Astigmatism is corrected by neutralizing the cylinder in the eye with an opposite cylinder lens. A minus cylinder neutralizes a plus cylinder, and a plus cylinder neutralizes a minus cylinder. The axis of the cylinder must be set in an identical position. If the eye is short on plus we give it a plus cylinder -- if it has too much plus we give it a minus cylinder. The purpose of cylinders is to make the eye spherical. IT IS THE SPHERICAL EYE WHICH GIVES BEST VISION.
Astigmatic eyes may well be divided into low grades and high grades -- the low grades are many -- the high grades are few. The low grades, mostly, have good vision with the naked eyes. The high grades, as arule, have poor vision without glasses. Most low grades of astigmatism are hyper or were spherical hyper eyes. They developed their astigmatism by misusing their eyes in near-seeing. Many children who wear weak minus cylinder glasses would do better if they wore a pair of plus spherical glasses for near-seeing.
There are many expensive instruments to find the exact amount of the cylinder needed, and the exact position of the axis. Unfortunately, in many cases, they are more confusing than revealing. They are especially confusing when minus cylinders are indicated.
Quite often older persons cannot accept or wear cylinders because they seem to distort the vision.
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