After 15 years in storage, a Dutch painting long slighted in the academic literature dramatically returned to public display on Monday at this city's Joslyn Art Museum as an authenticated work of Rembrandt van Rijn (1606-1669). The "Portrait of Dirck van Os" (c.1658) was recently confirmed as a work of the master by the world's foremost authority on Rembrandt, Amsterdam University professor Ernst van de Wetering, following conservation efforts to remove extensive repainting and layers of discolored varnish that previously obscured the picture's original paint surface. The culmination of a decades-long campaign by the Joslyn's staff to interest outside specialists in the painting's attribution, the unveiling marks a proud moment for one of America's outstanding regional museums. "People here sensed the underlying quality," says the Joslyn's executive director, Jack Becker, "but you need the scholarly community to rehabilitate a picture like this."