Credit has been given on many pages of this volume for assistance received in the course of its preparation; but I desire to express in this more formal manner my special gratitude, first of all, to my colleague, Dr. J. H. Moore, for contributing the valuable chapter on The Radial Velocity of a Star; also to Director E. C. Pickering and Miss Annie J.Cannon, of the Harvard College Observatory, for generously permitting me to utilize data from the New Draper Catalogue of Stellar Spectra; to Professor H. N. Russell, of Princeton University, Professor F. R. Moulton, of the University of Chicago, Professor E. E. Barnard, of the Yerkes Observatory, and Dr. H. D. Curtis, my colleague, for putting at my disposal published and unpublished material; and, finally, to Director W. W. Campbell, for his constant interest in and encouragement of my work. Nearly all of the manuscript has been read by Dr. Campbell and by Dr. Curtis, and several of the chapters also by Dr. Moore and by Dr. Reynold K. Young, and I am deeply indebted to them for their friendly criticism.<br><br>The book appears at this particular time in order that it may be included in the series of Semi-Centennial Publications issued by the University of California.