MLA

Eliot, George, 1819-1880. Letter. Box 12. 1880 Dec 28. https://collections.library.yale.edu/catalog/32365701.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 12"])(1880 Dec 28)[Letter]. https://collections.library.yale.edu/catalog/32365701.