ETTIET: AND MATTEu1.
Among the phenomena of light Which can he explained only by assuming sonic definite con nection between the ether and matter. are dis persion. both regular and anomalous: absorption and emission: color: fluorescence and phosphores cence: rotation of the plane of polarization by quartz and other bodies: metallic reflection; the various magnetic actions on light. etc.
DtspEnsioN. As has been already explained, dispersion is due to the fact that ether-waves of different wave-length have different indices of re fraction for any one material substance; that is, they have different velocities in it. As a rule the velocity decreases, i.e. the index of refrac tion increases as the wave-length decreases. This is called regular dispersion. There are, how ever, many substances for which this is not true; for instance, in fuchsine the indices of refraction of the red and yellow waves are great er than those for the blue and violet. If a spectrum of white light is formed on a screen by means of a slit, a prism of glass, and a lens, the colors will be arranged in the order red. yellow,
green, blue, violet—the red being deviated least, i.e. having the least index of refraction. If, however, a prism of fnchsine is used in place of the glass one, the order of colors will be blue, violet, red, yellow—the green being absent. Fuchsine is a substance having a green 'surface color' (see Count), and so this color is absent in the transmitted light. The resulting spectrum produced by a fuchsine prism is then exactly as if the ordinary spectrum had been cut in two by the removal of the green and the two halves dis placed over each other. In other words, with bodies possessing surface color of a definite wave length, the indices of refraction of the waves on the two sides of the absorbed waves are changed; if their indices are less than that of the absorbed color for ordinary media, they are increased, and vice versa. This is known as `anomalous dis persion.'