1960–1970: "The National Academy of Sciences", Prologue / B. L. Kropp to Richard P. Feynman, November 1959 / Richard P. Feynman to B. L. Kropp, November 9, 1960 / Richard P. Feynman to the National Academy of Sciences, February 20, 1961 / Detlev W. Bronk to Richard P. Feynman, July 15, 1961