Cole Swensen's beautifully exact and visionary meditations on specific paintings and sculptures give the venerable art of "ekphrasis" a new meaning. Whether focusing on a Siennese altar-piece, a 20th-century sculpture, or a river scene in the Loire Valley, Swensen "sees" with an uncanny X-ray vision that becomes the inward gaze. 'The difference, ' as Gertrude Stein once put it, 'is spreading.'