In regard to binary stars, it should be remembered that, in many cases, their distances and angles of position change so rapidly that any statement concerning them remains valid only for a few years at the most. There is also much confusion among the measurements announced by different authorities. In general, the most recent measurements obtainable in 1900 are given in the text, but the observer who wishes to study close and rapid binaries will do well to revise his information about them as frequently as possible. An excellent list of double stars kept up to date, will be found in the annual Companion to the Observatory, published in London. In the lunar charts the plan of inserting the names of the principal formations has been preferred to that usually followed, of indicating them only by numbers, accompanied by a key list. Even in the most detailed charts of the moon only a part of what is visible with telescopes can be shown, and the representation, at best, must be merely approximate. It is simply a question of what to include and what to omit; and in the present case the probable needs of the amateur observer have governed the selection--readiness and convenience of reference being the chief aim. It should, perhaps, be said here that the various chapters composing this book--like those of "Astronomy with an Opera-glass"--were, in their original form, with the single exception of Chapter IX, published in Appletons' Popular Science Monthly. The author, it is needless to say, was much gratified by the expressed wish of many readers that these scattered papers should be revised and collected in a more permanent form. As bearing upon the general subject of the book, a chapter has been added, at the end, treating on the question of the existence of planets among the stars. This also first appeared in the periodical above mentioned. In conclusion, the author wishes for his readers as great a pleasure in the use of the telescope as he himself has enjoyed. G. P. S. BOROUGH OF BROOKLYN, NEW YORK, _January, 1901_. CONTENTS CHAPTER I PAGE THE SELECTION AND TESTING OF A GLASS 1 How to get a good telescope--Difference between reflectors and refractors--How a telescope is made achromatic--The way to test a telescope on stars. CHAPTER II IN THE STARRY HEAVENS 19 Orion and its wonders, Lepus, Canis Major, Argo, Monoceros, Canis Minor, and the Head of Hydra. CHAPTER III FROM GEMINI TO LEO AND ROUND ABOUT 38 The zodiacal constellations Gemini, Cancer, and Leo, and their neighbors Auriga, the Lynx, Hydra, Sextans, and Coma Berenices. CHAPTER IV VIRGO AND HER NEIGHBORS 57 Crater and Corvus, Hydra, Virgo, the "Field of the Nebulae," Libra, Booetes, and the great Arcturus, Canes Venatici, and Corona Borealis. CHAPTER V