The RYB primary colors became the foundation of 18th-century theories of color vision as the fundamental sensory qualities blended in the perception of all physical colors and equally in the physical mixture of pigments or dyes. These theories were enhanced by 18th-century investigations of a variety of purely psychological color effects, in particular, the contrast between "complementary" or opposing hues produced by color afterimages and in the contrasting shadows in colored light. These ideas and many personal color observations were summarized in two founding documents in color theory: the "Theory of Colors" (1810) by the German poet and government minister Johann Wolfgang von Goethe, and "The Law of Simultaneous Color Contrast" (1839) by the French industrial chemist Michel-Eugène Chevreul.