all who hear it for the first time, and it
is not rare to hear it appealed to as an argument against the actual
existence of the ether. It does not seem, however, that such an
argument can be decisive. There is no reason for supposing that the
ether ought to be a sort of extension of the bodies we are accustomed
to handle. Its properties may astonish our ordinary way of thinking,
but this rather unscientific astonishment is not a reason for doubting
its existence. Real difficulties would appear only if we were led to
attribute to the ether, not singular properties which are seldom found
united in the same substance, but properties logically contradictory.
In short, however odd such a medium may appear to us, it cannot be
said that there is any absolute incompatibility between its
attributes.

It would even be possible, if we wished, to suggest images capable of
representing these contrary appearances. 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. The ideas
gathered from the most recent experiments also bring us to the same
conclusion.

Sec. 2. RADIATIONS

In the ether thus constituted there are therefore propagated
transverse vibrations, regarding which all experiments in optics
furnish very precise information. The amplitude