MLA

Eliot, George, 1819-1880. Letter. Box 12. 1881 Mar 5. https://collections.library.yale.edu/catalog/32365702.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 12"])(1881 Mar 5)[Letter]. https://collections.library.yale.edu/catalog/32365702.