MLA
Johnson, Herschel V. (Herschel Vespasian), 1812-1880. Letter from Herschel V. Johnson (Midway, Georgia) to Reverend Henry Cleveland, conferring an honorary degree to Cleveland for saving Oglethorpe University from General Sherman's destruction - December 3, 1864. December 3, 1864. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/665h9w0wt2-cor.
APA, 6th edition
Johnson, Herschel V. (Herschel Vespasian), 1812-1880. (December 3, 1864)[Letter from Herschel V. Johnson (Midway, Georgia) to Reverend Henry Cleveland, conferring an honorary degree to Cleveland for saving Oglethorpe University from General Sherman's destruction - December 3, 1864]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/665h9w0wt2-cor.
Chicago
Johnson, Herschel V. (Herschel Vespasian), 1812-1880, Letter from Herschel V. Johnson (Midway, Georgia) to Reverend Henry Cleveland, conferring an honorary degree to Cleveland for saving Oglethorpe University from General Sherman's destruction - December 3, 1864, December 3, 1864, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/665h9w0wt2-cor.