The manuscripts of father William Mitchell and daughter Maria Mitchell are not easily separated, and are treated here together. The collection includes William Mitchell's autobiography, his memoir of Judge Walter Folger of Nantucket, astronomical and meteorological observations, lectures, family and other correspondence.

Alexander Dallas Bache (1806-1867) was an important scientific reformer during the early nineteenth century. From his position as superintendent of the United States Coast Survey, and through leadership roles in the scientific institutions of the time, Bache helped bring American science into alignment with the professional nature of its European counterparts. In addition, Bache fostered the reform of public education in America. The Alexander Dallas Bache Collection consists of 91 letters written primarily by Bache. In most cases, these are brief notes replying to letters that are not part of the collection. The majority of items relate to Bache's work as superintendent of the U.S. Coast Survey, including letters of recommendation and introduction, and there is minor correspondence regarding the National Academy of Sciences, scientific matters, his travels in Europe, or personal matters. Of particular interest is the letter from Louis Agassiz in 1856 expressing his view of what the natural history museum of the future should be.