concave lenses (which curve inwards)
or shortsightedness (see
glasses are used
box, right). Most lenses
are single-vision, but bifocal, trifocal,
and varifocal lenses are becoming pop-
ular. Tinted lenses protect the eyes from
sunlight. (See also
contact lenses
. )
glass eye
eye, artificial
glass test
A test for
that involves press-
ing a clear glass against a rash. If the
rash remains visible through the glass, it
may be a form of
(leaking of
blood beneath the skin), w hich some-
times occurs in meningitis.
A condition in which the
intraocular pres-
(fluid pressure
inside the
causes impaired vision.
Aqueous humour
(watery fluid) is secreted into the front
of the eye by the
ciliary body
(a structure
behind the
to maintain the eye’s
shape and nourish the tissues. Excess
fluid drains away at the edge of the iris.
In glaucoma, however, this excess fluid
may not be able to escape. It causes com-
pression and obstruction of the blood
vessels that supply the
and the
This, in turn, may destroy nerve
fibres and cause gradual loss of vision.
The most common form of glaucoma is
chronic simple (open-angle) glaucoma.
It rarely occurs before the age of 40,
and tends to run in families. In this
hum our is gradually blocked over a
period of years, causing a slow rise in
pressure. There are often no symptoms
until visual loss is advanced.
In acute (closed-angle) glaucoma, the
outflow of aqueous humour is rapidly
blocked, and the pressure rises sudden-
ly. This may cause a severe, dull pain in
and above the eye, fogginess of vision,
and the perception of haloes around
lights at night. Nausea and vomiting
may occur, and the eye may become red
with a dilated pupil.
Congenital glaucoma is due to an ab-
normality in the drainage angles of the
eyes that develops before birth. Glaucoma
can also be caused by injury or an eye
disease such as
lens dislocation
A less common form, called normal-
pressure or low-tension glaucoma, occurs
in people whose intraocular pressure is
normal or only slightly raised but causes
Glasses compensate for certain visual defects in which the lens ofthe eye does not
focus light correctly on to the retina, at the back ofthe eyeball. For
h y p e r m e t r o p i a
(longsightedness) or
p r e s b y o p i a ,
convex (or plus) lenses are needed.
y o p i a
(shortsightedness) requires concave (or minus) lenses.
Before correction
Longsightedness occurs
when focusing power is
inadequate. Light from
distant objects is focused
on to the retina, butlight
from close objects is
focused behind it.
After correction
Convex magnifying (or plus)
lenses cause light raysto
converge (bend together). As
a result, theyfocusthe light
from close objects correctly
on to the retina.
Before correction
The focusing power of the eye
is too great. Light from close
objects is focused correctly;
however, light from distant
objects is focused in front of
the retina, and the objects
appear blurred.
After correction
Concave weakening (or
minus) lenses cause light
rays to diverge (bend apart).
As a result, they focusthe
light from distant objects
correctly on to the retina.
In astigmatism, the surfaces of
the cornea, rather than being a
hemisphere as normal, are
steeper in one direction than in
the others; as a result, the light
rays in one meridian (plane) are
out of focus. Lenses to correct
astigmatism are designed with
additional curvature in one
meridian, then set accurately in
the frame ofthe glasses so that
the steepest curves correspond
to the flattest meridian ofthe
cornea. As a result, they cancel
out the effects ofthe distortion
in the cornea. Both concave and
convex lenses can be designed
to correct astigmatism.
C o n v e x le n s -
C o n v e r g in g l i g h t
r a y s w it h in l e n s
D iv e r g in g li g h t
r a y s w it h in l e n s
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