These letters are to and from Planck. Correspondents include Hans Thacher Clarke, Gerard Peter Kuiper, and Raymond John Seeger.

Material collected by Muriel Rukeyser in the late 1940s and 1950s for a proposed biography of Franz Boas, including original manuscript materials of Boas, correspondence between Boas and Rukeyser, biographical data, early family letters and documents relating to him, school and military records, family reminiscences of Boas, and some professional correspondence. There is also material specifically related to the publication of Rukeyser's biography, including notes, a synopsis of the book, and correspondence with publishers and funding agencies. There are also some of Boas' and others' publications.

This collection contains correspondence, reports, notes, and notebooks. Clarke's participation in various organizations is documented, including the American Philosophical Society, American Chemical Society, American Otological Society, and the American Society of Biological Chemists. His chemistry research is detailed in thirteen notebooks, 1903-1971 (the 1903 volume is on photographic chemistry and processes). There is also one volume on the clarinet, for Clarke was an expert clarinetist. The personal and family correspondence is principally with Mrs. Dorothy Clarke Middleton and Mrs. Agnes Helfreich Clarke.