The watchman and southron. [volume] (Sumter, S.C.) 1881-1930, 1917-10-24

brain to the back of the eye. This
nerve is the medium of accurate in
formation as to the image formed on
the former 1t is consequently the
brain that sees, and the eye being
merely it's instrument of vision
The normal eye may, consequently
be defined as an optical apparatus of
such form that parallel rays of light
that is, rays proceeding from n dis
i tance of twenty feet or niore-are pre-
clsely focused on the retina without
any effort on the part of the eye, thus
imprinting on this sensitive MEM
brane sharply defined image of all
objects from which these rays ema-
nate. Very few eyes are so perfect
as to fit the above definition, and in
many eyes seemingly perfect 1t is due
to the muscles of the eye that cor
rect images are formed on the retina.
When the impcrfection is slight, the
extra muscular effort necessary may
be borne without discomfort; but
when the rays focus too far back Or
too far in front of the retina, the
rectification, and imperfect vision
with painful eyestrain, is the result.
Could we move the retina backward
or forward, we could easily remedy
such imperfections; but that can not
be done. and hence it is incumbent on
us to change by mechanical means
the direction of the rays Of light
which enter the eye. This the opto-
mertist can accomplish with ground
lenses of various kinds.
Defective Eyes.
Ther are three ways in which i'm
perfect eyes may deviate from nor
NAL, producing the three more preva-
lent defects of the eye: Mypoia, or
near sight, is a condition in which
the eyeball is relatively elongated
from before backward, in which
case the rays of light, instead of be-
ing focused upon the retina come to
focus before they reach this mem
brane. After coming to focus, the
rays cross and form a diffused cir
Gle on the retina; and thus a more or
less indistinct image is conveyed to
the brain. To enable nearsighted
person to read the book must be held
much closer to the face than it should
be; and minute objects at a distance
OF few feet. which are easily seen
by persons with normal eyes. are in
visible to persons who are nearsight-
ed.
When clear vision Of small print
cannot be obtained without holding
the book nearer than ten inches, spec
tacles are always required and fail
ure to use them will certainly result
in great injury to the sight. For this
defect the optometrist prescribes con-
cave glasses Of the proper strength to
throw the image of an object far
enough back to make it focus on the
retina. ShOrtsighted people generally
should wear spectacles all the time
they are awake--reading or walking,
at work or at play. This defect of
vision is generally hereditary, but is
often produced by reading in an i'm
perfect light when young.
Before the invention of bifocals
persons who were nearsighted had to
wear two pairs of spectacles-one
pair for seeing at short distances, and
the other for long distances The BIZ
fOcale spectaclos each one has two
lenses, of different focusing power,
combined in one. Today one-piece BIZ
calls can be procured with no visible
dividing lines between the segments.
Hypermetropa. or farsight is the
condition in which the cyeball is rel
atively too short, in which case the
rays of light, instead Of being focus
ed on the retina, do not come to a
focus until they get behind this men,
brane. Persons who are thus affict-
ed to a high degree cannot see OBJ
iects plainly, either close or distance,
without the aid Of convex lenses. Clear
images may be obtained for q time by
extra muscular effort, Lut such extra
effort always results in a sense of ia
tigue and blurred vision. Not infre-
quently it causes headaches, pain in
the eyes, nervous arrangement, or
! other physical ailment. The optome-
trist Is the only physician in such
cases, as glasses. not medicine, are
the radical cure for both the defec-
tive vision and it's physical and men,
tai manifestations.
Astigmatism is the condition of the