A description is given of James Clerk Maxwell’s strategy, in his early study of the additive mixing of light from coloured samples. He used the scattered daylight from known areas of card coated with artists’ pigments. Vermilion, emerald green and ultramarine were the optimum choice of standards for red, green and blue, respectively. They suited Thomas Young’s description of colour vision. Maxwell’s design of an analogue device – his ‘colour top’ – for varying the areas of the contributing pigments was remarkably simple. His meticulous observations with it allowed him to substantially further the understanding of perception of colour at the time, mid-19th century. The interpretation of a few very basic spectroscopic measurements on sunlight reflected from pigments are in line with Maxwell’s conclusions.
additive colour mixing of light, artists’ pigments, colour top, spectra
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