In my address at the Annual Meeting of the Mineralogical Society last year, in Plymouth, I gave an account of a new method of studying the optical properties of minerals, and of determining their indices of refraction. I then showed that an approximate knowledge of these characters would, in many cases, enable us to identify minerals with great confidence, and also would sometimes furnish us with valuable hints as to their general chemical composition. My remarks had reference exclusively to detached specimens of more or less considerable thickness, and I scarcely thought that it would ever be possible to apply this method successfully with comparatively high powers in the case of thin sections of rocks. I feared that certain errors, which appeared to be unavoidable, would then become so great in proportion to the difference in the indices of many of the commoner minerals, that it would be impossible to distinguish them with confidence.