EPOCH OF YOUNG AND FRESNEL. 453 



colours ; and he had the satisfaction to see Fresnel 

 re-discover, and M. Arago adopt, his views on dif- 

 fraction. He became engaged in friendly inter- 

 course with the latter philosopher, who visited him 

 in England in 1816. On January the 12th, 1817, 

 in writing to this gentleman, among other remarks 

 on the subject of optics, he says, " I have also been 

 reflecting on the possibility of giving an imperfect 

 explanation of the affection of light which con- 

 stitutes polarization, without departing from the 

 genuine doctrine of undulation." He then proceeds 

 to suggest the possibility of "a transverse vibra- 

 tion, propagated in the direction of the radius, the 

 motions of the particles being in a certain constant 

 direction with respect to that radius ; and this," he 

 adds, " is polarization" From his further explana- 

 tion of his views, it appears that he conceived the 

 motions of the particles to be oblique to the direc- 

 tion of the ray, and not perpendicular, as the theory 

 was afterwards framed; but still, here was the 

 essential condition for the explanation of the facts 

 of polarization, the transverse nature of the vibra- 

 tions. This idea at once made it possible to con- 

 ceive how the rays of light could have sides; for 

 the direction in which the vibration was transverse 

 to the ray, might be marked by peculiar properties. 

 And after the idea was once started, it was com- 

 paratively easy for men like Young and Fresnel 

 to pursue and modify it till it assumed its true and 

 distinct form. 



