MLA

Eliot, George, 1819-1880. Letter. Box 13. [1880] Dec 25. https://collections.library.yale.edu/catalog/10001626.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 13"])([1880] Dec 25)[Letter]. https://collections.library.yale.edu/catalog/10001626.