The English edition is based upon the second edition of the German version of the book. The author would like to thank Mr. A.H. Armstrong for providing the basic English manuscript of the text, his critical reading, and valuable comments. Thanks are also due to Mrs. A. Demmer, Mr. J. Matern, Mrs. B. Titze and Mrs. S. Pfetsch for preparing the camera ready manuscript and the figures. Springer- Verlag has generously supported the project and cooperating with them has been a great pleasure. Ulm, April 1992 K.J. Ebeling Preface to the First German Edition This book is a comprehensive introduction...