MLA

Gretton, H., active 1757, printmaker. John Watkins [graphic]. [approximately 1757]. https://collections.library.yale.edu/catalog/16830928.

APA, 6th edition

Gretton, H., active 1757, printmaker. ([approximately 1757])[John Watkins [graphic]]. https://collections.library.yale.edu/catalog/16830928.