Painting: The Man Who Left Home

It was as natural as two lumps in his cup of tea. The year was 1782, and there was Elkanah Watson, 24, a Massachusetts-born merchant visiting London with 100 guineas to burn. As he dined with the famous expatriate painter John Singleton Copley, Watson resolved to spend the money on a portrait of himself. Together they decided to include in the painting, as Watson wrote, "a ship, bearing to America the intelligence of the acknowledgment of Independence, with a sun just rising upon the stripes of the union, streaming from her gaff."

Prudently, the artist waited until the royal proclamation of...