PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 453 



FIG. 202. AUGUSTE FRESNEL. 



Malus, who was a declared partizan of the emission theory. A few years 

 afterwards (in 1817) we find Young taking a more hopeful view of the 

 capabilities of the undulatory theory, and a passage from one of his 

 letters to Arago, in which the subject is referred to, is highly interesting 

 as giving the first hint of the transverse vibrations, and showing that his 

 previous conception of the undulations of the ethereal medium corre- 

 sponded with that entertained regarding the undulations by which 

 sound is propagated. " I have been reflecting," says Young, " upon 

 the possibility of giving an imperfect explanation of the affection of 

 light which constitutes polarization, without departing from the genuine 

 doctrine of undulations. It is a principle of the theory that all undu- 

 lations are simply propagated through homogeneous mediums, in con- 

 centric spherical surfaces, like the undulations of sound, consisting 

 simply of the direct and retrograde motions of their particles in the 

 direction of the radius, with their concomitant condensations and rare- 

 factions. And yet it is possible to explain in this theory a transverse 



