EPOCH OF YOUNG AND FRESNEL. 463 



well as the general defect of its being an arbitrary 

 and unconnected hypothesis. 



Young's mode of accounting for the brilliant 

 phenomena of dipolarization appeared in the Quar- 

 terly Review for 1814. After noticing the disco- 

 veries of MM. Arago, Brewster, and Biot, he adds, 

 " We have no doubt that the surprize of these gen- 

 tlemen will be as great as our own satisfaction in 

 finding that they are perfectly reducible, like other 

 causes of recurrent colours, to the general laws of 

 the interference of light which have been established 

 in this country;" giving a reference to his former 

 statements. The results are then explained by the 

 interference of the ordinary and extraordinary ray. 

 But, as M. Arago properly observes, in his account 

 of this matter 17 , "It must, however, be added that 

 Dr. Young had not explained either in what circum- 

 stances the interference of the rays can take place, 

 nor why we see no colours unless the crystallized 

 plates are exposed to light previously polarized." 

 The explanation of these circumstances depends on 

 the laws of interference of polarized light which 

 MM. Arago and Fresnel established in 1816. They 

 then proved, by direct experiment, that when po- 

 larized light was treated so as to bring into view 

 the most marked phenomena of interference, namely, 

 the bands of shadows ; pencils of light which have 

 a common origin, and which are polarized in the 

 parallel planes, interfere completely, while those 



11 Enc. Brit. Supp. art. Polarisation. 



