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 6 = 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. 
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