MLA

Eliot, George, 1819-1880. Letter. Box 9. 1883 Aug 12. https://collections.library.yale.edu/catalog/32368888.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 9"])(1883 Aug 12)[Letter]. https://collections.library.yale.edu/catalog/32368888.