MLA

Simpson, Randolph Linsly, 1927-1992. [Alexander] Gardner's Gallery, Washington, D.C. Men on horseback and wagons loaded with goods crossing river. Railroad trestle in background. Stereograph, yellow card. Box 12, folder 549. circa 1862. https://collections.library.yale.edu/catalog/2111706.

APA, 6th edition

Simpson, Randolph Linsly, 1927-1992. (["Box 12, folder 549"])(circa 1862)[[Alexander] Gardner's Gallery, Washington, D.C. Men on horseback and wagons loaded with goods crossing river. Railroad trestle in background. Stereograph, yellow card]. https://collections.library.yale.edu/catalog/2111706.