MLA

Stieglitz, Alfred, 1864-1946. Letters. Box 91, folder 1800-1802. 1939 Jul-Sep. https://collections.library.yale.edu/catalog/2042813.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 91, folder 1800-1802"])(1939 Jul-Sep)[Letters]. https://collections.library.yale.edu/catalog/2042813.