The English painter and printmaker Thomas Gainsborough, one of Britain’s most prominent 18th-century artists, is famous for a portrait that rivals Leonardo’s Mona Lisa in mystery. It sees a boy dandy in blue satin, lips rosy and shoes beribbboned, who gazes out of the canvas with what could be shyness or intensity. His identity is uncertain. The Blue Boy was shown publicly for the first time in 1922, at London’s National Gallery, where it was on view for three weeks before it sailed to California, having been purchased by the railroad magnate Henry Edward Huntington. Now, for the first time in a century, the enigmatic boy is back in Trafalgar Square. —E.C.