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.

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.