212 FRESNEL. 



directly from the natural state to that of rays polarized 

 at right angles to each other, lose altogether the prop- 

 erty of interfering ; let them be modified afterwards as 

 to the routes they pursue in a thousand ways, or as to 

 the nature and thicknesses of the media they traverse ; 

 or even more, let them be brought back by suitable re- 

 flexions to the condition of parallel polarization ; nothing 

 of this kind can give them again the property of being 

 able to destroy each other. 



But if two rays already polai'ized in directions at right 

 angles to each other, and which in consequence cannot 

 act one on the other, have then received jjarallel polar- 

 ization, in passing out of their natural state, it will suffice, 

 in order that they again acquire the power of inter- 

 ference, to cause them to resume the kind of polarization 

 which they originally possessed.* 



* The question as to the nature and modifications of the vibrations 

 ■whose aggregate in their different stages, or phases, constitutes a 

 wave, may require a word or two of illustration. 



In the first instance, in the conception of waves, those who pursued 

 such a theory generallj- adopted the idea that the tethereal molecules 

 oscillated backwards and forwards in the line of the ray; thej' could 

 not admit the idea of their oscillating in any other direction. Yet, 

 oscillations in any direction occurring in regular succession, might 

 constitute a wave. 



The difficulty, when more fully examined, had reference to the 

 determination on admitted dynamical principles of the mode in which 

 the force propagating the ray and acting in its direction could give 

 rise to lateral disturbance. Yet it is easy to admit, as a rough illus- 

 tration, the case of a rope fastened at one end and agitated at the 

 other by the hand; when we can easily cause a series of waves to 

 run along it; but the particles of the rope really retain their original 

 distances from the hand, and merely move up and down in directions 

 transverse to its length. In a somewhat similar way, the EBthereal 

 molecules are, according to this theory, made to vibrate, or as Fres- 

 nel afterwards graphically expressed it, to " tremble laterally." 



At length. Young began to entertain the idea tliat the molecules 



