MLA

Richards, F. D. (Franklin Dewey), 1821-1899. Letter: to his parents, Phineas and Wealthy Dewey Richards. 1839 Jan 1. https://collections.library.yale.edu/catalog/16697578.

APA, 6th edition

Richards, F. D. (Franklin Dewey), 1821-1899. (1839 Jan 1)[Letter: to his parents, Phineas and Wealthy Dewey Richards]. https://collections.library.yale.edu/catalog/16697578.