MLA

Purdy, Richard Little, 1904-1990. "A Question of Priority" [Letter to The Westminster Gazette], printed version. Box 2, folder 73. 1893 May 10. https://collections.library.yale.edu/catalog/16482196.

APA, 6th edition

Purdy, Richard Little, 1904-1990. (["Box 2, folder 73"])(1893 May 10)["A Question of Priority" [Letter to The Westminster Gazette], printed version]. https://collections.library.yale.edu/catalog/16482196.