FBESNEL. ON DOUBLE REFRACTION. 261 



own plane AB. By the displacement of this layer, the same 

 effect -will be produced successively on the parallel layers A'B', 

 A"B", &c. ; and in this manner the transversal vibrations of the 

 incident wave may be transmitted throughout the whole extent 

 o'f the medium. 



The force which urges the point M along AB, in consequence 

 of the displacement of the layer E and of the superior layers 

 sliding in their own planes, is owing to this, that their material 

 elements are not contiguous ; if they were, each point M of the 

 layer AB would remain indifferent to a simple sliding of the 

 superior layers, which in that case would produce no alteration 

 in the action exerted by them on this point. But if the dis- 

 placement of these layers took place in the perpendicular direc- 

 tion G M, it is clear that the contiguity of the elements of each 

 of them would not prevent the force with which they tend to 

 repulse each point of A B from increasing in proportion as the 

 distance diminished ; so that, on this supposition, the resistance 

 opposed by the layers to their approximation would be infinitely 

 greater than the force necessary to give a sliding motion to an 

 indefinite layer. Without proceeding to this limit, which doubt- 

 less does not exist in nature, we may suppose that the resistance 

 of the aether to compression is much greater than the force op- 

 posed by it to the small displacements of these layers along their 

 own planes ; now, by help of this hypothesis it is possible to 

 conceive how the molecules of aether may have no sensible oscil- 

 lations except in a direction parallel to the surface of the lumi- 

 nous waves. 



How it may happen that the Molecules of jEther do not undergo 

 any sensible agitation in the direction of the Normal to the 

 Wave. 



The resistance to compression being in fact much greater than 

 the other elastic force put in play by the simple sliding of the 

 layers, the wave produced by the former will extend itself much 

 further than that which results from the second, during the same 

 oscillation of the illuminating particle by the vibrations of which 

 the aether is agitated ; thus, even if the small movements of the 

 molecules of this fluid were performed in such a manner that 

 their vires vivce were equally distributed between the two modes 

 of vibration, the vires vivce comprised in the wave of condensa- 

 tion or dilatation being distributed over a much greater extent 



