EPOCH OF YOUNG AND FRESNEL. 451 



The extension of this view, of the different elas- 

 ticity of crystals in different directions, to other 

 than uniaxal crystals, was a more complex and 

 difficult problem. The general notion was perhaps 

 obvious, after what Young had done ; but its appli- 

 cation and verification involved mathematical cal- 

 culations of great generality, and required also very 

 exact experiments. In fact, this application was 

 not made till Fresnel, a pupil of the Polytechnic 

 School, brought the resources of the modern ana- 

 lysis to bear upon the problem; till the pheno- 

 mena of dipolarized light presented the properties 

 of biaxal crystals in a vast variety of forms ; and 

 till the theory received its grand impulse by the 

 combination of the explanation of polarization with 

 that of double refraction. To the history of this 

 last-mentioned great step we now proceed. 



Sect. 4. Explanation of Polarization by the 

 Undulatory Theory. 



EVEN while the only phenomena of polarization 

 which were known were those which affect the two 

 images in Iceland spar, the difficulty which these 

 facts seemed at first to throw in the way of the 

 undulatory theory was felt and acknowledged by 

 Young. Malus's discovery of polarization by reflec- 

 tion increased the difficulty, and this Young did not 

 attempt to conceal. In his review of the papers 

 containing this discovery 8 he says, " The discovery 



8 Quart. Rev. May, 1810. 



GG2 



