EPOCH OF YOUNG AND FRESNEL. 459 



In the execution of his task, indeed, Fresnel was 

 forced upon several precarious assumptions, which 

 make, even yet, a wide difference between the theory 

 of gravitation and that of light. But the mode in 

 which these were confirmed by experiment, com- 

 pels us to admire the apparently happy boldness of 

 the calculator. 



The subject of polarization by reflection was 

 one of those which seemed most untractable ; but, 

 by means of various artifices and conjectures, it was 

 broken up and subdued. Fresnel began with the 

 simplest case, the reflection of light polarized in 

 the plane of reflection ; which he solved by means 

 of the laws of collision of elastic bodies. He then 

 took the reflection of light polarized perpendicu- 

 larly to this plane ; and here, adding to the general 

 mechanical principles a hypothetical assumption, 

 that the communication of the resolved motion 

 parallel to the refracting surface, takes place ac- 

 cording to the laws of elastic bodies, he obtains his 

 formula. These results were capable of comparison 

 with experiment ; and the comparison, when made 

 by M. Arago, confirmed the formulae. They ac- 

 counted, too, for Sir D. Brewster's law concerning 

 the polarizing angle (see Chap, vi.) ; and this could 

 not but be looked upon as a striking evidence of 

 their having some real foundation. Another artifice 

 which MM. Fresnel and Arago employed, in order 

 to trace the effect of reflection upon common light, 

 was to use a ray polarized in a plane making half 



