Eugene Chevreul’s 1839 book, On the Law of Simultaneous Contrast of Colors, guided the Impressionist practice of laying down strokes of pure, contrasting colors. Chevreul found that colors change in relation to the other colors near them. Complementary colors, or those directly opposite each other on his color wheel, create the most intense effects when placed next to each other, he wrote. Red-green or blue-orange combinations cause an actual vibration in the viewer’s eye so that color appears to leap off the canvas. No wonder viewers react emotionally to the glittering sunlight on Monet’s rivers or the splash of orange costume on Degas’ ballet dancers. "I want my red to sound like a bell!" Renoir said. "If I don’t manage it at first, I put in more red, and also other colors, until I’ve got it."