Few can doubt that J.M.W. Turner (1775-1851) was the greatest exponent of British watercolor in its golden age. An inveterate traveler in search of the ideal vista, he rarely left home without a rolled-up, loose-bound sketchbook, pencils, and a small traveling case of watercolors in his pocket. He exploited as no one before him the medium's luminosity and transparency, conjuring light effects on English meadows and Venetian lagoons and gauzy mists...