MLA

Clésinger-Sand, Solange, 1828-1899. Guérin, Alphonse. Box 1, folder 34. 1870, undated. https://collections.library.yale.edu/catalog/17444467.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 1, folder 34"])(1870, undated)[Guérin, Alphonse]. https://collections.library.yale.edu/catalog/17444467.