Looking for Leonardo: for centuries an altar panel in Florence, Italy, has been attributed to Andrea del Verrocchio. But are two of its figures actually by his student Leonardo da Vinci?

There's nothing unusual about discoveries of lost works by Leonardo da Vinci. Every few months, it seems, a story hits the news that yet another "Leonardo" has been unearthed--the lost fresco of the Battle of Anghiari, a terracotta bust discovered in the attic of a 14th-century palazzo, or a self-portrait embedded in the spidery script of one of his notebooks. A recent television documentary even made a claim for the artist's authorship of the Shroud of Turin.

Leonardo scholar Martin Kemp, emeritus professor of art history at Oxford University, calls the perpetrators of such dubious attributions "Leonardo loonies" and says he gets "bombarded" with them almost daily.

What is exceedingly rare, however, is for a noted Renaissance scholar to bring forth evidence, patiently argued and carefully annotated, that a work previously thought to be by a lesser light is actually an effort by the young Leonardo. That is the case with Gary M. Radke's recent announcement that two silver figures, from a 12 1/8-inch by 16 1/2-inch altar panel made for the Baptistery in Florence, Italy, were more likely created by Leonardo than by his teacher, Andrea del Verrocchio.

The two figures in question, an angelic-looking youth holding a salver at the far left of …