MLA

Eliot, George, 1819-1880. Letter. Box 13. 1881 Apr 3. https://collections.library.yale.edu/catalog/32365719.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 13"])(1881 Apr 3)[Letter]. https://collections.library.yale.edu/catalog/32365719.