MLA

Whitman, Walt, 1819-1892. Bainbridge, Thomas. Box 1, folder 24. 1870 May 15. https://collections.library.yale.edu/catalog/2053658.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 24"])(1870 May 15)[Bainbridge, Thomas]. https://collections.library.yale.edu/catalog/2053658.