James Clerk Maxwell performed a series of experiments which led to the first calculations of isoluminance across colors. Using an ingenious top--like device (see Figure 4), Maxwell was able to take advantage of the visual system's sensitivity to nearly identical stimuli in order to make the first psychophysical measurements of the shape of color perceptual space. Maxwell decided to use vermilion, emerald green, and ultramarine (red, green and blue) as primaries since,
``... if red, blue, and yellow, had been adopted, there would have been a difficulty in forming green by any compound of blue and yellow, while the yellow formed by vermilion and emerald green is tolerably distinct.'' [Maxwell 1857]
Figure 4. Maxwell's figure of the moveable paper disks from his top. The inner circle was moveable so that one could adjust the relative proportions of black and white paper which were mixed to form a gray. The outer disk was composed of three pieces of colored paper whose relative percentage of area could be adjusted so as to form measurable mixtures of three primary colors.
The top was spun at a high rate of speed, and sunlight was shown onto the surface of the spinning disks. After each spin of the top, the outer set of colored papers and inner set of black and white papers were readjusted until the outer annulus and the inner disk became the same shade of neutral gray. By choosing a variety of colors of papers, performing this experiment, and then using Grassmann's methods of analysis, Maxwell was able to calculate the relative position of a variety of reds, greens and blues in an isoluminant representational color space as shown in Figure 5.
Figure 5. Maxwell's calculations of the positions of colors on an equiluminant color surface.
A decade later, Helmholtz's lucid review of the previous work in perception of color [von Helmholtz 1866] provided a carefully crafted argument for the presence of three retinal color receptors along with spectral sensitivity curves for the primary colors associated with these receptors.
If we are to disentangle color perception from the physical qualities of light it is best that we spend some time discussing the physics of light as it is currently understood.