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The brain superimposes the two slightly different images and judges distances from the composite image.ĭisorders of the Eye. Because the eyes are set apart, each eye sees farther around an object on its own side than does the other. The eyes are situated in the front of the head in such a way that human beings have stereoscopic vision, the ability to judge distances. The small spot where it leaves the retina does not have any light-sensitive cells, and is called the blind spot. The optic nerve, which transmits the nerve impulses from the retina to the visual center of the brain, contains nerve fibers from the many nerve cells in the retina. White light stimulates all three sets of color cells any other color stimulates one or two. These are the primary colors in light, which, when mixed together, give white. There are three types of cones, each containing a substance that reacts to light of a different color, one set for red, one for green, and one for violet. The cones are responsible for color vision. They cover the full range of adaptation to light, the cones being sensitive in bright light, and the rods in dim light. When the object is close, the muscles relax and the elastic lens becomes thicker, bending the light rays and focusing them on the retina.ƒīecause the eye must function under many different circumstances, there are two types of nerve cells in the retina, with different shapes: the cones and the rods. This means that when the eye sees something in the distance, muscles pull the lens, stretching it until it is thin and almost flat, so that the light rays are only slightly bent as they pass through it. Unlike the lens of a camera, the lens of the eye focuses by a process called accommodation. (See also vision.) The refraction or bending of light rays so that they focus on the retina and can thus be transmitted to the optic nerve is accomplished by three structures: the aqueous humor, a watery substance between the cornea and lens the lens, a crystalline structure just behind the iris and the vitreous humor, a jelly-like substance filling the space between the lens and the retina.