Nickel, 96 colouring effect of, 197 steel, 27, 148 Nitrates, alkali, 44, 78 O. Objectives, apochromatic, 213 telescope, 218 Opal glass, 45, 52, 186 Opaque plate glass, 146 Open pots, 56 Optical glass, annealing, 235 chemical composition of, 217 cooling of, 233 cost of, 237 fining, 229 founding, 227 furnaces for, 225 hardness of, 220 moulding, 235 pressing, 93 range of, 216 raw materials for, 223 sorting, 235 stability of, 219 strain in, 221 stirring, 231 yield of, 236 properties of glass, 205 P. Painting on glass, 201 Parason, 102 Patent plate glass, 171 Paving stones, glass, 249 Pearl ash, 43 Phosphoric acid, 11 Phosphorus, 188 Photographic ghosts, 16 lenses, anastigmatic, 213 colour of, 209 Pipe, glass-maker’s, 89 sheet-blower’s, 158 warmer, 158 Plate glass, annealing kiln for, 135 bending of, 144 blown, 171 casting, 132 colour of, 33 figured rolled, 87 flatness of, 134 furnaces for, 133 grinding machines, 139 of, 137 mirrors, 145 opaque, 146 polishing machines, 141 polishing of, 137 raw materials for, 132 rolled, 86, 123 silvering, 146 sizes of, 143 strength of, 15 striæ in, 143 wired, 27, 147 Platinum, 27 Polishing, theory of, 141 Pontil, 98, 176, 239 Potash, 43 Potato, use of, in glass melting, 81 Ports, furnace, 67 Pots, burning of, 58 covered, 56 drying of, 58 for flint glass, 109 optical glass, 226 manufacture of, 56 open, 56 Pouring of glass, 85, 87 Pressed glass, 92, 118 composition of, 120 Presses for glass, 119 Proofs, 82, 231 Purity of materials, 36 Q. Quartz, 40 R. Range, limited, of vitreous bodies, 4 Recuperative furnaces, 66, 156 Red lead, 49 Refraction, double, in optical glass, 221 of light in optical glass, 209 Refractive index, 216 Regenerative furnace, 66, 155 Reichsanstalt, 10 Resistance to crystallisation of glass, 4 Rings for lighthouse lenses, 251 Rod, glass, 245 Rolled plate glass, 86, 123 annealing, 127 cutting, 128 defects of, 129 figured, 130 furnaces, 123 ladling, 124 raw materials for, 124 rolling, 126 sorting, 129 surface of, 122 Rolling of glass, 86 Rubies, artificial, 247 Ruby, copper, 184, 188, 198 flashed, 184 gold, 185 Rupert’s drops, 248 S. Salt-cake, 37, 42, 79, 189 Sand, 38 Sandstone, 39 Schott, 8, 19, 203, 213, 241 Scratches on sheet glass, 169 Searchlights, 250 Seed in sheet glass, 167 Selenium, colouring effect of, 190