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.