The Physics of Spectral Colours

Nicholas Odgen Rood (1831-1902)

In the 19th century a chromatic art theory was developed on the basis of natural science and led via the physics of spectral colours to the Pointillist movement. The eye of the beholder becomes a supplementary part of consciously composed image perception.

The painting physicist

Nicholas Odgen Rood had studied physics and also painted and was therefore interested both in scientific and technical as well as artistic findings about the arrangement of colours. Rood took up the work of the physicist J. C. Maxwell, who on the basis of experiments with rotating discs had shown that all colours can be produced from the mixing of three spectral colours provided that the light stimuli can be added as well as subtracted. Rood clarified what this means, or at least can mean, for the "artistic use of colour" in his book "Modern Chromatics", published in 1879.

Colour mixtures

By the middle of the 19th century three colour theories had essentially asserted themselves. For theories on spectral light, red, green and blue were regarded as primary colours; for a colour system dealing with pigments for use in art, handwork or industry, red, yellow and blue were undisputed as the basis of all other colour mixtures. However, there was still contention about the precise combinations – when is what mixed and how? Special attention was also paid to the contrast effects of colours. The phenomenon that complementary colours mutually reinforce one another or that the very same colour close to others creates different colour impressions was important both for art as well as for a new branch of colour research, psycho-physiology, later also called the psychology of perception. Thus the role played by the eye in mixing processes became more and more important.

Optical colour mixing

Rood clearly formulated the difference between the mixing of "beams of coloured light" and the mixing of pigmented "colour material". For the mixing of spectral light colours he stated that they can still increase their luminosity, and add their radiant components to still more light. In contrast the mixing of pigments leads to the wearing down and darkening of colour, it takes away, subtracts its light. For the artist who wants to increase the luminosity of his pictures, Rood therefore recommended no longer to mix the colours on the palette but to apply as pure colours as possible to the canvas in small points or lines. He said that the distribution of two colours as points which at a certain distance are mixed in the eye is almost the only practical possibility for the painter to achieve a real mixture not of colour material but of beams of coloured light. Rood's book "Modern Chromatics" was published in 1881 in French and, besides works by Maxwell and the chemist M. E. Chevreul, was very quickly absorbed by French Pointillism (e.g. Seurat).