In the case of
certain radiations the speed of propagation becomes nil, and the index
shows sometimes a maximum and sometimes a minimum. All those phenomena
are in close relation with those of absorption.
It is, perhaps, the formula proposed by Helmholtz which best accounts
for all these peculiarities. Helmholtz came to establish this formula
by supposing that there is a kind of friction between 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.
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