Secchi (Father), direct-vision star spectroscope, 431; stellar spectra, 433 Setting of stars (_see_ Stars) Sextants used by Tycho Brahe, 38, 50 Sidereal clock, 254, 266 (_see_ Clock) Sidereal day, 256 Sidereal time, 240, 254, 324 SIDEROSTAT, THE (Chap. XXIII.), 343-348, 461; at Lord Lindsay’s Observatory, 347 Signals for distributing “Greenwich time,” 278 Signals, time, 281, 283 Signs of the zodiac (_see_ Zodiac) Silver-on-glass reflector at the Paris Observatory, 316 Silvering glass specula, modes of, 137; silvered glass reflectors, 171 Simms, his introduction of the collimator in the spectroscope, 393, 425 Sirius, first observations of, 5; spectrum of, 432 Slough, Sir Wm. Herschel’s telescope at, 294 Smyth (Admiral), stars in Orion, 165; colours of stars, 351; star magnitudes, 381 Smyth (Prof. Piazzi), on the pyramids as astronomical instruments, 6; position of the vernal equinox, 34; clock arrangements at Edinburgh Observatory, 269 Sodium, discovery of its presence in the sun, 412 Solar photography, 459, 465 Solar spectroscope, 435; Browning’s and Grubb’s forms, 428 Solar spectrum, 390, 391, 392, 423, 433, 436, 438, 439; photographs of, 479, 480 Solar time, 253, 255 Solstices, first observations of the, 15, 16, 17, 22 Southing of stars, 234 SPACE MEASURERS (Book III.), 135-232; circle reading, 211; Digges’ diagonal scale, 213; the vernier, 214; micrometers, 218 Space-penetrating power of the telescope, 154; stars in Orion, a test of, 165 Spectroscope, construction of the, 393-400; automatic, 397; arranged for showing absorption, 409; attached to Newall’s refractor, 427; solar, Browning’s and Grubb’s forms, 428 Spectrum produced by prisms, irrationality of the, 86, 87 Spectrum, solar, 390, 391, 392 Spectrum analysis, principles of, 401-421 Specula, production of, 117, 120; casting, annealing, 121; curvature, 122; grinding, 127; polishing, 128; silvering, 137; mounting, 142, 169, 172; effective light, 169; repolishing, 171; cost as compared with object-glasses, 172 Spherical aberration, 87; diagram illustrating, 104, 105; its correction in eyepieces, 109, 111; of specula, 123, 124 Sprengel pump, 413 Spring governor of driving-clock for large telescopes, 319, 320 “Spurious disc” of fixed stars, 163 Standard clock at Edinburgh Observatory, 272 Standard sidereal clock of Greenwich Observatory, 267 Standard solar time clock of Greenwich Observatory, 267 STARS, CHEMISTRY OF THE (Chap. XXVII.-XXX.), 386-453 STARS, LIGHT AND HEAT OF (Chap. XXVI.), 377; variable, 377-385