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An all-reflector spectrometer is used to measure the refractive indices of the cubic crystals ZnS, GaP, SrTiO3, and Y3Al5O12; the tetragonal crystals rutile, CaWO4, SrMoO4, CaMoO4; the hexagonal crystals LiTaO3, CdSe, ZnO, and AlPO4; and the monoclinic crystal ZnWO4. Most of these are measured in the wavelength range 0.4 to 4 μ.
W. L. Bond (Sat,) studied this question.