Where to start with a collection like that held by the National Gallery? It's a truly daunting challenge, but fortunately the curators there have done some of the hard work already. This remarkable offering brings together 30 of their finest works, including Vermeer's "A Young Woman standing at a Virginal," Titian's "Bacchus and Ariadne," and Van Gogh's "Sunflowers." Visitors can click on each...