Gravity escapement, 200, 202 Greeks, their early use of the gnomon, 16 Greenwich, Royal Observatory; perspective view and plan of transit circle, 243, 245, 251; transit room, 251, 257; meridian of, 252; chronograph, 260-264; computing room, 267; standard sidereal clock, 267; mean solar time clock, 268; standard clock, 274; pendulum, 188; reflex zenith tube, 286; alt-azimuth, 290; equatorial, 310; thermopile, 384; photoheliograph, 469 “GREENWICH TIME” AND THE USE MADE OF IT (Chap. XVIII.), 271-283 Gregorian telescope, 149 Gridiron pendulum, 188, 189, 192 Grinding of lenses and specula, 127 Grubb; production and polishing of metallic specula, 121, 134; adjustment of object-glasses, 141; Cassegrainian and Newtonian reflectors, 102, 108, 301, 303; great Melbourne equatorial telescope, 108, 314, 315, 317, 324, 327; mode of mounting its speculum, 145-149; automatic spectroscope, 397; solar spectroscope, 428 Guinand, manufacture of flint-glass discs, 118 Guns fired as time-signals, 278 H. Haliburton, on ancient astronomical observations, 6 Hall; experiments with lenses, 85; manufacture of flint-glass discs, 118 Harcourt, Vernon, experiments with phosphatic glass, 123 Harrison’s gridiron pendulum, 188 HEAT OF STARS, DETERMINATION OF (Chap. XXVI.), 377-385 Heliometer, 224 Heliostat, 423, 458 Henry (Prof.), radiation of heat from sun-spots, 385 Herschel (Sir John), lenses corrected for aberration, 88; table of reflective powers, 169; star magnitudes, 381 Herschel, Sir William, his reflecting telescopes, 103, 108; his mode of polishing specula, 129; great telescope at Slough, 169, 294 Herschel-Browning direct-vision prism, 400 Hipparchus, trigonometrical tables constructed by, 17; discoveries of, 25-35; his measurement of space, 213 Hittorf, spectrum analysis, 413 Holmes (N. J.), his proposal of the electric time-gun, 278 Hooke, improvement in clock escapements, 196; micrometer, 221, 222; zenith sector invented by, 285; siderostat suggested by, 343 Horizon, the first astronomical instrument, 4, 7, 8; defined by Euclid, 12 Horological Institute, time-signals, 280 Hours, first reckoning of, 19; measurement of, 176 Hour circle of the equatorial telescope, 328, 335 Huen, island of, granted to Tycho Brahe, 38 Huggins (Dr.), telespectroscope, 429, 432 Huyghens; telescopes used by, 81; eyepiece, 110, 116, 212; application of the pendulum to clocks, 183; his measurements of space, 219, 223, 343; polarized light, 442 Hydrogen in the sun, 435 I. Iceland spar crystals; double refraction by, 226, 228; polarization of light, 442, 445, 447, 449, 450 Illuminating power of the telescope, 158, 166, 168, 169; stars in Orion, a test of, 164 Images, double, seen through Iceland spar, 227 Inclination of the earth’s axis, 14, 17