PREFACE TO REVISED EDITION, 1905.
In preparing the revised edition, Chapters I. and IV. have been rewritten and enlarged and the part relating to the determination of the plagioclases has been greatly amplified. Many additions have also been made to Chapter III. and the Becke method, for the determination of the relative indices of refraction of minerals, has been given in detail.
Some new and useful tables have been introduced; as tables of refractive indices (mean) and double refraction (maximum). A diagram has also been added, showing the relation existing between strength of double refraction, interference colors and thickness of section.
Professor E. Weinschenk’s admirable text-book, “Die Gesteinsbildenden Mineralien”, Freiburg, 1901, has been specially referred to, and the tables of refractive indices and double refraction have been compiled from Weinschenk’s new Tables.
Many new cuts have been added, among them being semi-ideal drawings, showing typical outlines of crystal sections, cleavage, optical orientation, etc. In describing the “Usual Appearance in Sections” of a mineral, it is of course only possible to mention the usual crystal form in which the mineral occurs in a rock. The crystal may be cut in any way by the plane of the section; but a general knowledge of the crystal forms will furnish an idea as to the outline, etc., that the mineral may show in the section.