ations. They are built up out
of our sensations or from these sensations as they exist in memory.

=Defects of Sense Organs.= The organs of sight and hearing are now by far
the most important of our sense organs. They enable us to sense things
that are at a distance. We shall therefore discuss defects of these two
organs only. Since sensations are the primary stuff out of which mind is
made, and since sight and hearing are the most important sense organs,
it is evident that our lives are very much dependent on these organs. If
they cannot do their work well, then we are handicapped. And this is
often the case.

The making of the human eye is one of the most remarkable achievements
of nature. But the making of a perfect eye is too big a task for nature.
She never makes a perfect eye. There is always some defect, large or
small. To take plastic material and make lenses and shutters and
curtains is a great task. The curvature of the front of the eye and of
the front and back of the crystalline lens is never quite perfect, but
in the majority of cases it is nearly enough perfect to give us good
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 horizon