The Newberry herald and news. (Newberry, S.C.) 1884-1903, 1902-05-16

knowledge of the laws of refraction
OF light. 1t is we'll known fact in
optics that a ray of light penetrat-
ing medium of lesser or greater
density than the one from which it
energies changes it's directions, or is
refraotod, A number of parallel rays
passing through medium of differ
ent density are refracted, but emerge
still parallel. But if the surfaces of
the medium are curved the rays
emerge divergent or con yergent, just
as the prism is conoave or convex.
Hence, rays of light passing through
the crystalline lens of the eve are
brought to a focus on the sensitive
retina behind. Here the image of
the external object is made in an in.
verted position
Thus you observe we do not see
objects, but their images; and we
leara from experience that the OBJ
ject is not upside down, but only the
image.
The more convex the lens the
shorter will be it's focusing power.
The clliary muscles have the prop
erty of rendering the lens more con-
vex or less convex according as we
are looking at near or distant objects.
This is called power of accommo-
dation. The abuse of this faculty
is one of the most fruitful sources of
evil in school life.
In the above remarks we have
been speaking of the perfectly
formed, or emmetropic, eye-one
that is able to focus parallel rays of
light accurately y upon the retina.
There are great many optical
defects that arise durings chool life
which should be readily recognized
by the teacher, and recommended
for correction.
The most common Of these are
long sight, or hypermetropia; short.
sight, or myopia, and astigmatism.
Long sight is produced by the axis
of the eyeball, the distance from the
cornea to the retina, being less than
the focal distance of the eye, and
hence the image falls behind the
retlna.
This condition is known as "f1at
eye" The same results are seen in
old age, only the cause is due to all
increased density and convexity Of
the lens.
This condition can be relieved by
a convex glass, which will bring the
rays of light more quickly to focus.
Just the opposite condition is met
with in near sight. Here the oxis is
too long and the image is focused in
front Of the retina. Of course con-
cave lens will correct such malforma-
ton. This is one of the most seri-
Gus conditions in errors of refraction.
If allowed to go on unaided most
appalling results may happen and
the child's vision become wreck.
1t is the most frequent defect met
with in the young, and as the eyes
are soft and elastic, the distortion
grows worse and worse as work is
imposed upon them. The increase
in such cases is very rapid from
the elementary to the high schools.
In some countries, as Germany, it is
more often met with than in others,
and more often seen in cities than in
towns or rural districts.
In the remarks upon long sight
and short, sight it was assumed that
the eye remained stationary. But
this is not true in nature, as the
ciliary muscles are constantly at
work rendering the crystalline lens
more or less convex as the case may
need or the distance Of the object
looked at may require. This brings
about the strain upon defective eyes,
and Il train Of symptoms such as
headache, pains in the eyes and gen
eral fatigue follow.
There remains yet one other con-
dition common among those who
have defective vision. This is known
as astigmatism. The normal shape
Of the cornea is that of section of q
sphere, and any variation from this
uniformity in curvature brings about
astigmatism, or different focal dis.