MLA

Clésinger-Sand, Solange, 1828-1899. Simonnet, Léontine (cousin). Box 2, folder 57. 1889 February 12. https://collections.library.yale.edu/catalog/17444489.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 2, folder 57"])(1889 February 12)[Simonnet, Léontine (cousin)]. https://collections.library.yale.edu/catalog/17444489.