MLA

Stieglitz, Alfred, 1864-1946. Letters. Box 90, folder 1792. 1938 May. https://collections.library.yale.edu/catalog/2042404.

APA, 6th edition

Stieglitz, Alfred, 1864-1946. (["Box 90, folder 1792"])(1938 May)[Letters]. https://collections.library.yale.edu/catalog/2042404.