London (CNN) -- The 500-year-old panel is not much bigger than an average flatscreen TV and the wood has split, but what it shows is truly extraordinary. A painting of Jesus Christ that, after centuries of neglect, has been identified as Leonardo da Vinci's "Salvator Mundi." It's a discovery so rare, so unlikely, that when New York art dealer and da Vinci expert Robert Simon of Robert Simon Fine Art first saw the painting, he didn't even consider the possibility. "The whole idea that it might be by him was almost an impossibility; it's kind of a dream," said Simon.
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Neglected for centuries, a Leonardo da Vinci painting is rediscovered
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Discovery so unlikely art historian and conservator thought they had "lost their minds"
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Main clues were original alterations in composition and painting of the curls
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"Salvator Mundi" now being exhibited for first time in London's National Gallery
She describes him using infrared reflectograms to see under the layers of paint, and discovering the thumb in the blessing hand and the stole @placeholder is wearing painted in different positions.