MLA

Backus, Isaac, 1724-1806. Collected papers. box 9, folder 21. 1771 September 12–1771 October 16. https://collections.library.yale.edu/catalog/33573514.

APA, 6th edition

Backus, Isaac, 1724-1806. (["box 9, folder 21"])(1771 September 12–1771 October 16)[Collected papers]. https://collections.library.yale.edu/catalog/33573514.