MLA

Thackeray, William Makepeace, 1811-1863. Unidentified artist, Portrait of William Makepeace Thackeray; graphite on paper. Box 2, folder 21. 1859. https://collections.library.yale.edu/catalog/2040462.

APA, 6th edition

Thackeray, William Makepeace, 1811-1863. (["Box 2, folder 21"])(1859)[Unidentified artist, Portrait of William Makepeace Thackeray; graphite on paper]. https://collections.library.yale.edu/catalog/2040462.