The late Dr. Pereira has placed the phenomena already described in the form of a most instructive diagram, which we borrow from his elaborate work on "Polarized Light." (Fig. 336.)
Fig. 336.
a. A ray of common or unpolarized light, incident on b. b. The polarizer (a plate of tourmaline). c. A ray of plane polarized light, incident on d. d. The doubly-refracting film of selenite. e. The extraordinary ray. o. The ordinary ray, produced by the double refraction of the ray c. g. The analyser (or doubly-refracting or Nicol's prism). e o. The ordinary ray. e e. The extraordinary ray, produced by the double refraction of the extraordinary ray, e. o o. The ordinary ray. o e. The extraordinary ray, produced by the double refraction of the ordinary ray, o.
The chromatic effects described are not confined to selenite objects only, but are obtained from glass, provided the particles are in a state of unequal tension, as in masses of unannealed glass of various forms. (Fig. 337.) Consequently, polarized light becomes a most valuable means for ascertaining the condition of particles otherwise invisible and inappreciable. One of the most beautiful experiments can be made with a bar of plate-glass, which refracts light singly until pressure is applied to the centre, in order to bend it into an arch or curve, when the appearance presented in Fig. 338 is apparent.
Fig. 337.
No. 1. Unannealed glass for the polariscope. Nos. 2 and 3. Appearance of the black cross and coloured circles in a square and circular piece of unannealed glass in the polariscope.
