MLA

[Portrait of John Watson] [Art Original]. 0AD. https://collections.library.yale.edu/catalog/11138691.

APA, 6th edition

[Portrait of John Watson] [art original]. (0 C.E.). [Portraits, Graphite drawings British 18th century, Ink drawings British 18th century, Watermarks (Paper)]. https://collections.library.yale.edu/catalog/11138691.