therefore, an electrical vibration could
not produce condensations of electricity. It was, in consequence,
necessarily transverse, and thus coincided with the vibration of
Fresnel; while the corresponding magnetic vibration was perpendicular
to it, and would coincide with the luminous vibration of Neumann.

Maxwell's theory thus establishes a close correlation between the
phenomena of the luminous and those of the electromagnetic waves, or,
we might even say, the complete identity of the two. But it does not
follow from this that we ought to regard the variation of an electric
field produced at some one point as necessarily consisting of a real
displacement of the ether round that point. The idea of thus bringing
electrical phenomena back to the mechanics of the ether is not, then,
forced upon us, and the contrary idea even seems more probable. It is
not the optics of Fresnel which absorbs the science of electricity, it
is rather the optics which is swallowed up by a more general theory.
The attempts of popularizers who endeavour to represent, in all their
details, the mechanism of the electric phenomena, thus appear vain
enough, and even puerile. 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 poi