The uniform thickness and random orientation of grains in most thin sections are highly desirable features for many types of optical measurements. Unfortunately, however, the precise measurement of indices of permanently mounted material is difficult. One way to overcome this difficulty, while preserving the thin section, is to dissolve the Canada balsam mounting material around the edges by immersing the section in xylene. The solution process is easily watched and is stopped by washing the xylene out with acetone (which dries rapidly) when the xylene just reaches the outer edge of the rock slice (about 15 minutes for most sections). The section is then dried, and immersion oils are inserted under the edge of the cover glass and into contact with the rock. Indices of the various minerals are determined by using different oils sequentially and washing each of them out with acetone. Sections in which the rock slice is attached to the underlying slide by a mounting medium not easily soluble in xylene should have the outer edges of the rock “shaved” off before the cover glass is attached, and the cover should be cemented by Canada balsam. By removing edges which might be coated by an insoluble mounting medium, contact between immersion oil and mineral is assured.