Nova Cassiopeiæ, 252; Coronæ, 252; Cygni, 253; Andromedæ, 253; Ophiuchi, 252; Aurigæ, 253; spectrum of, 253; changes into planetary nebula, 254; Persei, 254; photographs of, 254; nebulosity round, 254; Geminorum, 255; colour, spectrum of, 255 O Object-glass, treatment of, 19, 20; testing of, 20-23 Observation, methods of solar, 65-67 Olbers discovers Pallas and Vesta, 149; theory of asteroid formation, 150, 152 Oppolzer, E. von, discovers variability of Eros, 152 Opposition, 130 (note); of Mars, 130, 131 Orion, 237; great nebula of, 261-263 Orionis [theta], observation of, 240; [iota], naked-eye double, 241; [theta], multiple star, 243; [sigma], multiple star, 243 P Palisa discovers asteroids, 151 Pallas, discovery of, 149; diameter of, 152 Peck, 'Constellations and How to Find Them,' 238; star-charts, 278 Pegasi [kappa], short-period binary, 245 Pegasus, 237 Perihelion of planets, 131 (note) Period, synodic, of moon, 103 Perrine discovers Jupiter's sixth and seventh satellites, 167 Perseid, meteors, 214, 224, 225 Perseus, sword-handle of, 257 Petavius cleft, 119 Peters discovers asteroids, 151 Phillips, Rev. T. E. R., polar cap of Mars, 134; canals of Mars, 137; clouds on Mars, 140 Phobos satellite of Mars, 143 Phosphorescence of dark side of Venus, 97 Photosphere, 75 Piazzi discovers Ceres, 149 Pickering, E. C., number of lucid stars in northern hemisphere, 233; parallax of Orion nebula, 262 Pickering, W. H., on lunar ray systems, 120, 121; changes on moon, 126; on polar cap of Mars, 134, 135; discovers Saturn's ninth and tenth satellites, 187; photographs Orion nebula, 262 Planetary nebulæ, 266; spectra of, 266; nebula in Draco, 266 Plato, 117, 126 Pleiades, number of stars in, 233, 256, 257; nebula of, 257 Pleione, 256 Polarizing eye-piece, 66 Pollux, 234 Præsepe, 256 Procellarum Oceanus, 111 Proctor, 2; method of finding Mercury, 82; on state of Jupiter, 166 Proctor on the Saturnian system, 181; visibility of Cassini's division, 182; on Challis's search for Neptune, 199; Star Atlas, 278 Procyon, 234 Projecting sun's image, 67 Projections on terminator of Mars, 141 Prominences, 73, 74 Ptolemäus, 112 Pulkowa, 30-inch refractor, 8, 9 R Radiant point of meteors, 223, 224; number of, 225; stationary, 229 Ranyard Cowper on parallax measures, 231 Regulus, 234 Reversing layer seen by Young, 74; spectrum photographed by Shackleton, 75; depth of, 75 Riccioli observes duplicity of [zeta] Ursæ Majoris, 240 Rigel, 232, 234; colour of, 235 Ritchey, 5-foot reflector Yerkes Observatory, 12 Roche's limit, 186