This book was written twenty years ago, but we are leaving it
unchanged-not because little has happened to the subject, but because so
much has happened that any change would require very major rewriting. On
the other hand, no other book on the subject seems to have appeared and
we hope this paperback edition will be of some use to a younger
generation-if not for another twenty years, at least until someone else
has the patience to write a new book. Although we have added no new
material we have corrected some misprints. We have added new references
only where they correct actual errors. Weare grateful to Drs. G.
Feinberg, M. Inokuti, Y. Kim, and A. Petermann for pointing out errors.
Preface. Nearly aU of this book is taken from an article prepared in
19;6 for a recent volume of the Encyclopedia of Physics. This article,
in tum, is partly based on Dr. NORBERT ROSENZWEIG'S translation of an
older article on the same subject, written by one of us (HAB) about 25
years ago for the GEIGER-SCHEEL Handbuch der Physik. To the article
written last year we have added some Addenda and Errata. These Addenda
and Errata refer back to some of the 79 sections of the main text and
contain some misprint corrections, additional references and some notes.