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The Science of Human Nature - A Psychology for Beginners by William Henry Pyle
page 49 of 245 (20%)
vision. However, in about one third of school children the defect is
great enough to need to be corrected by glasses.

The principle of the correction of sight by means of glasses is merely
this:[1] When the focusing apparatus of the eye is not perfect, it can
be made so by putting in front of the eye the proper kind of lens. There
is nothing strange or mysterious about it. In some cases, the eye
focuses the light before it reaches the retina. Such cases are known as
nearsightedness and are corrected by having placed in front of the eyes
concave lenses of the proper strength. These lenses diverge the rays and
make them focus on the retina. In other cases, the eye is not able to
focus the rays by the time they reach the retina. In these cases, the
eyes need the help of convex lenses of the proper strength to make the
focus fall exactly on the retina.

[1] The teacher should explain these principles and illustrate by
drawings. Consult a good text in physiology. Noyes' University of
Missouri Extension Bulletin on eye and ear defects will be found most
useful.

Another defect of the eye, known as astigmatism, is due to the fact that
the eye does not always have a perfectly spherical front (cornea). The
curvature in one direction is different from that in others. For
example, the vertical curvature may be more convex than the horizontal.
Such a condition produces a serious defect of vision. It can be
corrected by means of cylindrical lenses of the proper strength so
placed before the eye as to correct the defect in curvature.

Still another defect of vision is known as presbyopia or farsightedness
due to old age. It has the following explanation: In early life, when we
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