chapter. In palliation can be only pleaded the extent and complexity of the subject, and the impossibility of entering far into technical details in a work designed mainly for the general use. In treating such a subject it is impossible always to avoid the use of language more or less technical, except at the expense of precision and completeness of statement. An effort has however been made to limit the use of such language to the necessities of the case. The most gratifying experience associated with the work has been the cordial assistance and support rendered by a number of the author's friends and colleagues, who have supplied him with the material necessary to the presentation of their latest researches. Professor Campbell has supplied nearly all the material relating to spectroscopic binary systems, completed and revised the list of those objects, and freely placed at the author's disposal photographs taken at the Lick Observatory, including the frontispiece to the volume. Professor Kapteyn has supplied a large mass of material, published and unpublished, relating to his researches in stellar statistics, of which, however, only inadequate use could be made. Professor Pickering has permitted the free use of the treasures contained in the circulars and other publications of the Harvard Observatory, and Sir William Huggins has communicated the results of his latest studies in the life-history of the stars. Drs. A. A. Common and Isaac Roberts have each supplied a specimen of their photographs of nebulæ, and Father Sidgreaves, S. J., of his photographs of spectra taken at the Stonyhurst College Observatory. Professor Barnard has allowed the use of his photographs of the Milky Way. CONTENTS. Extension of Research into the Southern Hemisphere- The Revelations PAGE I The Brightness of a Star Depends on Distance-Ancient System of Mag- nitudes Modern Conception of Magnitude-Effect of Color on Magnitude Photographic Magnitudes-Photometric Surveys of the Principles of Spectrum Analysis-Description of the Visible Spectrum- Special Lines and Wave-Lengths - Classification of Stellar Spectra Apparent and Real Motions-Swiftness of the Motions-Stars of Large Proper Motion-Moving Systems of Stars-Radial Motions of the Stars-The Motion of the Sun-Position of the Solar Apex-Speed Periodic Stars-Light Curve of a Star-The Omicron Ceti Type-The Algol Type- The Beta Lyra Type - Combination of the Two Types-Variations of Eta Aquila-Classification of Variable Stars -Possible Secular Variations of the Brillancy of Stars - Spectra of |