Among the most delectable paintings in the Metropolitan Museum of Art’s collection is a smallish canvas of a man playing a guitar, seated on a stone bench in what seems to be a garden. His posture—legs crossed, torso angled, head tipped back—seems exaggerated. His clothes—a tight-fitting silk suit in marzipan-hued stripes, ruffled collar and cuffs, an enormous silk beret and matching cloak in deep rose—seem as extravagant. At first, we simply accept these qualities, fascinated by the ravishing color and the fluent touch that conjure up gleaming silk, softly gathered lace, tightly stretched stockings, and the crisp rosettes on narrow shoes. Gradually, though, we become aware of just how artificial everything about the elaborately dressed musician really is. His position becomes a pose, his gorgeous suit a costume, and the “garden” in which he plays his instrument a painted backdrop ...