Man's physical universe

xanabras

440 ENERGY MAY BE PROPAGATED BY VIBRATIONS

Near-sightedness and far-sightedness are the results of elongated

and shortened eyeballs respectively. People who have these defects

either cannot bring near or distant objects into focus, or they subject

their eyes to undue strain in so doing. Such defects are corrected by the

use of concave or convex lenses.

Another common eye trouble is astigmatism. Astigmatism will

cause more discomfort and eyestrain than any other type of eye defect.

The eyeballs in this case are slightly cylindrical in shape, so that two

Fig. 214.

The diflferent types of eye defects and the types of lenses required to

correct these defects.

equally distant lines at right angles to each other will not be in the

same focus. Astigmatism is corrected by use of a cylindrical lens.

Double vision or unequal focus, which at its worst is represented by

crossed eyes, is due to muscle unbalance. Extreme cases may be overcome

by delicate operations on the muscles, and less severe cases can

be corrected by means of prismatic lenses.

Aniseikonia, Greek for "unequal images," is a condition in which

the images received by the two eyes differ in size and shape. People

afflicted with this defect lack depth perception and distance sense

and should not operate automobiles or airplanes. The eye strain resulting

from this disorder frequently causes headaches and stomach or

nervous disorders. Aniseikonia can be corrected by "tailor-made"

lenses.

STUDY QUESTIONS

1. Mention three types of eye defects. What is the cause in each case? How is

each type corrected?

2. W'hat is the principle involved in red reflectors?

3. What is meant by reflection and refraction of light?

4. Why does the sky appear light on the earth's surface in the day, while it is

very dark at higher altitudes?

5. Why does a pencil placed in a glass of water appear to be bent at the surface

of the water?

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