In a similar way I have been able to determine the indices of refraction of various substances, both solid and liquid, for electric radiation. In the case of solids, two semi-cylinders, separated by a suitable parallel air-space, are placed on the spectrometer-circle, the receiver being placed opposite the radiator. The trouble of following the deviated ray is obviated, and the critical angle determined with considerable accuracy. The index of refraction of glass thus found is 2·04; that of commercial sulphur, 1·73.
Double Refraction and Polarisation
I now proceed to demonstrate some of the principal phenomena of polarisation, especially in reference to the polarisation produced by crystals and other substances, and by solid dielectrics when subjected to strain due to pressure or unequal heating.
As the wave-length of electric radiation is many thousand times the wave-length of light, there is a
Fig. 18. Polarisation Apparatus.
K, crystal-holder; S, a piece of stratified rock; C, a crystal; J, jute polariser; W, wire-grating polariser; D, vertical graduated disc, by which the rotation is measured.
