Like a far-off celebrity glimpsed at a party, the star of Tate Britain’s new exhibition, John Singer Sargent’s Madame X (1883-84), appears at a distance, perfectly framed by a doorway, sending hearts aflutter. Marmoreal, lavender-tinged skin; a nose as sharp as a ninja’s throwing star; that sumptuous black bodice held up only by a pair of chains: her features and attire are unmistakable.