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Poincare, Lucien

"The New Physics and Its Evolution"

Various authors have done so.
Thus, M. Boussinesq assumes that the ether behaves like a very
rarefied gas in respect of the celestial bodies, because these last
move, while bathed in it, in all directions and relatively slowly,
while they permit it to retain, so to speak, its perfect homogeneity.
On the other hand, its own undulations are so rapid that so far as
they are concerned the conditions become very different, and its
fluidity has, one might say, no longer the time to come in. Hence its
rigidity alone appears.
Another consequence, very important in principle, of the fact that
vibrations of light are transverse, has been well put in evidence by
Fresnel. He showed how we have, in order to understand the action
which excites without condensation the sliding of successive layers of
the ether during the propagation of a vibration, to consider the
vibrating medium as being composed of molecules separated by finite
distances. Certain authors, it is true, have proposed theories in
which the action at a distance of these molecules are replaced by
actions of contact between parallelepipeds sliding over one another;
but, at bottom, these two points of view both lead us to conceive the
ether as a discontinuous medium, like matter itself.


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