454 HISTORY OF OPTICS. 



We may judge of the difficulty of taking firmly 

 hold of the conception of transverse vibrations of 

 the ether, as those which constitute light, by ob- 

 serving 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 vibra- 

 tions 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," 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 wont to relate 9 that 

 when he and Fresnel had obtained their joint ex- 

 perimental results, of the non-interference of oppo- 

 sitely polarized pencils, 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, tho 

 second part of the Memoir was published in Fres- 

 nel's name alone. What renders this more remark- 



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



