The crystallographic-optical theory upon which the determination of the indices of refraction of a birefringent mineral by the immersion method is based, is very adequately treated in a number of texts, so it is not the purpose of this paper to approach the subject from that standpoint. But the application of this theory to the routine determination of minerals is an art which the student acquires chiefly by practice. The student finds himself confronted with the necessity of doing three things simultaneously, no one of which in itself would give him much difficulty, but taken together they contrive to delay his mastery of the petrographic microscope. The first essential, of course, is an understanding of the theory upon which the determinations of the indices of refraction are based; secondly, he must interpret his observations under the microscope in terms of this theory; and, thirdly, he must at the same time develop a manipulative technique or skill in the use of the microscope as a mechanical instrument. The present treatment is an attempt to bridge the gap between theory and practice.