A miscellaneous collection of letters written by naturalist Louis Jean Rodolph Agassiz concerning a wide range of topics: natural history and naturalists, geology, mineralogy, fossils, publications, expeditions, and the Museum of Comparative Zoology at Harvard, which he founded.

A gifted mathematician, Polish-born Stanislaw Ulam made contributions to set theory, topology, mathematical logic, and number theory, but is most widely remembered for his work in fostering the technical development of thermonuclear weapons. He was associated with Los Alamos Scientific Laboratories for most of the years between 1943 and 1965, and thereafter with the University of Colorado. These papers include personal and professional correspondence, manuscripts of both published and unpublished works, and memorabilia.

After receiving his doctorate from Harvard in 1923, the physicist John Clarke Slater did postgraduate work at Cambridge University and on the continent working n quantum theory with both Niels Bohr and Werner Heisenberg. Slater was appointed to the head of the department of physics at MIT in 1930, which he and Karl Compton worked effectively to transform into one of international stature. His own work on the electromagnetic theory of microwaves was fundamental to the development of radar systems. During the Second World War, he was affiliated with the radiation laboratory and after he helped found the solid state and molecular theory group, the interdisciplinary Center for Materials Science and Engineering, the Research Laboratory of Electronics, and the Laboratory for Nuclear Science. After his retirement from MIT in 1966, Slater moved to the University of Florida, remaining active until his death in 1976. The Slater Papers contains a wealth of information on the development of physics at MIT, as well as Slater's post-1966 work at the University of Florida. There are about 133 (7 linear ft.) research notebooks, 1944-1976, and a long series (30 linear ft.) of folders, containing lectures, scientific notes, drafts of manuscripts and papers, correspondence during his collaboration with the Los Alamos Labs, 1966-1970, and extensive correspondence relating to the National Academy of Science. Information about American-Swedish exchange in quantum science is located in the correspondence with Per-Olov Löwdin.

Leonard Carmichael was a psychologist whose most significant contributions were made in the fields of child psychology and biopsychology. Educated at Tufts and Harvard, he taught at Princeton, Brown, and Rochester, later returning to Tufts to serve as its President from 1938-1952. An outstanding administrator, Carmichael served as executive officer for the Smithsonian Institute from 1952-1964, and was subsequently vice president for research and exploration at the National Geographic Society. In his later years, he served as President of the American Philosophical Society from 1970-1973. This large and varied collection reflects the entire range of Carmichael's prolific career. It includes a variety of types of records, including correspondence, committee reports and records, long series of articles and speeches, drafts of papers and books, and pocket notebooks. All phases of his professional life are covered, from his years in academia, to his time at the Smithsonian Institute and the National Geographic Society, as well as his involvement in a large number of professional organizations.

One of the seminal figures in the emergence of the Modern or Neo-Darwinian Synthesis during the mid-twentieth century, George Gaylord Simpson (1902-1984) helped define the unique contribution made by vertebrate paleontology to the life sciences. A specialist in Mesozoic and early Cenozoic mammals, Simpson's contributions to the fusion of Darwinian natural selection and Mendelian genetics were both empirical and theoretical, culminating in his major works Tempo and Mode in Evolution and The Meaning of Evolution. From his posts at the American Museum of Natural History (1927-1959), Columbia University (1945-1959), Harvard (1959-1967), and the University of Arizona (1967-1984), Simpson became one of the most influential paleontologists of the century, helped in part by his ability to write successfully for both a technical, professional audience and a popular audience. The Simpson Papers include a comprehensive assemblage of professional and personal correspondence, reflecting nealy all phases of Simpson's career. Written with charm, wit, and a sense of literary style, the correspondence touches on all aspects of modern paleontology, providing an important perspective on the emergence of contemporary evolutionary theory, biogeography, systematic theory and methodology, the relationship of science and religion, and creationism, as well as more general issues in scientific epistemology and social and political issues. The collection also includes autobiographical data and writings, lectures, class notes and papers, research data, material on his scientific expeditions (diaries in carbon form, photos, notes, etc.), publication material (he was author of some 800 publications), extensive photographic material, diplomas, and medals.