Chevreul's chromatic diagram, 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). Each dark line on this diagram represents a different colour, the lighter lines are mixtures of the two encompassing darker lines. The 20 marks on each line are degrees of graduation; changes from brightness (centre) to darkness (outer edge). Complementary colours are next to each other. Chromatic diagrams help to show how the perception of a colour changes depending upon its surrounding colours. This occurs because the brain exaggerates differences in hue and brightness, making juxtaposed colours appear more different than they actually are. This phenomenon is known as simultaneous contrast.