MLA

Stevenson, Robert Louis, 1850-1894. Lang, Andrew. Box 8, folder 126. [1892 June 20]. https://collections.library.yale.edu/catalog/17297007.

APA, 6th edition

Stevenson, Robert Louis, 1850-1894. (["Box 8, folder 126"])([1892 June 20])[Lang, Andrew]. https://collections.library.yale.edu/catalog/17297007.