It is useless to find out to what material body the ether may be compared, if we content ourselves with seeing in it a medium of which, at every point, two vectors define the properties.
For a long time, therefore, we could remark that the theory of Fresnel simply supposed a medium in which something periodical was propagated, without its being necessary to admit this something to be a movement; but we had to wait not only for Maxwell, but also for Hertz, before this idea assumed a really scientific shape.

Hertz insisted on the fact that the six equations of the electric field permit all the phenomena to be anticipated without its being necessary to construct one hypothesis or another, and he put these equations into a very symmetrical form, which brings completely in evidence the perfect reciprocity between electrical and magnetic actions.

He did yet more, for he brought to the ideas of Maxwell the most striking confirmation by his memorable researches on electric oscillations.
Sec.4.ELECTRICAL OSCILLATIONS The experiments of Hertz are well known.

We know how the Bonn physicist developed, by means of oscillating electric discharges, displacement currents and induction effects in the whole of the space round the spark-gap; and how he excited by induction at some point in a wire a perturbation which afterwards is propagated along the wire, and how a resonator enabled him to detect the effect produced.
The most important point made evident by the observation of interference phenomena and subsequently verified directly by M.
Blondlot, is that the electromagnetic perturbation is propagated with the speed of light, and this result condemns for ever all the hypotheses which fail to attribute any part to the intervening media in the propagation of an induction phenomenon.
If the inducing action were, in fact, to operate directly between the inducing and the induced circuits, the propagation should be instantaneous; for if an interval were to occur between the moment when the cause acted and the one when the effect was produced, during this interval there would no longer be anything anywhere, since the intervening medium does not come into play, and the phenomenon would then disappear.
Leaving on one side the manifold but purely electrical consequences of this and the numerous researches relating to the production or to the properties of the waves--some of which, those of MM.

Sarrazin and de la Rive, Righi, Turpain, Lebedeff, Decombe, Barbillon, Drude, Gutton, Lamotte, Lecher, etc., are, however, of the highest order--I shall only mention here the studies more particularly directed to the establishment of the identity of the electromagnetic and the luminous waves.
The only differences which subsist are necessarily those due to the considerable discrepancy which exists between the durations of the periods of these two categories of waves.