MLA

Speck, William A. (William Alfred), 1864-1928. ALS : Emmendingen, to Johann Heinrich Merck. Box 15, folder 611. 1784 Apr 10. https://collections.library.yale.edu/catalog/10257596.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 15, folder 611"])(1784 Apr 10)[ALS : Emmendingen, to Johann Heinrich Merck]. https://collections.library.yale.edu/catalog/10257596.