Page:Proceedings of the Royal Society of London Vol 59.djvu/174

160

A List of the Presents received was laid on the table, and thanks ordered for them.

Pursuant to notice, Professor Albert Gaudry, Professor F. Kohlrausch, Professor S. P. Langley, Professor Sophus Lie, and Professor E. Metschnikoff were balloted for and elected Foreign Members of the Society.

The following Papers were read:—

The indices of refraction of transparent substances have been determined by the usual optical methods. There is still a large number of substances like the various rocks, wood, brick, coal-tar, and others which are not transparent to light, so that their indices could not be obtained. These substances are, however, transparent to electric radiation; and the present investigation was undertaken to find a direct method of determining their indices with a sufficient amount of accuracy.

Even in the case of optically transparent substances, the indices are only known for the narrow range of light waves. For greater wave lengths, the index is inferred from Cauchy’s formula. Professor Langley has, however, shown that this formula fails to give trustworthy results when applied to the dark radiations in the infra-red portion of the spectrum. It does not, therefore, seem at all likely that the above formula will give accurate results when applied to the electric radiation.

For the determination of the index for the electric ray, the prism method is unsuitable. In the well-known Hertz’s experiment with the pitch prism, the deviation of the refracted rays extended from 11° to 34°. The approximate value of μ, = 1·69, obtained from this experiment, is probably higher than the true value by about 15 to 20 per cent.