MLA

Zincke, Christian Frederick, 1685-1767, artist. [A gentleman in brown coat called Horace Walpole, 4th Earl of Orford] [art original].. [1730s?]. https://collections.library.yale.edu/catalog/15815250.

APA, 6th edition

Zincke, Christian Frederick, 1685-1767, artist. ([1730s?])[[A gentleman in brown coat called Horace Walpole, 4th Earl of Orford] [art original].]. https://collections.library.yale.edu/catalog/15815250.