MLA

Van Vechten, Carl, 1880-1964. Singer, Israel Joshua. Box 376, folder 5436. 1938 April 27. https://collections.library.yale.edu/catalog/10054077.

APA, 6th edition

Van Vechten, Carl, 1880-1964. (["Box 376, folder 5436"])(1938 April 27)[Singer, Israel Joshua]. https://collections.library.yale.edu/catalog/10054077.