The chapters correspond approximately to a series of six lectures
delivered at the University of Chicago in August 1904, at the hospitable
invitation of President Harper. They afforded me the opportunity of seeing
something of this wonderful University, only a dozen years old and yet so
amazingly vigorous; and especially of its observatory (the Yerkes
observatory, situated eighty miles away on Lake Geneva), which is only
eight years old and yet has taken its place in the foremost rank. For
these opportunities I venture here to put on record my grateful thanks.

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. C