American Folk Art Museum: ‘Artist and Visionary: William Matthew Prior Revealed’ (through May 26) William Matthew Prior (1806-1873), the subject of this fascinating exhibition, painted portraits in the “American primitive” mode. His paintings of solemn, pie-faced tots, stiffly posed yet animated by some inner mystery, are especially compelling. But he had the chops to paint like a well-trained academician too. He was also an abolitionist who, at some risk to his reputation, painted portraits of black people; they are among his best works. He was also into spiritualism, and he was a savvy entrepreneur. He was, altogether, a most remarkable character.