MLA
Summers, Thomas O. (Thomas Osmond), 1812-1882. Letter from Thomas O. Summers (Tuscaloosa, Alabama) to Robert Paine (Aberdeen, Mississippi) - October 7, 1865. October 7, 1865. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/2085hqc0sh-cor.
APA, 6th edition
Summers, Thomas O. (Thomas Osmond), 1812-1882. (October 7, 1865)[Letter from Thomas O. Summers (Tuscaloosa, Alabama) to Robert Paine (Aberdeen, Mississippi) - October 7, 1865]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/2085hqc0sh-cor.
Chicago
Summers, Thomas O. (Thomas Osmond), 1812-1882, Letter from Thomas O. Summers (Tuscaloosa, Alabama) to Robert Paine (Aberdeen, Mississippi) - October 7, 1865, October 7, 1865, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/2085hqc0sh-cor.