Shortly after Langley became Secretary, a new system of dividing outgoing correspondence into subjects was devised as follows (some of the units are not represented in the Archives, hence the gaps in series numbers): 1) General, 59 volumes, 1892-1907; 2) Museum, 18 volumes, 1892-1907; 4) Ethnology, 7 volumes, 1892-1907; 5) National Zoological Park, 9 volumes, 1890-1907; 6) Astrophysical Observatory, 14 volumes, 1892-1907; 8) Publication Requests, 3 volumes, 1900-1907; 11) Assistant in Charge of Office, 8 volumes, 1891-1907; 12) New Museum, 1 volume, 1903-1907; 13) Advisory Committee on Printing and Publication, 1 volume, 1906-1907; 20) Aerodromics, 10 volumes, 1891-1907; 25) Hodgkins, 6 volumes, 1892-1907. All are chronologic within divisions. There are also copies of Museum Business Letters, 2 volumes, 1888-1889, and Internal and Confidential Correspondence, 9 volumes, 1887-1906.
Volumes are indexed individually. Additional finding aids to this correspondence exist in the form of card indexes, one of which is bound. A second index, combining incoming and outgoing correspondence for the period, provides access mostly by proper name and usually contains an abstracted version of the letter. Deterioration of the letterpress correspondence has affected the legibility of the microfilm edition. A volume list follows.