256 FRESNEL. 



As to the system of waves, the interferences are so 

 natural a deduction from it, that we have some reason to 

 be astonished that experimenters should have discovered 

 them before theory had indicated them. To convince 

 ourselves of this, it suffices to remark that a wave, in 

 propagating itself through an elastic medium, communi- 

 cates to the molecules of which it is composed an oscil- 

 latory motion, in virtue of which they displace themselves 

 successively in two opposite directions : this being under- 

 stood, it is evident that a series of waves will destroy 

 completely the effect of another series, if at every 

 point in the fluid the motion in one direction which the 

 first wave produces alone, shall coincide with the motion 

 in the opposite direction which would result from the 

 sole action of the other wave. The molecules solicited 

 at the same time by equal forces diametrically opposed, 

 will then remain at rest, for as long a period as they 

 would have freely oscillated if under the action of one 

 wave alone. Motion has destroyed motion ; now motion 

 is light. 



I will not push further this enumeration, because we 

 can already judge on how many points the antagonists of 

 the emission theory have been successful in their attacks. 

 Experiments so numerous, so varied, so delicate, as those 

 I have referred to, do not alone testify all the importance 

 which the question seems to them to possess ; they must 



perpendicularly, might be reflected internally at that angle, and, 

 passing to the opposite side, be reflected again internally at the same 

 angle ; after two reflexions it would emerge, consisting of two pencils 

 polarized at right angles to each other, and having a difference of 

 phase d = 90, and would thus possess a circular polarization ; or if 

 the inclination was any other than 45 and 6 differing from 90, the 

 polarization would be elliptic of different degrees; all which conclu- 

 sions are fully verified by experiments as before noticed. 



