MLA
Benham, Henry Washington, 1813-1884. Letter from Union general Henry Washington Benham to Mrs. Townsend expressing his regret that he cannot assist at a reception on February 20th at the Academy of Music opera house in New York City - January 26, 1864[?]. circa January 26, 1864. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/12683bk4nj-cor.
APA, 6th edition
Benham, Henry Washington, 1813-1884. (circa January 26, 1864)[Letter from Union general Henry Washington Benham to Mrs. Townsend expressing his regret that he cannot assist at a reception on February 20th at the Academy of Music opera house in New York City - January 26, 1864[?]]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/12683bk4nj-cor.
Chicago
Benham, Henry Washington, 1813-1884, Letter from Union general Henry Washington Benham to Mrs. Townsend expressing his regret that he cannot assist at a reception on February 20th at the Academy of Music opera house in New York City - January 26, 1864[?], circa January 26, 1864, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/12683bk4nj-cor.