A method for determining the refractive indices of transparent isotropic or anisotropic solids is presented. The method utilizes null position measurements of light deflected by a sample immersed in a liquid of known index. Measurements of absolute deflection angles are not involved. Natural crystal growth faces, even at right angles, can serve as prism faces, thus eliminating the need for extensive surface preparation. Consequently, this method is particularly well suited for measuring the refractive index of small samples with high indices. Using a simple refractometer, results were obtained for two biaxial and one isotropic material with high refractive indices: Ni 2SiO4 with the spinel structure (maximum dimension of 0.12 mm), Ni2SiO4 (olivine structure), and GdAlO3 (perovskite structure). Uncertainties of less than 0.1% are obtained from visual observations of the refracted light. Increased accuracy is possible by improving the calibration of the immersion liquid refractive index and the detection system.
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