MLA

Eliot, George, 1819-1880. Letter. Box 9. 1880 Dec 26. https://collections.library.yale.edu/catalog/32368886.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 9"])(1880 Dec 26)[Letter]. https://collections.library.yale.edu/catalog/32368886.