MLA

Eliot, George, 1819-1880. Letter. Box 13. 1873 Feb 26. https://collections.library.yale.edu/catalog/2013094.

APA, 6th edition

Eliot, George, 1819-1880. (["Box 13"])(1873 Feb 26)[Letter]. https://collections.library.yale.edu/catalog/2013094.