Object-glasses, production of, 118, 119; correction of colour, 88; correction for spherical aberration, 126; mode of polishing, 128; mode of centring, 140; illustrations of defective adjustment, 141; adjustment of, 163; its perfection in modern telescopes, 166, 305; cost of production, 172; divided, for duplication of image, 225 Object-glass prism, 426 Observatories [_see_ Alexandrian Museum, Cambridge (U.S.), Cincinnati, Edinburgh, Greenwich, Huen (Tycho Brahe’s), Kew, Lord Lindsay’s, Mauritius, Melbourne, Meudon, Paris, Potsdam, Vienna, Washington] Observing chair for equatorial telescopes, 339 Optical action of the eye, 67; long and short sight, 69, 71 Optical qualities of telescopes, permanence of, 170 Optic axis in crystals of Iceland spar, 228 “Optick tube,” telescope so first called, 55, 139-151 Orion, first observations of, 5; Orion and the neighbouring constellations, 156; nebula of, 157, 158; stars in, a test for power of telescopes, 164-166; facilities for observing, 164 P. Parallactic rules, 51; used by Ptolemy, 35; by Tycho Brahe, 38, 41 Parallax of the moon, observed by Ptolemy, 35 Paris Observatory, reflecting equatorial telescope, 314, 315, 337; siderostat, 344; photoheliograph, 469 Pendulum, 183, 185, 187, 188 Personal equation, 259 Phosphatic glass for lenses, 123 PHOTOGRAPHY, CELESTIAL (Chap. XXXI., XXXII.), 454-483 Photography, stellar, 172 Photoheliograph, for photographs of the sun, 460, 470; for transit of Venus (1874), 461 Photometry, 373, 377 PHYSICS, ASTRONOMICAL (Book VI.), 371 PHYSICAL INQUIRY, GENERAL FIELD OF (Chap. XXV.), 371-376 Picard, transit circle, 284 Pisces, its position in the zodiac, 34 Pitch employed in polishing lenses and specula, 128, 132 Plane of the ecliptic, 13, 14 Planets, in Ptolemy’s system, 3; first observations of conjunction, 4, 5; motions observed by Autolycus, 9; in Tycho Brahe’s system, 46; Saturn seen with object-glasses of 3¾ and 26 inches, 160, 161; as telescopic objects, 350; photographs of, 465 Pleiades, the first observations of, 5 Plücker, spectrum analysis, 413 Pogson, star magnitudes, 381, 382 Pointers of pre-telescopic instruments, 35, 49, 214, 216 Polar axis of the equatorial, 299, 302, 308, 311, 312, 324, 328, 329, 346 Polariscope, 441-453 Polarization of light, 441-453 Pole, North, 238; diagram illustrating how it is found, 249 Pole star, first observations of, 6; observations of Euclid, 10, 14; its position, 238 Polishing lenses and specula, 128, 171; Lord Rosse’s polishing machine, 131; Mr. Lassell’s, 132 Posidonius, measurement of the earth’s circumference, 8 Position circle, 353 Position micrometer, 353, 358 Post Office Telegraphs, for distribution of Greenwich time, 275 Potsdam, photoheliograph at, 469 Precession of the equinoxes, 33 Prime-vertical, 285