MLA

Boyesen, Hjalmar Hjorth, 1848-1895. Letter : to [Howard] Pyle. Box 43, folder GROUP 729, F-1. [18]89 Jan 17. https://collections.library.yale.edu/catalog/10081317.

APA, 6th edition

Boyesen, Hjalmar Hjorth, 1848-1895. (["Box 43, folder GROUP 729, F-1"])([18]89 Jan 17)[Letter : to [Howard] Pyle]. https://collections.library.yale.edu/catalog/10081317.