PREFACE TO FOURTH REVISED EDITION, 1913.
This edition is practically that of 1908, with necessary minor revisions and corrections, especially the data referring to indices of refraction and strength of double refraction. The values of the refractive indices α and γ are recorded as well as the mean values. References are also given to “The Methods of Petrographic-Microscopic Research,” by F. E. Wright, 1912, for exact (quantitative) methods of determining crystal constants.