surface-temperature of, 256 Surface-temperatures of planets, 21 ff., 346, 347 Surface-temperatures of stars, 254, 257, 263 ff., 270, 271 of sun, 256 Taylor’s comet, 251 Telescope, 1 Galileo’s, 2, 13 Herschels’, 25 Lord Rosse’s, 30 100-inch, 2, 20, 68, 72, 73 200-inch, 2, 73 Temperature, scale of, 102 Temperature-radiation, 140, 255 Thermodynamics, 326 ff. first and second laws of, 326, 328 Thomson, Sir J. J., 106 Tidal friction, 224 ff. Tidal theory of solar system, 236 ff. Tides in binary stars, 224, 226 Titan (satellite of Saturn), 23, 196 Transit of Venus, 34 Transport of energy in a star, 307 Ultra-violet radiation, 116, 143, 144 Universe, age of, 334, 336 apparent expansion of, 84, 86 ff. beginning of, 77, 332 ff. final end of, 329 ff. size of, 77, 84, 87 structure of, 74 ff., 87 Uranium atom, 110 disintegration of, 116, 121, 153 Uranus, 16, 17, 18, 19, 20 birth of, 240, 243, 244 rotation of, 225 satellites of, 23, 240, 243, 244 temperature of, 21 _V_ Puppis, 278, 317 van Maanen, A., 273 van Maanen’s star, 268, 274, 278, 292 van Rhijn, P. J., 26 Variable stars, 55 ff., 222, 271, 319 Vega (α Lyrae), 33 Velocities of stars, 36, 68, 163 ff. Venus, 4, 5, 16, 17, 19, 34 absence of satellites, 243 atmosphere on, 197, 344 birth of, 240, 243 life on, 344, 347 phases of, 4, 5 rotation of, 224 temperature of, 22, 347 transit of, 34 Virgo, nebula N.G.C. 4594 in, 71, 72, 204, 208 and Plate XV (p. 204) Visual binaries, orbits of, 170, 171 origin of, 169, 284 Wave-length of radiation, 115, 122 Wave-mechanics, 136, 305 White dwarf stars, 280, 283, 288, 317, 318, 319, 320, 323, 348, 349 Wilson, C. T. R., 113, 117 Wireless transmission, 115 Wolf 359, 36, 264, 269, 270 X-rays, 116, 138, 144, 307, 311 Zero (absolute) of temperature, 102 Zodiacal light, 238 Zwicky, F., 87 CAMBRIDGE: PRINTED BY W. LEWIS, M.A. AT THE UNIVERSITY PRESS