MLA

Whitman, Walt, 1819-1892. Crossman, H. F. to Elmira Crossman. Box 1, folder 38. 1867. https://collections.library.yale.edu/catalog/2053672.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 38"])(1867)[Crossman, H. F. to Elmira Crossman]. https://collections.library.yale.edu/catalog/2053672.