tween the ether and
matter, which, like that exercised on a pendulum, here produces a
double effect, changing, on the one hand, the duration of this
oscillation, and, on the other, gradually damping it. He further
supposed that ponderable matter is acted on by elastic forces. The
theory of Helmholtz has the great advantage of representing, not only
the phenomena of dispersion, but also, as M. Carvallo has pointed out,
the laws of rotatory polarization, its dispersion and other phenomena,
among them the dichroism of the rotatory media discovered by M.
Cotton.

In the establishment of these theories, the language of ordinary
optics has always been employed. The phenomena are looked upon as due
to mechanical deformations or to movements governed by certain forces.
The electromagnetic theory leads, as we have seen, to the employment
of other images. M.H. Poincare, and, after him, Helmholtz, have both
proposed electromagnetic theories of dispersion. On examining things
closely, it will be found that there are not, in truth, in the two
ways of regarding the problem, two equivalent translations of exterior
reality. The electrical theory gives us to understand, much better
than the mechanical one, that _in vacuo_ the dispersion ought to be
strictly null, and this absence of dispersion appears to be confirmed
with extraordinary precision by astronomical observations. Thus the
observation, often repeated, and at different times of year, proves
that in the case of the star Algol, the light of which takes at least
four years to reach us, no sensible difference in coloration
accompanies the changes in brilliancy.

Sec. 2. THE THEORY OF LORENTZ

Purely mechanical considerations have therefore failed to give an
entirely satisfactory interpretation of the phenomena in which even
the simplest relations between matter and the ether appear. They
would, evidently, be still more insufficient if used to explain
certain effects produced on matter by light, which could not, without
grave difficulties, be attributed to movement; for instance, the
phenomena of electrification under the influence of certain
radiations, or, again, chemical reactions such as photographic
impressions.

The problem had to be approached by another road. The electromagnetic
theory was a step in advance, but it comes to a standstill, so to
speak, at the moment when the ether penetrates into matter. If we wish
to go deeper into the inwardness of the phenomena, we