In a portion of the first chapter it will be obvious that I am indebted to Miss Clerke's "History of Astronomy in the Nineteenth Century"; in the second to Professor R. A. Sampson's Memoir on the Adams MSS.; in the third to Rigaud's "Life of Bradley." There are other debts which I hope are duly acknowledged in the text. My grateful thanks are due to Mr. F. A. Bellamy for the care with which he has read the proofs; and I am indebted for permission to publish illustrations to the Royal Astronomical Society, the Astronomer Royal, the editors of _The Observatory_, the Cambridge University Press, the Harvard College Observatory, the Yerkes Observatory, and the living representatives of two portraits. H. H. TURNER. UNIVERSITY OBSERVATORY, OXFORD, _November 9, 1904_. CONTENTS PAGE CHAPTER I URANUS AND EROS 1 CHAPTER II THE DISCOVERY OF NEPTUNE 38 CHAPTER III BRADLEY'S DISCOVERIES OF THE ABERRATION OF LIGHT AND OF THE NUTATION OF THE EARTH'S AXIS 86 CHAPTER IV ACCIDENTAL DISCOVERIES 121 CHAPTER V SCHWABE AND THE SUN-SPOT PERIOD 155 CHAPTER VI THE VARIATION OF LATITUDE 177 INDEX 221 LIST OF PLATES PLATE I. PORTRAIT OF J. C. ADAMS _To face page_ 22 II. PORTRAIT OF A. GRAHAM " " 22 III. PORTRAIT OF U. J. LE VERRIER " " 60 IV. PORTRAIT OF J. G. GALLE " " 60 V. CORNER OF THE BERLIN MAP BY THE USE OF WHICH GALLE FOUND NEPTUNE " " 82 VI. ASTRONOMERS ROYAL _Frontispiece_ VII. GREAT COMET OF NOV. 7, 1882 _To face page_ 122 VIII. THE OXFORD NEW STAR " " 142 IX. NEBULOSITY ROUND NOVA PERSEI " " 146 X. SUN-SPOTS AT GREENWICH, FEB. 18 AND 19, 1894 " " 158 XI. SUN-SPOTS AT GREENWICH, FEB. 20 AND 21, 1894 " " 162 XII. NUMBER OF SUN-SPOTS COMPARED WITH DAILY RANGE OF MAGNETIC DECLINATION AND DAILY RANGE OF MAGNETIC HORIZONTAL FORCE " " 164 XIII. GREENWICH MAGNETIC CURVES, 1859-60 " " 166 XIV. GREENWICH MAGNETIC CURVES, 1841-1860 " " 166 XV. SUN-SPOTS AND TURNS OF VANE " " 170 ERRATA Page 133, line 27, _for_ "200 stars" _read_ "200 stars per hour." " 145, See note on page 220.