MLA

Whitman, Walt, 1819-1892. Bainbridge, William F.. Box 1, folder 25. n.d.. https://collections.library.yale.edu/catalog/2053659.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 25"])(n.d.)[Bainbridge, William F.]. https://collections.library.yale.edu/catalog/2053659.