The retina of the eye provides the essential foundation for visual perception. It contains millions of nerve cells that analyze the visual image and transmit signals to the brain. Much like a camera, the eye forms a physical image of an object in the external world on the surface of the retina at the back of the eye. The next essential step is to respond to the image. This is the job of cells known as photoreceptors. The photoreceptors are extremely small—about 1 to 3 microns in diameter. There are about 130 million of these photoreceptors packed side-by-side, much like pixels in a digital camera. There are two classes of photoreceptors. Rods are the receptors for night vision. Cones are the receptors for daylight and ...