MLA

Nicholls, Norton, 1741?-1809. Norton Nicholls to Jane Floyer Nicholls. Eton. Concerns Eton College; Sir Lucas Pepys. Box 1, folder 18. 1758 Jul 28. https://collections.library.yale.edu/catalog/17439815.

APA, 6th edition

Nicholls, Norton, 1741?-1809. (["Box 1, folder 18"])(1758 Jul 28)[Norton Nicholls to Jane Floyer Nicholls. Eton. Concerns Eton College; Sir Lucas Pepys]. https://collections.library.yale.edu/catalog/17439815.