Art: Duveen on da Vinci

"The right eye is dead," said Sir Joseph Duveen, dolefully. "Dead," he intoned, "very dead."

Surely Leonardo da Vinci had never painted a "dead" eye. Leonardo studied artillery, muscle fibres, ladies' lips, everything that quivered with life, mechanical or protoplasmic. He was the inspired archetype of the small boy who wants to know how things work. Sir Joseph Duveen could not believe that the painted "dead" eye was by Leonardo, nor, for that matter, that any part of the canvas had been colored by that amazing Florentine.

But there was another consideration: if...