J. M. W. Turner captured his country in flux. Caught within the romance of his oils and watercolors is the artist’s native Britain as he saw it: at war, transitioning from sail to steam, charging ahead into capitalism. As the world transformed around him, Turner reacted by altering his style, and in 150 key works viewers can track the changes. Brushstrokes get wilder. Colors clash, less blended, reflecting the dizzying pace of the modernization taking place. At the time, Turner’s new approach shocked his contemporaries. Today, it’s the reason he’s considered Britain’s greatest landscape painter. —J.V.