THE TELESCOPE _McGraw-Hill Book Co. Inc._ PUBLISHERS OF BOOKS FOR Coal Age ▿ Electric Railway Journal Electrical World ▿ Engineering News-Record American Machinist ▿ Ingeniería Internacional Engineering & Mining Journal ▿ Power Chemical & Metallurgical Engineering Electrical Merchandising [Illustration: Galileo’s Telescopes. (_Frontispiece_) (_Bull. de la Soc. Astron. de France._)] THE TELESCOPE BY LOUIS BELL, PH.D. CONSULTING ENGINEER; FELLOW, AMERICAN ACADEMY OF ARTS & SCIENCES; PAST-PRESIDENT, THE ILLUMINATING ENGINEERING SOCIETY; MEMBER, AMERICAN ASTRONOMICAL SOCIETY FIRST EDITION MCGRAW-HILL BOOK COMPANY, INC. NEW YORK: 370 SEVENTH AVENUE LONDON: 6 & 8 BOUVERIE ST., E. C. 4 1922 COPYRIGHT, 1922, BY THE MCGRAW-HILL BOOK COMPANY, INC. THE MAPLE PRESS YORK PA PREFACE This book is written for the many observers, who use telescopes for study or pleasure and desire more information about their construction and properties. Not being a “handbook” in two or more thick quartos, it attempts neither exhaustive technicalities nor popular descriptions of great observatories and their work. It deals primarily with principles and their application to such instruments as are likely to come into the possession, or within reach, of students and others for whom the Heavens have a compelling call. Much has been written of telescopes, first and last, but it is for the most part scattered through papers in three or four languages, and quite inaccessible to the ordinary reader. For his benefit the references are, so far as is practicable, to English sources, and dimensions are given, regretfully, in English units. Certain branches of the subject are not here discussed for lack of space or because there is recent literature at hand to which reference can be made. Such topics are telescopes notable chiefly for their dimensions, and photographic apparatus on which special treatises are available. Celestial photography is a branch of astronomy which stands on its own feet, and although many telescopes are successfully used for photography through the help of color screens, the photographic telescope proper and its use belongs to a field somewhat apart, requiring a technique quite its own.