MLA

Eliot, George, 1819-1880. Letter. Box 1. 1861 Mar 23. https://collections.library.yale.edu/catalog/17298271.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 1"])(1861 Mar 23)[Letter]. https://collections.library.yale.edu/catalog/17298271.