Art: Lost & Found

Among the old masterpieces from Germany now touring the U.S. (last week they were in Philadelphia) was one painting that was almost out of place, it looked so modern. It was a scene bathed in sickly torchlight, chill as a tomb, still as death—a stark and somber painting called Saint Sebastian Mourned by Saint Irene. To most gallerygoers the name under it—Georges de la Tour—meant nothing.

Thirty-five years ago the name would have stumped even the experts. But last week it was on many lips. In its first purchase for two years, Manhattan's conservative...