MLA

Eliot, George, 1819-1880. Letter. Box 10. 1884 Dec 9. https://collections.library.yale.edu/catalog/10001535.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 10"])(1884 Dec 9)[Letter]. https://collections.library.yale.edu/catalog/10001535.