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 polarized in directions at right 
angles to each other, and which in consequence cannot 
act one on the other, have then received parallel 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 generally adopted the idea that the ethereal molecules 
oscillated backwards and forwards in the line of the ray; they 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 zthereal 
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 that the molecules 
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