282 THOMAS YOUNG. 



pondering the question, Young was at length forced to 

 the conclusion that the vibrations concerned in the pro- 

 pagation of light were executed at right angles to the 

 direction of the ray. By this assumption of transverse 

 vibrations, which removed all difficulty, Young also 

 removed the ether from the class of aeriform bodies, 

 and endowed it with the properties of a semi-solid. 



Fresnel's memoir on Diffraction, upon which, as 

 already stated, Arago had reported, initiated a lasting 

 friendship between the two illustrious Frenchmen. 

 They subsequently worked together. Fresnel, the 

 more adventurous and powerful spirit of the two, came 

 independently to the same conclusion that Young had 

 previously enunciated. But so daring did the idea of 

 transverse vibrations appear to Arago so inconsistent 

 with every mechanical quality which he could venture 

 to assign to the ether that he refused to allow his 

 name to appear in conjunction with that of Fresnel on 

 the title-page of the memoir in which this heretical 

 doctrine was broached. Still, the heresy has held 

 its ground, and the theory of transverse vibrations, 

 as applied to the luminiferous ether, is now univer- 

 sally entertained. 



Fresnel died in the fortieth year of his age. 



Allow me to wind up this section of our labours by 

 reference to a German estimate of Young's genius. 

 ' His mind,' says Helmholtz, ' was one of the most pro- 

 found that the world has ever produced ; but he had 

 the misfortune of being too much in advance of his age. 

 He excited the wonder of his contemporaries, who, how- 

 ever, were unable to rise to the heights at which his 

 daring intellect was accustomed to soar. His most im- 

 portant ideas lay, therefore, buried and forgotten in the 

 folios of the Eoyal Society, until a new generatiou 



