Bill Gronseth, age 13, of Duluth, Minn., for his question:
WHAT IS 20/20 VISION?
Doctors tell us that good habits are important in using our eyes. To read comfortably, good light is very necessary. We should hold the printed pages at least 14 inches from our eyes. The eyes should also be rested from time to time since the muscles that focus the eyes for reading become tired after long use and do not work properly. Guard against eyestrain.
Our eyes help us to carry on almost all of the activities we take part in daily. The eyes are the most important organs for finding out about the world around us.
A test for visual acuity, or sharpness of vision, is usually given with a standard eye chart that is made up of rows of letters. The letters in each line are smaller than those in the line above.
The chart is placed 20 feet from the patient who is going to be tested. The patient then reads each line first with both eyes and then with the right eye covered. A final test is to read as far down the chart as possible with the left eye covered.
If the person being tested can read letters of the size normal for a person 20 feet away, his vision is recorded as 20/20/. If, at 20 feet, he can read only the letters that the normal eye can see at 40 feet, his vision is expressed as 20/40.
Some people have sharper eyesight than normal. A person who can see letters from 20 feet that the normal eye can see only from 15 feet has 20/15 vision.
In countries where the metric system is used, the chart is read from a distance of 6 meters. Under this system, normal vision is expressed as 6/6.
Astigmatism, which results in blurred vision, and color blindness, or the inability to see certain colors, are other defects in vision that may be tested with charts of various kinds.
The inside of the eye is examined by doctors using instruments called ophthalmoscopes. An ophthalmoscope shows conditions within the eye as clearly as if they were on the surface. It allows the doctor to examine the blood vessels in the eye. The condition of these vessels reveals certain general conditions of the body's circulatory system.
Vision is the ability of the eye to form an image of an object and send the image to the visual centers of the brain.
The eyes work very much like a camera. They each obtain pictures in similar ways. Each has a lens to focus the image.
The iris of the eye and the diaphragm of the camera each regulate the amount of light received. The retina of the eye and the film of the camera are each sensitive to light.