MLA

Whitman, Walt, 1819-1892. Unidentified. Box 1, folder 19. 1885 Aug 19. https://collections.library.yale.edu/catalog/2053653.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 19"])(1885 Aug 19)[Unidentified]. https://collections.library.yale.edu/catalog/2053653.