In this case
the eye is made farsighted or hyperopic in one meridian, and is
normal in the other. Or again, the rays may be focused in front of the
retina in one meridian, and directly on the retina in the other; this
would be an example of nearsighted or myopic astigmatism.
Farsightedness and nearsightedness are then both caused by
astigmatism, although in this case not by the length of the eyeball,
but by inequality in the curvature of the front part (cornea) of the
eyeball. For example, in simple astigmatism one of the principal
meridians is hyperopic (turning the rays so that they focus behind the
retina) or myopic (bending the rays so that they focus in front of the
retina), while the other meridian is normal. In mixed astigmatism, one
of the principal meridians is myopic, the other hyperopic; in compound
astigmatism the principal meridians are both myopic, or both
hyperopic, but differ in degree; while in irregular astigmatism, rays
of light passing through different parts of the outer surface of the
eyeball are turned in so many various directions that they can never
be brought to a perfect focus by glasses.

It is not by any means possible for a layman to be able always to
inform himself that he is astigmatic, unless the defect is
considerable. If a card, on which are heavy black lines of equal size
and radiating from a common center like the spokes of a wheel, be
placed on a wall in good light, it will appear to the astigmatic eye
as if certain lines (which are in the faulty meridian of the eyeball)
are much blurred, while the lines at right angles to these are clear
and distinct. Each eye should be tested separately, the other being
closed. The chart should be viewed from a distance as great as any
part of it can be seen distinctly. All the lines on the test card
should look equally black and clear to the normal eye.

Astigmatism is corrected by a cylindrical lens, which is in fact a
segment of a solid cylinder of glass. The axis of the cylindrical lens
should be at right angles to the defective meridian of the eye, in
order to correct the astigmatism. Eye-strain is caused by astigmatism
in the same manner that it is brought about in the simple farsighted
eye, i. e., by constant strain on the ciliary muscle, which regulates
the convexity of the crystalline lens. For it is possible for the
inequalities of the front surface of the eyeball or of the lens to be
offset or counterbalanced by change in the convexi