A translation of The Collected Papers of Albert Einstein is being published by Princeton University Press, under the supervision of Diana Kormos Buchwald, General Editor, and Jennifer Nollar James. The translation of Volume 15, contain- ing selected texts, was prepared by Ann Hentschel and Mary Jane Teague. Klaus Hentschel and Andreas Aebi were consultants. Alice Calaprice was the copy editor, and Beth Gianfagna proofread the manuscript. The entire editorial team of the Einstein Papers Project contributed to revisions, typesetting, indexing, and prepar- ing camera ready copy. The translation volumes, produced in paperback, are intended to be read in conjunction with the documentary edition, as they contain none of the editorial commentary. NOTE ON THE TRANSLATION