When Maxwell began his work the theory of colours was exciting considerable interest. Sir David Brewster had recently developed a new theory of colour sensation which had formed the basis of some discussions, and in 1852 von Helmholtz published his first paper on the subject. According to Brewster, the three primitive colours were red, yellow and blue, and he supposed that they corresponded to three different kinds of objective light. Helmholtz pointed out that experiments up to that date had been conducted by mixing pigments, with the exception of those in which the rotating disc was used, and that it is necessary to make them on the rays of the spectrum itself. He then describes a method of mixing the light from two spectra so as to obtain the combination of every two of the simple prismatic rays in all degrees of relative strength. From these experiments results, which at the time were unexpected, but some of which must have been known to Young, were obtained. Among them it was shown that a mixture of red and green made yellow, while one of green and violet produced blue. In a later paper (_Philosophical Magazine_, 1854) Helmholtz described a method for ascertaining the various pairs of complementary colours--colours, that is, which when mixed will give white--which had been shown by Grassman to exist if Newton’s theory were true. He also gave a provisional diagram of the curve formed by the spectrum, which ought to take the place of the circle in Newton’s diagram; for this, however, his experiments did not give the complete data.