The Immersion Method
The immersion method permits measurement of refractive index by comparison of the unknown crystal with liquids of known refractive index. A determination of refractive index usually suffices for the identification of isotropic crystals. Refractive index determinations are also fundamental in identification of anisotropic crystals, and the techniques of this chapter are applicable to these as well. Anisotropic crystals, however, have either two or three principal refractive indices, and determination of these requires alignment of principal vibration directions with the polarizer, as well as certain other techniques described in succeeding chapters. Anisotropic crystals also possess other optical and crystallographic characteristics such as optic angle and extinction angle, which are usually needed for identification, and may be determined at the same time as refractive index.
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