With rare exceptions only two principal indices of refraction of a biaxial crystal can be determined directly on the universal stage by the immersion method, since mechanical limitations prevent rotating all the way to the appropriate position for the third principal index. This usually must be determined by extrapolation from an intermediate index determined at a chosen rotation toward the third position. The extrapolated value, however, is subject to error, for the index at the intermediate position is measured where the change of index per degree of rotation is large. If the universal stage and lens system are not in optimum adjustment, the “corrected optical rotation” of the mineral grain and in turn the extrapolated value of the third principal index are erroneous.

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