MLA

Eliot, George, 1819-1880. Letter. Box 13. 1881 Mar 16. https://collections.library.yale.edu/catalog/10001630.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 13"])(1881 Mar 16)[Letter]. https://collections.library.yale.edu/catalog/10001630.