The terms axes and directions of elasticity, used throughout this book, are very commonly employed in petrographical literature of the present time. These axes and directions should probably more correctly be called axes and directions of vibration or extinction. The reasons for or against the elastic condition of the “ether” are of more interest, however, to the physicist than to the petrographer. An optical scheme is appended, with the minerals grouped according to their common optical characters. The writer’s thanks are due to Dr. A. J. Moses, Professor of Mineralogy, and to Mr. J. F. Kemp, Professor of Geology, for kind suggestions offered during the preparation of this book. LEA MCI. LUQUER. DEPARTMENT OF MINERALOGY, COLUMBIA UNIVERSITY, N. Y. CITY, October, 1898. TABLE OF CONTENTS PAGE. CONVENTIONS AND ABBREVIATIONS ix CHAPTER I. INTRODUCTORY OPTICS FOR OPTICAL MINERALOGY. Ordinary Light.—Plane Polarized Light.—Effects Produced by Crystal Sections on Transmitted Light.—Amorphous Bodies.—Isotropic Crystals.—Anisotropic Crystals.—Double Refraction.—Uniaxial Crystals.—Biaxial Crystals.—Principal Vibration Directions.—Axial Angle.—Dispersion. 1 CHAPTER II. PETROGRAPHICAL MICROSCOPE. Reflector.—Polarizer.—Nicol Prism.—Condensing Lens.—Rotating Stage.—Objectives.—Analyzer.—Eye-pieces. 7 CHAPTER III. INVESTIGATION OF MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS. Opaque Minerals. Transparent Minerals: With transmitted light: _Form_, _Color_, _Index of Refraction and Relief_, _Becke Method_, _van der Kolk Method_, _Cleavage_, _Fracture_, _Inclusions_. With polarized transmitted light: _Pleochroism_. With crossed nicols: _Isotropic Character_, _Anisotropic Character_, _Interference Colors_, _Extinction_ and _Extinction Angles_, _Vibration Directions of Faster and Slower Rays_, _Order of Interference Color_, _Strength of Double Refraction_, _Determination of Minerals and Thickness of Section_, _Structure_. With convergent light: _Uniaxial Interference Figures_ and _Optical Character_, _Biaxial Interference Figures_ and _Optical Character_, _Determination of Axial Angle_, _Distinctions between Orthorhombic, Monoclinic and Triclinic Sections_. Resumé of Uses of Parallel and Convergent Polarized Light. 13 CHAPTER IV. MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS. Amorphous Minerals.—Isometric Minerals.—Tetragonal Minerals.—Hexagonal Minerals.—Orthorhombic Minerals.—Monoclinic Minerals.—Triclinic Minerals.—Mineral Aggregates. 51