In the retina are specialized receptor ~nerve~ cells, called rods and cones, that contain light-sensitive chemicals. The ~nerve~ cells get their names because of their shapes. The rods are about 1/400 of an inch (0.06 millimeters) long and about 1/100 of an inch (0.25 millimeters) thick. The cones are shorter and thicker. There are about 120 million rod cells in each |eye|. They work in dim light and are used for seeing black and white. The cone cells number about 7 million in each |eye| and function in full light. They provide color vision. The cones contain a pigment known as visual purple or rhodopsin which, it is believed, is broken down and bleached by light. This breaking down process sets off an electrical charge which transmits the light in the form of nervous impulses to the |brain| by way of the |optic nerve|. These impulses are interpreted by the visual cortex to give us the sensation of sight. Color blindness is due to defects in the cone cells. Different cones are sensitive to different colors, so that a color blind person may be able to perceive some colors but not others. Color blindness is inherited and rarely occurs in women. Proper nutrition is important to preserve good eyesight. The rod and cone cells use a form of vitamin A to help convert light into nerve signals. The vitamin combines with proteins to make a light-sensitive chemical in the rods and color sensitive chemicals in the cones. People who are get too little vitamin A can not see well at night.
