For decades, a mysterious black stain has been spreading across the face of an anonymous woman in Australia [below]. She is the subject of a painting by Edgar Degas, the French Impressionist painter, and since the 1920s, the oil paints in her portrait have gradually faded, revealing the hints of another, hidden portrait underneath.

Until recently, attempts to capture the image underlying “Portrait of a Woman” with conventional X-ray and infrared techniques have only yielded the shadowy outline of another woman. In a study published on Thursday, however, a team of researchers reports that they have revealed the hidden layer underneath the painting, which hangs in the National Gallery of Victoria in Australia, at a very high resolution. It seems to be a portrait of Emma Dobigny, a model who was a favored subject of Degas. [...]

To get their high-resolution image, the research team used a type of particle accelerator called a synchrotron. Synchrotrons are sources of extremely high-energy light. They work by directing that light, which is a million times brighter than the sun, into an X-ray beam that’s one tenth the diameter of a human hair.