ability to distinguish among various wavelengths of light waves and to perceive the differences as differences in hue. The normal human eye can discriminate among hundreds of such bands of wavelengths as they are received by the colour-sensing cells (cones) of the retina. There are three types of cones, each of which contains a distinctive type of pigment; one cone absorbs longer wavelengths (red light), another middle wavelengths (green light), and the third type shorter wavelengths (blue-violet light). A given colour stimulates all three types of receptors with varying effectiveness, and the pattern of these responses determines the colour perceived. In 1986 researchers identified the genes that correspond to the red, green, and blue pigments.