OF NATURAL PHILOSOPHY. 261 



tion, of which, in the hands of M. Fresnel, a late 

 eminent French geometer, it also furnished a com- 

 plete explanation, and that, too, in cases to which 

 Newton's hypothesis could not apparently be made 

 to apply, and through a complication of circum- 

 stances which might afford a very severe test of 

 any hypothesis. 



(289.) A simple and beautiful experiment on the 

 interferences of polarized light due to Fresnel and 

 Arago enabled them to bring Dr. Young's law to 

 bear on the colours produced by crystallized plates 

 in a polarized beam, and by so doing afforded a 

 key to all the intricacies of these magnificent but 

 complex phenomena. Nothing now was wanting to 

 a rational theory of double refraction but to frame 

 an hypothesis of some mode in which light might 

 be conceived to be propagated through the elastic 

 medium supposed to convey it in such a way as not 

 to be contradictory to any of the facts, nor to 

 the general laws of dynamics. This essential idea, 

 without which every thing that had been before 

 done would have been incomplete, was also fur- 

 nished by Dr. Young, who, with a sagacity which 

 would have done honour to Newton himself, had 

 declared, that to accommodate the doctrine of 

 Huyghens to the phenomena of polarized light it is 

 necessary to conceive the mode of propagation of 

 a luminous impulse through the ether, differently 

 from that of a sonorous one through the air. In the 

 latter, the particles of the air advance and recede ; 

 in the former, those of the ether must be supposed 

 to tremble laterally. 



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