LONDON, March 1 — The first painting donated in 1856 to the new National Portrait Gallery here was of William Shakespeare, already well enshrined as the nation's literary idol. For the gallery, the oil recorded as NPG 1 seemed like a singularly apt founding work for its collection. And now, as the museum celebrates its 150th anniversary, it is again in the limelight.

But does this so-called Chandos portrait actually depict Shakespeare? Indeed, do any of dozens of other "Shakespeare" paintings and engravings offer a true likeness of the man who was born in Stratford-on-Avon in 1564 and died there in 1616?

These are the central questions addressed in "Searching for Shakespeare," a fascinating exhibition on view here through May 29. For this inquiry, the National Portrait Gallery has for the first time united the six oils most frequently said to portray Shakespeare. For further comparison, it is also presenting the 1623 engraving of him in the First Folio of his collected plays, as well as a plaster cast of the bust that was placed above his grave in Holy Trinity Church in Stratford sometime between 1620 and 1623.

And the answer? Well, for all the light that Shakespeare threw on human nature, his own life remains shadowy: his education, the "lost years" between 1585 and 1592, his relations with his wife and children and, yes, even his appearance are very much matters of conjecture.

Still, of all the competing paintings, the Chandos portrait has emerged as the strongest contender. "It's not absolutely watertight," said Tarnya Cooper, the gallery's curator for 16th-century painting, who organized the show, "but the evidence has increased. It is a portrait that probably represents Shakespeare, but will we ever have watertight evidence?"