a means of determination of the index of refraction of plates of different substances (p. 44).
5. Double-refraction and Polarisation
The shortness of wage-length of electric radiation has enabled investigations on the polarising action of even small crystals. The polariser and analyser are made of fine wire gratings; they can also be made of a number of crystals such as Nemalite, Chrysotile, Serpentine and Epidote. The most perfect polariser and analyser are constructed of jute-fibres or of a book, both of which produce complete polarisation of the electric ray. Crystals of tetragonal, orthorhombic, hexagonal, monoclinic and triclinic systems produce double refraction and polarisation. Locks of human hair and vegetable fibres also produce polarisation (p. 96).
6. Double refraction by strained dielectric
Various stratified rocks exhibit very strong double refraction when the plane of stratification producing polarisation, is inclined at 45° to the crossed polariser and analyser, there being no effect when this plane is parallel to either the polariser or the analyser. Unannealed glass and ebonite also produce double refraction and polarisation of the electric ray (p. 95).
7. Double absorption and double conductivity
Certain crystals like Nemalite, Chrysotile and Epidote exhibit selective transparency to polarised radiation; these are transparent in one direction but opaque in a direction,which is perpendicular to the first. Complete polarisation is due to their property of double absorption. Investigations showed that they also exhibit unequal
