MLA

Eliot, George, 1819-1880. Letter. Box 10. 1859 Jun 26. https://collections.library.yale.edu/catalog/10966110.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 10"])(1859 Jun 26)[Letter]. https://collections.library.yale.edu/catalog/10966110.