the theory at every step 

 since the commencement of Fresnel's labours. 



But anything further which has been done in 

 this way, may be treated of more properly in re- 

 lating the verification of the theory. And we have 

 still to speak of the most numerous and varied 

 class of facts to which rival theories of light were 

 applied, and of the establishment of the undulatory 

 doctrine in reference to that department; I mean 

 the phenomena of depolarized, or rather, as I have 

 already said, ^'polarized light. 



Sect. 5. Explanation of Dipolarization by the 

 Undulatory Theory. 



WHEN Arago, in 1811, had discovered the colours 

 produced by polarized light passing through certain 

 crystals 15 , it was natural that attempts should be 

 made to reduce them to theory. M. Biot, animated 

 by the success of Malus in detecting the laws of 

 15 See p. 420. 



