Art: Jefferson at 33

The founding fathers of the U.S. make a somewhat solemn gallery in the mind. Remembered mostly from portraits painted late in their hardworking, often harsh lives, they seem austere, wrinkled and careworn. Now a miniature portrait of one of the greatest of them, Thomas Jefferson, has come to light, showing him as he really appeared in the fateful summer of 1776.

The scarred little picture has rested unidentified for more than a century in the collection of the Historical Society of Pennsylvania. It is the work of a minor Swiss artist named Pierre Eugène...