For weeks, Dutch art lovers have been quietly celebrating the 100th anniversary of Vincent van Gogh without causing much of a stir in the world's art circles. But last week one of the big exhibitions produced the kind of unexpected treasure-trove that always sets the experts to buzzing.
The discovery came in Amsterdam, where nine wise-eyes were examining a display of 19th century paintings once owned by Van Gogh's younger brother, Theo. Among them were 15 works by "unknown" artists, and one of them in particular caught the scholars' attention. A small (16 in. by 12 in.) portrait of a frock-coated man,...