MLA

Mulchinock, William Pembroke, 1820-1864. Letter : Box 2557, New York, to H[enry] W[adsworth] Longfellow. Box 45, folder GROUP 900, F-1. 1850 November 12. https://collections.library.yale.edu/catalog/17234168.

APA, 6th edition

Mulchinock, William Pembroke, 1820-1864. (["Box 45, folder GROUP 900, F-1"])(1850 November 12)[Letter : Box 2557, New York, to H[enry] W[adsworth] Longfellow]. https://collections.library.yale.edu/catalog/17234168.