It is many years, however, since any book has dealt with the telescope itself, apart from the often repeated accounts of the marvels it discloses. The present volume contains neither pictures of nebulæ nor speculations as to the habitibility of the planets; it merely attempts to bring the facts regarding the astronomer’s chief instrument of research somewhere within grasp and up to the present time. The author cordially acknowledges his obligations to the important astronomical journals, particularly the Astro-physical Journal, and Popular Astronomy in this country; The Observatory, and the publications of the Royal Astronomical Society in England; the Bulletin de la Société Astronomique de France; and the Astronomische Nachrichten; which, with a few other journals and the official reports of observatories form the body of astronomical knowledge. He also acknowledges the kindness of the various publishers who have extended the courtesy of illustrations, especially Macmillan & Co. and the Clarendon Press, and above all renders thanks to the many friends who have cordially lent a helping hand—the Director and staff of the Harvard Observatory, Dr. George E. Hale, C. A. R. Lundin, manager of the Alvan Clark Corporation, J. B. McDowell, successor of the Brashear Company, J. E. Bennett, the American representative of Carl Zeiss, Jena, and not a few others. LOUIS BELL. BOSTON, MASS., _February, 1922_. CONTENTS PAGE PREFACE vii CHAP. I. THE EVOLUTION OF THE TELESCOPE 1 II. THE MODERN TELESCOPE 31 III. OPTICAL GLASS AND ITS WORKING 57 IV. THE PROPERTIES OF OBJECTIVES AND MIRRORS 76 V. MOUNTINGS 98 VI. EYE-PIECES 134 VII. HAND TELESCOPES AND BINOCULARS 150 VIII. ACCESSORIES 165 IX. THE TESTING AND CARE OF TELESCOPES 201 X. SETTING UP AND HOUSING THE TELESCOPE 228 XI. SEEING AND MAGNIFICATION 253 APPENDIX 279 INDEX 281 THE TELESCOPE CHAPTER I THE EVOLUTION OF THE TELESCOPE In the credulous twaddle of an essay on the Lost Arts one may generally find the telescope ascribed to far antiquity. In place of evidence there is vague allusion of classical times or wild flights of fancy like one which argued from the Scriptural statement that Satan took up Christ into a high mountain and showed him all the kingdoms of the earth, that the Devil had a telescope—bad optics and worse theology.