MLA

Eliot, George, 1819-1880. Letter. Box 10. 1880 Dec 28. https://collections.library.yale.edu/catalog/10001509.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 10"])(1880 Dec 28)[Letter]. https://collections.library.yale.edu/catalog/10001509.