A small crystal or crystal fragment mounted on the tip of a spindle in a suitable holder on the polarizing microscope stage may be oriented quickly and accurately for measurement of all its principal indices of refraction by the immersion method. Independent measurements or estimations of other properties, such as optic angle, optic sign, dispersion, pleochroism, and the relation of the indicatrix to cleavage or crystal faces may be made on the same fragment. Besides its usefulness in determininative mineralogy, this inexpensive device has much to recommend it as a teaching aid in optical crystallography, since, with its help, the student rapidly acquires facility and confidence in the application of interference figures and extinction angles to mineralogic and petrographic problems. The device used here is based on that of Rosenfeld (1950) but is of somewhat more rugged construction and provides for reading of angular rotations about the spindle axis. For mounting the fragment a mixture of ordinary carpenter's glue and molasses is recommended. Either orthoscopic or conoscopic illumination may be used to orient the fragment for measurement of its principal indices. The orthoscopic procedure can give results to the limit of accuracy of the immersion method employed, while the conoscopic procedure is usually faster and gives results sufficiently accurate for all but the most exacting work, besides providing additional optical information.

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