The beauty, mystery, joy, and inspired observation of the human spirit that paintings evoke result from a complex of intuitive and cognitive choices made by the artist. An understanding of the genesis of these choices can be as elusive as the resulting imagery, but great paintings seem to initiate in us a curiosity about the ideas, methods, and mate- als used by their creators. In the twentieth century connoisseurship has been enriched by the application of methods of scientific analysis. The results of these investigations into the physical properties of paintings have shed new light on...