physiological features across individuals. This is supported by the fact that color judgments can vary depending on differences in the color environment across long periods of time, but these same chromatic and achromatic judgments are held constant if the color environment is the same, despite aging and other individual physiological factors affecting the retina. Like the Bezold–Brücke effect, the Abney effect suggests a non-linearity between the cone responses (LMS) to the stage of hue perception. Colorimetric purity The saturation, or degree of paleness of a color, is related to colorimetric purity. The equation for colorimetric purity is: . In this equation, equals the luminance of the colored light stimulus, is the luminance of the white light stimulus to be mixed with the colored light. The above equation is a way of quantifying the amount of white light that is mixed with the colored light. In the case of pure spectral color, with no white light added, equals one and equals zero. This means colorimetric purity would equal one, and for any case involving the addition of white light, the colorimetric purity, or the value of , would be less than one. The purity of a spectral color stimulus can be altered by adding white, black, or gray stimulus. However, the Abney effect describes the change in colorimetric purity by the addition of white light. In order to determine the effect that changing the purity has on the perceived hue, it is important that purity be the only variable in the experiment; luminance must be kept constant. Hue discrimination The term hue discrimination is used to describe the change in wavelength that must be obtained in order for the eye to detect a shift in hue. An expression defines the required wavelength adjustment that must take place. A small (< 2 nm) change in wavelength causes most spectral colors to appear to take on a different hue. However, for blue light and red light, a much larger wavelength shift must occur in order for a person to be able to identify a difference in hue. History The original article describing the Abney effect was published by Sir William de Wiveleslie Abney in Proceedings of the Royal Society of London, Series A in December 1909. He decided to do quantitative research following the discovery that the visual observations of color did not match the dominant colors obtained photographically when using models of