Eye Strain

Eyestrain

Eyestrain may result from a need for eyeglasses or from use of out- grown glasses. It may also result from unfavorable conditions under which the eyes arc used.

 Improper lighting, especially for reading or close work, frequent cause of strain

 Do not face the light; it should comefrom theback of you and from the side so you are not in shadow, make sure light bulbs are strong enough (75 to 100 watts) and free of dust. Hold a book or paper about 16 to 18 inches away from you and slightly below eye level. Reading when lying on your back in bed or propped up on an elbow may strain the eyes, and so will reading for long periods in a vibrating vehicle. Avoid glare, of course. 

And rest your eyes from time to time by shifting focus, looking off into the distance. Night Blindness Inability to see well or at all in dim light can mean something wrong not alone with the eyes but with the entire system. Night blindness, as it is called, is a threat to safety, particularly on the highway, because a driver may have 20/20 vision and not realize that his vision is somewhat impaired at night. The condition produces no discernible changes in eye tissues, so it cannot be diagnosed unless the patient tells the physician that he has difficulty in reading road signs at night or has trouble picking out objects in dimly lighted streets. 

It is not normal to have trouble seeing in dim light after a brief period, two to three minutes, of adjustment. If you become aware of such a problem, discuss it with your physician. It can be treated, sometimes simply by addition of vitaminA to the diet. Sunlight Good sunglasses can protect your eyes if you are in bright sunlight. Poor ones may only add to your troubles if you wear them for long. Don't wear glasses with scratches or irregularities. Some glasses are too lightly tinted to do much good. It's true that good sunglasses are expensive. 

The best have ground and polished lenses, and are worth the investment. Even though you wear sunglasses, do not look directly into the sun. Don't wear the glasses indoors and at night no matter how fashionable it may seem because accidents result from the reduced visual perceptiveness. If you use regular glasses, it is worthwhile to have a pair of sunglasses ground to your prescription rather than clip a pair of possibly inferior sunglasses over your carefully made regular glasses. 

Sense Organs- Eye

THE SENSE ORGANS

THE EYE

The human eye, which presents us with a world of space and depth and a continual variety of sights, is packs into a single cubic inch of space more than 150 million light receptors. The eye is often compared to a camera. The outermost layer of the eyeball forms the white of the eye and at the front becomes the completely transparent cornea. 

The next layer is the iris, which has an adjustable aperture, the pupil, which becomes larger or smaller depending upon the amount of light entering. Just behind the iris and its pupillary opening is the oval-shaped and elastic lens, which bulges out when its muscles contract, and flattens when they relax, thus adjusting to properly bend and focus light rays on the retina. (The lens muscles are the fastest and best-coordinated in the body.)

The retina, which lines the eye and serves as the "film," contains the light receptors that react to incoming images. The responses of the receptors are transmitted along a million nerve fibers which form one outgoing cable, the optic nerve, at the back of each eye. The exit of the optic nerve leaves a "blind" spot-an area without receptors. It has been estimated that 85 percent of everything we learn comes to us through our eyes.

 It's the brain that does the actual seeing; the eyes are light-transmit- ting machines. This is the process: Light rays strike an object and are reflected to the eyes. The rays pass through the cornea, the clear front window, the aqueous humor (a watery liquid behind the cornea), the pupil, and the lens.

The lens bends and focuses the rays on the retina. As the rays impinge on light-sensitive pigments in the retina, chemical reactions take place that send impulses through the optic nerve to the brain. Actually, images are received upside down because the lens inverts them, but the brain has learned to interpret them in accord with reality. 

Central vision-what you use when you look straight at an object-is sharpest. But you also have side, or peripheral, vision. And while peripheral vision is not very acute, it is important; without it, you would bump into things and be unaware of objects approaching from the side. You can demonstrate side vision with a simple experiment: With both eyes open, hold your right thumbnail 16 inches in front of your face.

Have someone hold a wrist watch at arm's length to your left and gradually move it toward your thumb. Without moving your eyes, you will be able to identify the watch as a watch probably when it is about 15 inches away from your thumb. Chances are, though, that you will not be able to tell time until the watch is about two inches away. 

Interestingly enough, each of us has his own individual view of the world. That's because, for one thing, the eyes can transmit millions of impulses per second but the brain chooses details on the basis of individual past experience, mood, and interests at the moment. How we see things also can be affected by their meaning for us.