Vision is the ability to be able to see. In mammals the organ that is responsible for vision is the eye. Light enters the eye and it is then focused on the retina by the lens. The retina is a layer of cells that cover the interior surface of the eye. The retina contains two different types of cells--the rods and the cones. Both of these are known as photoreceptors because they respond to light. There are three million cones and 100 million rods in the human eye. Each of these types of cell performs a different function. The cones are responsible for color vision and they require high levels of light to operate. The rods are responsible for low light level vision and they operate in black and white. The cones are concentrated in the center of the back of the eye, at its...