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 turn, 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. The aim of this b. ook is two-fold. First, to act as a reference work on calcu­ lations pertaining to
hydrogen-like and helium-like atoms and their comparison with experiments. However, these calculations involve a vast array of approxi­ mation methods, mathematical tricks and physical pictures, which are also useful in the
application of quantum mechanics to other fields. In many sections we have given more general discussions of the methods and physical ideas than is necessary for the study of the H- and He-atom alone. We hope that this book
will thus at least partly fulfill its second aim, namely to be of some use to graduate students who wish to learn "applied quantum mechanics". A basic knowledge of the principles of quantum mechanics, such as given in the
early chapters of SCHIFF's or BoHM's book, is presupposed.