Double image, 2 Double refraction, 2, 27 estimation of strength of, 30 strength of, measured by von Federow mica wedge, 35 table (maximum), 142 Effects produced by crystals on transmitted light, 2 ▄Elæolite▄, 69 Elasticity, axes of, 4 directions of, ix Electric motor for grinding machines, 117 Electro-magnetic separation, 134 Emery, grades of, for grinding, 124 ▄Enstatite▄, 75 ▄Epidote▄, 94 Etched figures, 130 Extinction, 31 angles, 5, 31 tests for, 32 wavy, 31 Extraordinary ray, 3 Eye-pieces, 11 Faster and slower rays, test for vibration directions of, 33 ▄Fayalite▄, 78 ▄Feldspar Group▄, 98 Feldspathoids, 69 ▄Fibrolite▄, 73 Finishing and cleaning sections, 126 Focusing, 11 Form of crystals, 13 Fracture, 24 ▄Garnet▄, 53 Gelatinizing silica, test for, 130 Glass slides, 125 ▄Glaucophane▄, 88 ▄Graphite▄, 61 Gravity (specific) separation, 133 Gridiron twinning, 37 Grinding, 123 Grinding and cutting machines, 117 apparatus for sections with parallel faces, 118 plates or laps, 121 ▄Gypsum▄, 80 Gypsum test-plate, 33 Hardness of grains, 132 ▄Haüyne▄, 56 ▄Haüynite▄, 56 Haüynite and noselite, micro-chemical distinction between, 139 Heating sections to redness, 131 Heavy solutions, 133 ▄Hematite▄, 61 Hexagonal crystals, 4 High relief, 17 ▄Hornblende▄, 84 ▄Hyalosiderite▄, 78 ▄Hydrargillite▄, 115 Hydrofluosilicic acid test, 135 ▄Hypersthene▄, 75 Idiomorphic, 13 ▄Idocrase▄, 60 ▄Ilmenite▄, 62 Inclusions, 24 Index of refraction, ix, 17 by Becke’s method, 19 by Schrœder van der Kolk’s method, 22 by Sorby’s method, 19 Indicators for specific gravity separation, 133 Indices of refraction (mean), table of, 142 Interference color, determination of order of, 35 Interference colors, 28 Interference colors (abnormal), 30 Interference figures, 40 biaxial, 43 uniaxial, 40 Investigation of microscopic and optical characters of minerals, 13 ▄Iolite▄, 78 Iron, test for, by Borichy’s method, 137 test for, by Behren’s method, 139 Isolating crystals or mineral fragments for testing, 132 Isometric crystals, 2 Isotropic character, 26, 40 crystals, 2 ▄Kaolin▄, 115 ▄Labradorite▄, 103 Laps for grinding, 121 ▄Leucite▄, 54 Leucoxene, 62 Light, ordinary, 1 plane polarized, 1 ▄Limonite▄, 51 ▄Lithia Mica▄, 92 Machines for cutting and grinding, 117 Magnesium, test for, by Borichy’s method, 137 test for, by Behren’s method, 138 ▄Magnetic iron ore▄, 52 ▄Magnetic pyrites▄, 51 ▄Magnetite▄, 52 Measuring strength of double refraction by von Federow mica wedge, 35 Mechanical and chemical tests, 129 ▄Melilite▄, 61 ▄Menaccanite▄, 62 Methods of preparing sections, 117 ▄Mica Group▄, 89 Mica plate (quarter undulation), 33 Mica wedge, von Federow, 35 Micro-chemical reactions, 134 ▄Microcline▄, 102 Microlites, 17 Microscope (petrographical), 7