Microscopic and optical characters of minerals, 51 Minerals and thickness of section, determined by table and diagram, 36 Monoclinic crystals, 4 principal vibration directions in, 5 ▄Monoclinic pyroxenes▄, 80 Mounting sections, 124 solutions, 125 ▄Muscovite▄, 89 ▄Natrolite▄, 79 Negative optical character, biaxial, 46 uniaxial, 42 ▄Nepheline▄, 69 ▄Nephelite▄, 69 Nicol prism, 7 ▄Nosean▄, 56 ▄Noselite▄, 56 Noselite and haüynite, micro-chemical distinction between, 139 Nose-piece for microscope, 10 Objectives, 10 Oblique extinction, 31 Obtuse bisectrix, 5 ▄Oligoclase▄, 103 ▄Olivine▄, 77 ▄Opal▄, 51 Opaque minerals, characters of, 13 Optical, anomalies, 27 character, biaxial, 46 character, uniaxial, 42 classification into systems, 141 distinctions between orthorhombic, monoclinic and triclinic sections (perpendicular to bisectrices), 48, 141 distinctions between tetragonal and hexagonal sections (perpendicular to optic axis), 141 and microscopic characters of minerals, 51 principal section, 3, 4 scheme, 145 Optic axes, 4 Optic axis, 3 Optics for optical mineralogy, 1 Order of consolidation of minerals in plutonic rocks, 144 Order of interference color, determination of, 35 Ordinary light, 1 ray, 3 ▄Orthite▄, 96 ▄Orthoclase▄, 98 Orthorhombic crystals, 4 principal vibration directions in, 4 ▄Orthorhombic pyroxenes▄, 75 Parallel and convergent light, résumé of uses of, 49 Parallel extinction, 31 Parallel face sections, grinding apparatus for, 119 ▄Pargasite▄, 85 ▄Perofskite▄, 57 ▄Perovskite▄, 57 Petrographical apparatus, 127 Petrographical microscope, 7 Phenocryst, 14 ▄Phlogopite▄, 89 ▄Picotite▄, 53 ▄Piedmontite▄, 95 ▄Plagioclases▄, 103 Plane polarized light, 1 Plates, grinding, 121 Pleochroic halos, 59 Pleochroism, 25 test for, 25 ▄Pleonaste▄, 53 Polarization colors, 28 Polarized light, characters observed by, 25 plane, 1 Polarizer, 7 test for vibration plane of, 9 Polishing sections, 124 Polysynthetic twinning, 37 Positive optical character, biaxial, 46 uniaxial, 42 Potassium, test for, by Borichy’s method, 136 test for, by Behren’s method, 138 Preparation of sections, 117 Principal section, optical, 3, 4 Principal vibration directions, 3, 4 Prism, nicol, 7 Pseudomorphic structure, 39 ▄Pyrite▄, 51 ▄Pyrites▄, 51 ▄Pyroxenes, monoclinic▄, 80 ▄Pyroxenes, orthorhombic▄, 75 ▄Pyrrhotite▄, 51 Quarter-undulation mica plate, 33 ▄Quartz▄, 63 wedge, 34 Reflected light, characters observed by, 13 Reflector, 7 Refraction, double, 2, 27 Refraction, index of, ix, 17 Refractive indices (mean), table of, 142 Relief, 17 test for, 17, 18 Resorption border, 15 Résumé of uses of parallel and convergent light, 49 Rotating stage of microscope, 10 ▄Rutile▄, 57 ▄Sagenite▄, 58 ▄Sanidine▄, 102 ▄Saussurite▄, 96, 113 Saws, 119