X-ray examination of a recently discovered painting -- judged by some authorities to be a self-portrait by Vincent van Gogh -- revealed an underimage of a woman' s face. Either van Gogh or another painter covered the first painting with the portrait now seen on the surface of the canvas. Because the face of the woman in the underimage also appears on canvases van Gogh is known to have painted, the surface painting must be an authentic self-portrait by van Gogh.
The conclusion above is properly drawn if it is assumed that any painted canvas incorrectly attributed to van Gogh would not contain an underimage of a subject that appears in authentic paintings by that artist.