Impressionist artists wanted to replicate the colours of the natural world. They noticed that shadows are not simply black, and that clouds are not simply white, but made up of a multitude of colours. The artists were interested in colour theory, and in particular, the research of Michel-Eugène Chevreul, whose colour wheel (1839) helped to illustrate the concept of complementary colours. For example, when orange and blue are placed beside each other, like in this detail from Monet's painting, the intensity and hue of each colour is enhanced. This gave rise to theories about the optical combination of colours that was important for Impressionist techniques and especially for Neo-Impressionist painters.