MLA
Duncan, James A. Letter from James A. Duncan (Ashland, Virginia) to Robert Paine - May 25, 1876. 1876. Pitts Theology Library, Box 1, Folder 8. https://digital.library.emory.edu/purl/4816m906vp-cor.
APA, 6th edition
Duncan, J. A. (1876). Letter from James A. Duncan (Ashland, Virginia) to Robert Paine - May 25, 1876 (Box 1, Folder 8) [Letters (correspondence)]. Pitts Theology Library. https://digital.library.emory.edu/purl/4816m906vp-cor.
Chicago
Duncan, James A. “Letter from James A. Duncan (Ashland, Virginia) to Robert Paine - May 25, 1876.” Letters (correspondence). Robert Paine Papers, 1828-1882, 1876. Box 1, Folder 8. Pitts Theology Library. https://digital.library.emory.edu/purl/4816m906vp-cor.