δ Cephei, 56, 223 δ-rays, 113 Darwin, Sir George, 224 de Sitter, cosmology of, 79 ff., 84, 86, 174 Democritus, 93 Density, of matter in space, 77, 331, 332 of nebulae, 208 of stars, 265-8, 273, 282 Diameters of stars, 260, 263-8, 272, 277-80 Diffraction grating, 114 Dimensions, of earth, 34 of galactic system, 65 of solar system, 20, 34 of universe, 77, 83, 86, 87 Discontinuity, in physics, 124 ff. of matter, 93 Displacements of spectral lines, (de Sitter), 81 (Doppler), 52 (Einstein), 78, 323 Distances, of globular clusters, 63 of nebulae, 71, 73 of stars, 33, 35, 36, 61, 62, 63 Doppler effect, 52 Dwarf and Giant stars, 276 Dwarfs, white, 280, 283, 284, 292, 317, 319, 320, 322, 323, 335 Earth, age of, 14, 151, 155, 173, 247 as a planet, 16, 19, 230, 237, 243 birth of, 237, 243 dimensions of, 34 future of, 225, 226, 250, 346, 352 -moon system, 34, 225, 226, 240, 243, 250 orbit of, 35, 36, 227, 346 Eccentricity of binary orbits, 171 of ellipse (defined), 46 Eclipsing binaries, 54, 55, 56, 58 Eddington, A. S., 67, 84, 179, 184, 187, 224, 287, 288, 295 Einstein, A., (Cosmology), 74, 77, 79, 84 (Gravitation), 45, 75, 79, 161 (Quantum theory), 124, 126, 127 (Relativity), 74 ff., 186, 261 Electricity, positive and negative, 107 attraction and repulsion, 107, 129 Electromagnetic energy, 120, 121 Electron, 107, 112, 119, 121 orbits in atom, 108, 128 Ellipse, defined, 46 as atomic orbit, 129 as gravitational orbit, 46, 47, 227 Ellis, C. D., 139 Emden, R., 286, 288 Energy, 98, 120, 189, 326 ff. availability of, 326, 328 conservation of, 99, 189 source of stellar, 184 weight of, 120 Epicycles, 2, 3, 4 Equipartition of energy, in a gas, 156 stellar, 160, 164, 170, 172 Eros, 34 Ether, existence of, 338 Evaporation, 94 Evening stars, 17 Evolution, from primaeval chaos, 202, 205, 229, 230 stellar, 313 ff. Expanding universe, 84, 86 ff. Extra-galactic nebulae, 30, 70, 82 densities of, 208 distance of, 71, 82 evolution of, 204 ff. and Plates XVI to XXI in order number of, 72 rotation of, 205, 206 size of, 31 velocities of, 82, 84, 86 weights of, 71, 215 Faraday, M., 122 Final end of universe, 329 ff. Fireball, 248 Fission of stars, 169, 217, 219, 222, 229 Fizeau, H. L., 261 Fluorescence, 115, 327 Fraunhofer, J. von, 105 Free-path of molecules, 98 Frequency of radiation, 115, 126, 127 Full radiator, 123, 255, 261