Within the retina (the thin, light-sensitive inner layer at the rear of the eye) are about 150 million rods and seven million cones -- specialized cells made up of chemicals that react to different wavelengths in light. Located mainly in the periphery of the retina, the rods do not perceive color. The cones, which do perceive color, are responsible for fine detail in the center of vision. They enable us to read words on a page and recognize a familiar face from across the room. Cones are most active in bright light, while rods are most sensitive in the dark; this is why it is hard to detect colors and fine details in the dark. The cones are located primarily in the macula, a remarkably small part of the retina that gives us sharp central vision. The best vision -- for reading or detailed work -- comes from the fovea, at the center of the macula. The rest of the retina delivers peripheral vision (side vision), which is less sharply focused.