MLA

Backus, Isaac, 1724-1806. Collected papers. box 8, folder 38. 1769 September 12. https://collections.library.yale.edu/catalog/33566961.

APA, 6th edition

Backus, Isaac, 1724-1806. (["box 8, folder 38"])(1769 September 12)[Collected papers]. https://collections.library.yale.edu/catalog/33566961.