MLA

Purdy, James, 1914-2009. Schwarzschild, Bettina. Box 4, folder 112-15. 1963. https://collections.library.yale.edu/catalog/32210557.

APA, 6th edition

Purdy, James, 1914-2009. (["Box 4, folder 112-15"])(1963)[Schwarzschild, Bettina]. https://collections.library.yale.edu/catalog/32210557.