In answer to your question about the alteration of surface in quartz crystals, I place the glass or quartz plate whose refractive index χ is to be determined between two right-angled flint-glass prisms, with oil of cassia, and measure the angle θ, at which total reflection begins from the hypothenuse of the first flint-glass prism; the angle θ can easily be calculated from i, μ the refractive index of the flint-glass, and P the angle of the prism. Sunlight falls from a collimator with a slit upon the system of flint-glass prisms, and after passing through the second prism is examined by a direct vision set of prisms, I turn the system of flint-glass prisms until the spectrum appears to be broken off at a definite Fraunhofer line, measure the angle i, and thus obtain—