The double refraction of crystals may be due (1) to anisotropy of theions taken separately; (2) to the anisotropic arrangement of the ions in the lattice, which results in an anisotropic force even if the ions are undisturbed; (3) to an anisotropic Lorentz-Lorenz force due to the anisotropic arrangement of the ions. Ewald and Bragg have assumed (3) alone to be present. Here calculations are made for effect (3) in cubic crystals under stress (forced double refraction, photo-elasticity). The effect seems not to be explained entirely by (3). Incidentally it is shown how the Lorentz-Lorenz force is built up in the unstrained crystal. It turns out that the force−4πP+4π/3P is almost entirely due to the plane passing through the considered ion.
© 1928 Optical Society of AmericaFull Article | PDF Article
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