the spectrum, while the order of the other colors was different from that in the normal spectrum. In the spectrum of fuchsin the colors in order, beginning with the one most deviated, were violet, red, orange, and yellow. Other substances give rise to anomalous dispersion in which the order of the colors is different.
In order to account for these phenomena, the ordinary theory of light is extended by the assumption that the ether and molecules of a body materially interact upon one another, so that the vibrations in a light-wave are modified by the vibrations of the molecules of a transparent body through which light is passing. This hypothesis, in the hands of Helmholtz and Ketteler, has been sufficient to account for most of the phenomena of light.
