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... interpreted, and sight occurs. Light must pass through the covering layers of the retina to reach the layer of rods and cones. There are about 75 to 150 million cones in the human retina. Rods do not detect lines, points, or color.
They perceive only light and dark tones in an image. The sensitive rods can determine outlines of objects in almost complete darkness. They make it possible for people to see in darkness or at night. Cones are the keenest of the retina's receptor cells.
They detect the fine lines and points of an image. The cones, for example, make it possible to read these words. There are three types of cones that receive color sensations. One type absorbs light best in wavelengths of blue-violet and another in wavelengths of green; a third is sensitive to wavelengths of yellow and red.
How Two Eyes Can Work Together Most individuals use both eyes to see an object. This type of sensory perception is known as binocular vision. Thus, two images of the object are formed - one of the retina of each eye. Impulses from both images are sent to the brain. Through experience these impulses are interpreted as two views of the same object. Because the eyes are about 2 1 / 2 inches apart from pupil to pupil and therefore are looking at the object from different angles, the 2 views are not exactly alike.
This is known as the stereoscopic effect. If the object is far away, the difference between the images is slight. If it is a few inches away, the difference is very great. The brain makes good use of the phenomenon. It learns to judge the distance of an object by the degree of difference between the images it receives from the two eyes.
In the same way, the brain perceives what is called perspective. It estimates differences in distance between two different objects or between two parts of the same object. The eyes are turned up, down and sideways by long muscles. At one end these muscles are attached to the bony walls of the eye socket.
They are regulated with the most delicate precision so that normally they turn both eyes toward the same object at exactly the same time. Organic Disorders The conjunctiva, the membrane lining the inner surface of the eyelids and the exposed surface of the sclera, can become irritated and inflamed. This is called conjunctivitis, and is caused by viral infections or by exposure to smoke, dust, or similar irritants. A common disorder of the eyelid, particularly in children, is a sty - an infection in the small glands of the eyelash. It is caused by the growth of bacteria and results in reddening and swelling of the entire eyelid. A cataract is a cloudy discoloration in the lens of the eye.
It can develop until the entire lens is covered with a thin, milky film. Treatment for cataract usually involves surgery, after which the patient is fitted with special glasses. Glaucoma is a fairly common disorder caused by an increase in pressure within the eyeball. It can result from heredity, tumors, and other causes. Headache, blurred vision, and eye pain are symptoms of glaucoma. Many treatments are used, and it is sometimes necessary to perform surgery to relieve the pressure.
Retinitis pigmentosa is an inherited eye disease in which the retinal pigments degenerate. In the course of the disease, the rods are destroyed early, causing night blindness in youth. Deterioration of the retina is progressive. Eventually the affected person sees objects as if looking through a narrow pipe. This stage is followed by complete blindness. Diabetes, treated or untreated, may cause serious eye complications.
The most common condition occurs in people who have had the disease for a long time. The blood vessels of the retina expand and bleed into the retina. In later stages the bleeding becomes more extensive and eventually the retina becomes detached. These changes invariably lead to blindness.
The actual cause of the changes in vessels of the retina is still unknown. Both benign and malignant eye tumors are usually extremely serious. They not only damage the eye but may also invade the brain. Optical Defects Some eyes are abnormally long from front to back.
The lens, even when stretched to the utmost, cannot bring distant objects to a focus on the retina. Nearsightedness, or myopia, is the result. This defect is corrected by wearing a concave, or negative, spectacle lens. This lens, together with the convex lens of the eye, makes an optical system of longer focus.
When the distance between the front and the back of the eye is too short, the lens cannot bring near objects to a focus. This is a form of farsightedness called hypermetropia. The condition is corrected by shortening the focus with a convex or positive lens. In many eyes the cornea is deformed so that its surface is oval instead of spherical. Light rays are distorted at the entrance of the eye.
This produces a blurred image and is known as astigmatism. To correct it, glasses are given a nonspherical or cylindrical curvature. Cross-eyes and walleyes are produced when both eyes do not work together because of weakness of the eye muscles. The images formed on the two retinas are so unlike that they cannot be blended in the brain. Thus, a double image is perceived. The condition is known as diplopia, or double vision.
Prismatic lenses are prescribed to correct this defect. Imperfections in the cones of the retina, resulting from heredity or disease, cause defective color vision. This is known as color blindness, or Daltonism. In total color blindness, everything appears in shades of gray.
In its more common from, color blindness is the inability to distinguish between reds and greens. Persistent headaches, blurred vision, and painful inflammation of the eyelids are symptoms that may indicate serious eye disorders. Particles lodged in the eye should be removed without delay. Glasses are prescribed to strengthen vision and to reduce strain and fatigue.
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Research essay sample on Blurred Vision Color Blindness
