It is now universally recognised that, at least to as close an approximation as is practically testable under given conditions, a match in hue can be established in normal colour-vision between any visible light and a mixture of three suitably chosen spectrum lights. The latter are selected usually from portions of the spectrum in the close neighbourhood of three definite wave-lengths. Though considerable variation in the choice of these three standard lights is possible, it is found to be most suitable to place them in the pure red, the pure green, and the pure blue regions of the spectrum respectively.