Prepared at the request of the AAPT Editorial Board for Resource Letters. This is a supplement to one of a series of Resource Letters on different topics intended to guide college physicists, astronomers, and other scientists to some of the literature and other teaching aids that may help improve course contents in specified fields. No Resource Letter is meant to be exhaustive and complete; in time there may be more than one letter or supplement on some of the main subjects of interest. Comments on these materials as well as suggestions for future topics will be welcomed. Please send such communications to Professor Arthur R. Quinton, Editor, Resource Letter Board, Department of Physics and Astronomy, University of Massachusetts, Amherst, Massachusetts 01002.Reprints: When ordering, request Resource Letter TLA−2, enclose 50 cents per copy (not in stamps) together with a stamped and self−addressed envelope to Arnold A. Strassenburg, Executive Officer, American Association of Physics Teachers, Drawer AW, Stony Brook, New York 11790.