MLA

Eliot, George, 1819-1880. Letter, to an unidentified recipient. Box 8. 1880 Dec 23. https://collections.library.yale.edu/catalog/10001309.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 8"])(1880 Dec 23)[Letter, to an unidentified recipient]. https://collections.library.yale.edu/catalog/10001309.