This readiness for innovation has undoubtedly its dangers and drawbacks. To the historian, above all, it presents frequent occasions of embarrassment. The writing of history is a strongly selective operation, the outcome being valuable just in so far as the choice what to reject and what to include has been judicious; and the task is no light one of discriminating between barren speculations and ideas pregnant with coming truth. To the possession of such prescience of the future as would be needed to do this effectually I can lay no claim; but diligence and sobriety of thought are ordinarily within reach, and these I shall have exercised to good purpose if I have succeeded in rendering the fourth edition of _A Popular History of Astronomy during the Nineteenth Century_ not wholly unworthy of a place in the scientific literature of the twentieth century. My thanks are due to Sir David Gill for the use of his photograph of the great comet of 1901, which I have added to my list of illustrations, and to the Council of the Royal Astronomical Society for the loan of glass positives needed for the reproduction of those included in the third edition. London, _July_, 1902. PREFACE TO THE FIRST EDITION The progress of astronomy during the last hundred years has been rapid and extraordinary. In its distinctive features, moreover, the nature of that progress has been such as to lend itself with facility to untechnical treatment. To this circumstance the present volume owes its origin. It embodies an attempt to enable the ordinary reader to follow, with intelligent interest, the course of modern astronomical inquiries, and to realize (so far as it can at present be realized) the full effect of the comprehensive change in the whole aspect, purposes, and methods of celestial science introduced by the momentous discovery of spectrum analysis. Since Professor Grant's invaluable work on the _History of Physical Astronomy_ was published, a third of a century has elapsed. During the interval a so-called "new astronomy" has grown up by the side of the old. One effect of its advent has been to render the science of the heavenly bodies more popular, both in its needs and in its nature, than formerly. More popular in its needs, since its progress now primarily depends upon the interest in, and consequent efforts towards its advancement of the general public; more popular in its nature, because the kind of knowledge it now chiefly tends to accumulate is more easily intelligible--less remote from ordinary experience--than that evolved by the aid of the calculus from materials collected by the use of the transit-instrument and chronograph.