TO THE FIRST ENGLISH EDITION. In preparing this translation, I have
taken the liberty of including footnotes in the main text or inserting
them in small type at the appropriate places. I have also corrected
minor misprints without special mention .. The Chapters and Sections of
the original text have been called Parts and Chapters respectively,
where the latter have been numbered consecutively. The subject index was
not contained in the Russian original and the authors' index represents
an extension of the original list of references. In this way the reader
should be able to find quickly the pages on which anyone reference is
discussed. The transliteration problem has been overcome by printing the
names of Russian authors and journals also in Russian type. While
preparing this translation in the first place for my own informa- tion,
the knowledge that it would also become accessible to a large circle of
readers has made the effort doubly worthwhile. I feel sure that the
reader will share with me in my admiration for the simplicity and
lucidity of presentation.