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 of these vibrations is exceedingly small, even in relation to the wave-length, small as these last are.