J. H. JEANS DORKING _1 May 1929_ PREFACE TO SECOND EDITION In preparing a second edition, I have taken advantage of a great number of suggestions made by correspondents and reviewers, to whom I offer my sincerest thanks. I have also inserted discussions of the new planet Pluto, the rotation of the galaxy, the apparent expansion of the universe, and other subjects which have become important since the first edition was published, and in general have tried to bring the book up to date. J. H. JEANS DORKING _2 August 1930_ CONTENTS PAGE _Introduction_—The Study of Astronomy 1 _Chapter_ I Exploring the Sky 16 II Exploring the Atom 92 III Exploring in Time 150 IV Carving out the Universe 194 V Stars 253 VI Beginnings and Endings 325 _Index_ 355 PLATES The Discovery of Pluto FRONTISPIECE PLATE FACING PAGE I The Milky Way in the neighbourhood of the Southern Cross 23 II Planetary Nebulae: 1. N.G.C. 2022 2. N.G.C. 6720 (The Ring Nebula) 3. N.G.C. 1501 4. N.G.C. 7662 28 III The Nebula in Cygnus 29 IV The Great Nebula _M_ 31 in Andromeda 30 V The Nebula N.G.C. 891 in Andromeda seen edge-on 31 VI The “Horse’s Head” in the Great Nebula in Orion 37 VII The Trifid Nebula _M_ 20 in Sagittarius 44 VIII Stellar Spectra 51 IX The Globular Cluster _M_ 13 in Hercules 63 X The Region of ρ Ophiuchi 65 XI Magnification of part of the outer regions of the Great Nebula _M_ 31 in Andromeda 70 XII Magnification of the central region of the Great Nebula _M_ 31 in Andromeda 71 XIII The tracks of α- and β-particles 113 XIV Collisions of α-particles with Atoms 117 XV The Nebula N.G.C. 4594 in Virgo The Nebula N.G.C. 7217 204 XVI A sequence of Nebular Configurations: 1. N.G.C. 3379 2. N.G.C. 4621 3. N.G.C. 3115 4. N.G.C. 4594 in Virgo 5. N.G.C. 4565 in Berenice’s hair 207