Maxwell's discs

Maxwell's discs are an apparatus used in studies of colour perception. They were developed by the eminent physicist James Clerk Maxwell. Maxwell was intrigued by James David Forbes's use of color tops. By rapidly spinning the top, Forbes created the illusion of a single color that was a mixture of the primaries:

"[The] experiments of Professor J. D. Forbes, which I witnessed in 1849… [established] that blue and yellow do not make green, but a pinkish tint, when neither prevails in the combination…[and the] result of mixing yellow and blue was, I believe, not previously known."

- James Clerk Maxwell

Maxwell took this a step further by using a circular scale around the rim with which to measure the ratios of the primaries, choosing vermilion (V), emerald (EG), and ultramarine (U).

Initially, he compared the color he observed on the spinning top with a paper of different color, in order to find a match. Later, he mounted a pair of papers, snow white (SW) and ivory black (Bk), in an inner circle, thereby creating shades of gray. By adjusting the ratio of primaries, he matched the observed gray of the inner wheel, for example:

$$0.37V+0.27U+0.36EG=0.28SW+0.72BK$$

To determine the chromaticity of an arbitrary color, he replaced one of the primaries with a sample of the test color and adjusted the ratios until he found a match. For pale chrome (PC) he found $$0.33PC+0.55U+0.12EG=0.37SW+0.63BK$$. Next, he rearranged the equation to express the test color (PC, in this example) in terms of the primaries.

This would be the precursor to the color matching functions of the CIE 1931 color space, whose chromaticity diagram is shown above.

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