The light reflected on a body, differing due to the length of the radiation wave or electromagnetic radiations of which the wave is made of, that is the physiologic sensation produced by these radiations on the retina of the eye, on the basis of that which we perceive to be their difference. Throughout time, colours have been subject to research and study, but it is only since the middle of the 19th Century that they result analyzed on the basis of new discoveries in physics and chemistry with relative classifications that have remained valid even today. The scientific research conducted in the preceding century was of great importance, especially the work carried out by Newton, who was the first to experiment the refraction and the decomposition of a ray of solar light: all of the colours are contained in the light, as one observes by orienting a luminous beam in a prism where the seven colours of the spectrum appear: red, orange, yellow, green, blue, indigo and violet. These were then organized in a chromatic circle or disk in order to evidence their relationships. See primary colour, secondary colour, tertiary colour, complementary colour, warm colour and cold colour.