Now we know that in each principal plane of a crystal the ray polarized in that plane obeys the ordinary law of refraction, and therefore its velocity is the same in whatever direction in that plane it is propagated. If polarized light consists of electromagnetic disturbances in which the electric displacement is in the plane of polarization then a<sup>2<\sup> = b<sup>2<\sup> = c<sup>2<\sup> (93) If on the contrary the electric displacements are perpendicular to the plane of polarization [lambda] = [mu] = [nu] (94) We know, from the magnetic experiments of Faraday, Plucker, &c that in many crystals [lambda] [mu] [nu] are unequal The experiments of Tyndall (Phil Mag 1852) on electric induction through crystals seem to show that a b & c may be different The inequality however of [lambda] [mu] [nu] is so small that great magnetic forces are required to indicate their difference, and the differences do not seem of sufficient magnitude to account for the double refraction of the crystals. On the other hand, experiments on electric induction are liable to error on account of minute flaws or portions of conducting matter in the crystal. Further experiments on the magnetic and dielectric properties of crystals are required before we can decide whether the relation of these bodies to <s>permanent<\s>magnetic and electric forces is the same when these forces are permanent or alternating with the rapidity of the vibrations of light.
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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