MLA

Stewart, Robert, and artist. Admiral Sir Charles Hardy. Robt Stewart, 1779. https://collections.library.yale.edu/catalog/2015696.

APA, 6th edition

Stewart, R., & artist. (1779). Admiral Sir Charles Hardy [Pictorial works, Portraits, Mezzotints]. Robt Stewart. https://collections.library.yale.edu/catalog/2015696.