“Each human retina contains two sorts of photoreceptors: rods and cones. The 120 million rods (so called because of how they look under a microscope) absorb light waves across the entire visible spectrum. . . . [R]ods register distinctions only between light and dark—that is, distinctions in value. Distinctions in hue are triggered by a different set of receptors: the cones. The retina’s five to seven million cones (again, that’s what they look like when seen through a microscope) are clustered at the focal point opposite the eye’s lens.”

—Bruce R. Smith, The Key of Green, 2009.