Page:Collected Physical Papers.djvu/41



The indices of refraction of transparent substances have been determined by the usual optical methods. There are, however, numerous substances like the various rocks, wood, brick, coal-tar, and others which are not transparent to light, so that their indices could not be determined. These substances are, nevertheless, transparent to electric radiation; the present investigation was therefore undertaken to find a direct method of determining their indices with a high degree 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 infrared region of the spectrum. It does not, therefore, seem at all likely that it will give any reliable 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 Hertzs' experiment with the pitch prism, the deviation of the refracted rays extended from 11° to 34°. The approximate value of $$\mu$$ = 1.69, obtained from this experiment, is probably higher than the true value by about 15 to 20 per cent.

For the accurate measurement of deviation, the effect produced by radiation on the receiver should undergo