Georges de La Tour's haunting depiction of a repentant Mary Magdalen gazing into a mirror by candlelight; Jean Sim on Chardin's perfectly balanced image of a young boy making a house of cards; Jean Honor Fragonard's monumental suite of landscapes showing aristocrats at play in picturesque gardens--these are among the familiar and beloved masterpieces in the National Gallery of Art, which houses one of the most important collections of French old...