“A surprising feature of the chromasphere is the circular character of the color wheel. Since the physical variable that corresponds to hue is the wavelength of light, and the wavelengths that human eyes respond to vary from 700 nanometers (red) to 400 nanometers (violet), one might have expected that the subjective sensation of color would likewise be spread out in a linear fashion fading away to invisibility at two limiting hues. However, unlike the physical spectrum, the visual spectrum loops back on itself, forming a color circle rather than a color line. The loop is closed via a nonspectral purple – a particular ratio of red and violet light. The circular character of the color wheel is explained by the fact that in normal eyes there are three different color receptors, whose sensitivities peak in red, green, and blue light, respectively. The relative stimulation of these three receptors defines a unique position within the color solid. The reason that our psychological color space appears to have more dimensions than the physical spectrum is that our eyes do not detect color as a single note, but as a three-note chord, perceiving a kind of optical harmony. If we happened to possess eyes with four color receptors, the subjective color space would no doubt be four-dimensional, the pure colors being spread over the surface of a sphere rather than a circle. Although a few poets have speculated about new color sensations outside the human range, it is impossible for us to imagine what a new color would actually look like, visually imprisoned as we are inside the normal human chromasphere. One boon a new science of mind might be able to grant would be the literal explosion of our visual horizons, with one or more color receptors, preferably lying outside our present visual range, whose outputs combined with those of our present receptors would allow us to perceive a vastly extended color space of more than three dimensions.”