EPOCH OF YOUXG AXD FKESXEL. 101 



out departing from the genuine doctrine of undulation." He then 

 proceeds to suggest the possibility of "a transverse vibration, propa- 

 gated 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 explanation of his views, 

 it appears that he conceived the motions of the particles to be oblique 

 to the direction 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 

 vibrations. This idea at once made it possible to conceive how the 

 rays of light could have sides ; for the direction in which the vibra- 

 tion was transverse to the ray, might be marked by peculiar properties. 

 And after the idea was once started, it was comparatively easy for men 

 like Young and Fresnel to pursue and modify it till it assumed its true 

 and distinct form. 



We may judge of the difficulty of taking firmly hold of the concep- 

 tion of transverse vibrations of the ether, as those which constitute 

 light, by observing how long the great philosophers of whom we are 

 speaking lingered within reach of it, before they ventured to grasp it. 

 Fresnel says, in 1821, " When M. Arago and I had remarked (in 1816) 

 that two rays polarized at right angles always give the same quantity 

 of light by their union, I thought this might be explained by supposing 

 the vibrations to be transverse, and to be at right angles when the rays 

 are polarized at right angles. But this supposition was so contrary to 

 the received ideas on the nature of the vibrations of elastic fluids 1 ' 

 that Fresnel hesitated to adopt it till he could reconcile it better to his 

 mechanical notions. "Mr. Young, more bold in his conjectures, and 

 less confiding in the views of geometers, published it before me, though 

 perhaps he thought it after me."' And M. Arago was afterwards Avon, 

 to relate 10 that when he and Fresnel had obtained their joint experi- 

 mental results of the non-interference of oppositely-polarized pencil-, 

 and when Fresnel pointed out that transverse vibrations were the only 

 possible translation of this fact into the undulatory theory, he himself 

 protested that he had not courage to publish such a conception ; and 

 accordingly, the second part of the Memoir was published in Fresnel's 

 name alone. What renders this more remarkable is, that it oci-nrivd 

 when M. Arago had in his possession the very letter of You MI:, in 

 which he proposed the same suggestion. 



' I take the liberty of stating this from personal knowledge. 



