In the field of optical mineralogy, the birefringence or partial birefringence of the mineral section is determined by observing interference colors in crossed-polarized light of petrographic polarizing microscope with the diaphragm below the stage open, and using a low- or moderate-power objective lens. Using the Michael-Levy color chart, this can be illustrated diagrammatically. With the aid of the chart if one is available, it can be found that the values of birefringence are usually represented by lines radiating from the lower left hand corner of the diagram. The thickness of the mineral section is represented by the horizontal lines, and the interference colors are the ones in vertical lines. As illustrated, basically the technique involves the determining of the thickness of the mineral section and the interference color.