Some of the simpler measurements of optical mineralogy are so precise and powerful that they give satisfaction to beginning students. Not long after mastering the strike and dip of rock surfaces with the Brunton compass, many geology students are able to determine precisely the identity of quartz, or the anorthite content of plagioclase, or the magne sium ratio of pyroxene with the polarizing or petrographic microscope, by means of measuring refractive index to better than one part in a thousand. Very little training and almost no theory are needed to achieve these skills. But there...