Chevreul illusion. Diagrams from 'De la loi du contraste simultane des couleurs et de l'assortiment des objets colores' (The principles of harmony and contrast of colours) by the French chemist Michel Chevreul (1786-1889). These diagrams show how the brain exaggerates differences in colour. When O (top left) and P (top right) are placed next to each (top centre) O appears lighter and P appears darker. When many different shades are put next to each other (along bottom), this leads to the perception of more shades than are actually present.
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