“The Madame X Files,” by Hamish Bowles, was originally published in the January 1999 issue of Vogue.

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John Singer Sargent’s 1884 portrait of Virginie Avegno Gautreau, universally acclaimed as Madame X, is a definitive study in image-making. La Gautreau flaunts her otherworldly looks and her chosen role as that exotic ornament to society, a professional beauty. She is a sphinx without a secret, “prophetic of all the sophisticated chic of Vogue, ” as Philippe Jullian, historian of fin-de-siècle culture, noted in 1965. But who was this fascinator whose mystery remains compelling more than a century after Sargent captured it in sensual oil paints?

John Singer Sargent, whose career is celebrated in a

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