A prolific and decorated scholar, a talented and hard-working administrator, and an encouraging and supportive mentor, Frederick Mosteller was one of the leading statisticians of the twentieth century. He wrote and edited over 50 books and published nearly 350 papers. He was instrumental in establishing the Statistics Department at Harvard University, where he taught and administered for over 50 years. Mosteller contributed to numerous studies in wide-ranging fields from social sciences to healthcare to education. His work was widely recognized and hailed as influential, both in its breadth and content. He was celebrated as both a scholar and leader in statistics. The Mosteller Papers are an important source for scholars in multiple disciplines, both for those interested in particular ground-breaking studies, and for those interested in the history of statistics and statistical methodology. The Mosteller Papers provide an interesting insight into the work of Frederick Mosteller. Including correspondence, manuscripts, printed matter, and awards and certificates, the collection is comprised of twenty linear feet of material from Mosteller's professional life. The collection contains expansive records of Mosteller's time at Harvard University, as a scholar, a teacher, and an administrator. It includes insight into his thoughts, his works, and his activities. Throughout his long and illustrative career, Mosteller collaborated with a wide variety of experts in diverse fields, and contributed to numerous committees and studies, both within academia and in a wider milieu, and the papers attest to that.

Principal correspondents in the lettes of John Fries Frazer include Alexander Dallas Bache, Louis Agassiz, Joseph Henry, and Titian Ramsay Peale who discuss mainly general scientific topics, although personal and family matters are discussed with Bache. Topics include the University of Pennsylvania, boiler explosions, chemistry, education, the National Academy of Sciences, the Smithsonian Institution, fossils, magnetism, solar eclipses, the American Philosophical Society, weights and measures, the U.S. Coast and Geodetic Survey, scientific instruments, the American Civil War, electricity, the United States Mint, the Franklin Institute, scientists of the period, natural history, and publications.

The largest portion of correspondence is with Charles Edwin Bessy on botanical matters, including botanical publications in the United States and Europe, and plants and shell specimens.

This collection includes letters to Maclure, journals of travel in Europe and America, notes and essays. Also included are manuscripts, transcripts, and notes relating to Maclure.

Simon Flexner, born in 1863, one of the nation's leading experts in pathology and bacteriology, was most renowned for his research on cerebrospinal meningitis, polio and infantile paralysis. Arguably though, Flexner's stewardship of the Rockefeller Institute was his greatest contribution to medical and scientific research. His rise in the medical community began in the late nineteenth century in Louisville, Kentucky, where despite not having completed even the seventh grade, Flexner taught himself basic bacteriology by conducting experiments at home using a microscope borrowed from the pharmacy where he served as an apprentice. Granted a medical degree from the University of Louisville School of Medicine in 1889, he went on to a pathology fellowship at the newly opened John Hopkins School of Medicine. Within two short years of leaving Louisville, Flexner received an assistant of pathology appointment at Johns Hopkins. It was a quick ascent and the beginning of a long and brilliant career that included a prestigious appointment at the University of Pennsylvania and then a directorship at the new Rockefeller Institute where he realized his lifelong dream of creating a dynamic and productive research laboratory. The Rockefeller Institute became instantly famous worldwide as the preeminent research facility for virology and under Flexner's direction produced invaluable contributions in pathology, bacteriology, and immunology. This collection does not reflect the early phases of Flexner's career at Johns Hopkins but does document an early interest in meningitis and other infectious diseases with science-related correspondence, laboratory notebooks, and administrative correspondence with the New York City and State Departments of Health. There is abundant material on Flexner's directorship of the Rockefeller Institute for Medical Research, including Flexner's search for staff, an involved process which is detailed in correspondence with the scientists, many of whom became quite famous. Also included is material relating to the other institutions and Rockefeller philanthropies with which Flexner was involved. (Among the most significant correspondence, however, may be that which documents the support of the General Education Board and the Rockefeller Foundation in the development and subsequent reorganization of medical schools following brother Abraham Flexner's scathing report on medical education in the United States and Canada). This collection would be of great interest to anyone interested in the history of bacteriology, histology, and immunology or the general history of modern medicine and philanthropy.

After a brief career in the Congregational church, J. Peter Lesley (1819-1903) left the ministry in 1852 to work full-time as a geologist. Having gained experience in structural geology and stratigraphy with the First Pennsylvania Geological Survey (1836-1842), Lesley became an expert in the geology of coal, working for the Pennsylvania Railroad and other corporations and conducting some of the first systematic studies of the state's hydrocarbon resources. A long-time professor at the University of Pennsylvania, he was elected to the APS in 1856, serving variously as its librarian, secretary, and vice president, and he was selected as Director of the Second Pennsylvania Geological Survey (1875-1889). The Lesley Papers include letters to and from Peter Lesley and his wife, Susan on geology, coal and iron mining, abolition, educational reform, organized charity, and Unitarianism. The collection offers important insight into academic and applied geology in late 19th century Pennsylvania, the development of the coal and iron industries, as well as into the Lesleys' progressive social and intellectual milieu. It is divided into three series: Lesley's general correspondence, his correspondence with his brother Joseph, and microfilms of Lesley's research notes.