The curator Nicholas Cullinan has had the bold and lovely idea of interspersing paintings, drawings and sculptures by Cy Twombly with paintings and drawings by Nicolas Poussin, in a compact exhibition at the Dulwich Picture Gallery. Arcadian Painters, he bills the pairing, his working premise being that the veteran American artist (who died on 5 July 2011) shares with the 17th-century Frenchman a devotion to classical antiquity. Whether or not you feel that such an affinity comes through visually, the experiment in juxtaposition gives you much to reflect on