While determining the refractive index of fluorite by the method of minimum deviation the writer introduced a nicol prism before the telescope of the goniometer to determine the plane of polarization of the refracted ray. On rotating the nicol no difference in the intensity of light could be observed, but on examining the reflected ray nearly complete polarization was obtained so that it was thought that the prism was not cut at the angle that should give polarization if Brewster’s law applies to isometric crystals. A new prism was then cut from a crystal of fluorite from Madoc, Ontario, so that the angle between the reflected ray and the refracted ray should be ninety degrees. With this new prism the same phenomena were observed. A second nicol was now introduced between the collimator and the crystal so that the signal was extinguished. On rotating the two nicols simultaneously extinction was obtained throughout a complete rotation so that it would appear as though we may accept without question the statement that a ray of light passing through fluorite vibrates with equal facility in all directions at right angles to the direction of propagation and is not polarized.

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