MLA

Allen, George, active 1750-1771. Some occasional thoughts on genius, with an apology for so long deferring the publication of Observations in the art of painting; humbly offered to the public consideration, and the subscribers for that treatise more particularly; in a letter to one of the profession. 1750.. https://collections.library.yale.edu/catalog/17480179.

APA, 6th edition

Allen, George, active 1750-1771. (1750.)[Some occasional thoughts on genius, with an apology for so long deferring the publication of Observations in the art of painting; humbly offered to the public consideration, and the subscribers for that treatise more particularly; in a letter to one of the profession]. https://collections.library.yale.edu/catalog/17480179.