I. Saturn _Frontispiece_ OPPOSITE PAGE II. The Moving Nebula surrounding Nova Persei, 1901-1902 14 III. Representative Stellar Spectra 24 IV. Spectra of the Major Planets 52 V. Venus, 1896-1897 82 VI. Asteroids: Major Axes of Orbits 98 VII. Saturn—A Drawing showing Agglomerations 108 VIII. Spectrogram of Jupiter, Moon Comparison 152 IX. Spectrogram showing Water-vapor in Atmosphere of Mars 160 X. Tree Fern 176 XI. Ten Views of Mercury, showing Effect of Libration 222 XII. Spectrogram of Saturn 232 CUTS APPEARING IN TEXT PAGE Algol and its Dark Companion 4 Nova Persei 11 Spectrum of Nova Persei 12 The Moving Nebula surrounding Nova Persei, 1901 13 Great Nebula in Orion 17 Great Nebula in Andromeda 18 Nebula M. 100 Comæ 19 Nebula ♅ I. 226 Ursæ Majoris 20 Nebula ♅ V. 24 Comæ. Showing Globular Structure 21 Nebula M. 101 Ursæ Majoris 23 The Radiant of a Meteoric Shower 37 Diagram explaining Proportionate Visibility of Meteors 38 The Mart Iron 41 Section of Meteorite showing Widmannstättian Lines 42 Meteorite, Toluca 43 Nebula ♅ V. 14 Cygni 45 Nebula N.G.C. 1499 Persei 46 Nebula N.G.C. 6960 in Cygnus 47 Nebula M. 51 Canum Venaticorum 48 Orbits of the Inner Planets 59 Sulla Rotazione di Mercurio.—Di G. V. Schiaparelli 64 Map of Mercury. Lowell 69 Venus. October, 1896-March, 1897 78 Venus. April 12, 1909. 79 Diagram: Convection Currents in Atmosphere of Venus 81 Diagram: Shift in Central Barometric Depression 81 Spectrogram of Venus, showing its Long Day 87 Spectrogram of Jupiter, giving the Length of its Day by the