This collection of Morton letters forms part of a bound volume of incoming correspondence. It is a companion volume to the original letters in the Morton collection at the American Philosophical Society, which were also originally bound. Most of the subjects of the two collections overlap, but additional subjects in the microfilmed collection include American Indians, archaeology, Egyptology, and phrenology.

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.