Page 311 - My FlipBook

MANAGEMENT OP LIGHT AND CAEE OF THE EYES. 175
are soft and their forms are controlled by slight muscular effort.
As the person grows older the substance of the lens grows harder
aud a continuous increase of muscular effort is required for the
control of the near focus. The boy or girl of twelve years may
readily obtain perfect focus at six inches or even less. At
twenty-five years ten inches distance is required. At thirty-five,
fifteen inches and the maintenance of this requires more and
more muscular effort. At this latter age the dentist begins to
need help. This is a statement of something like averages for
normal eyes. But within the range of normality considerable
personal differences must be allowed. Some will need help
almost from the beginning of dental practice, while others will
be able to go longer without help. There are variations among
persons that are so wide as to become abnormal, producing far-
sightedness and near-sightedness, or hypermetropia and myopia.
In hypermetropia of considerable degree one is unable, or soon
becomes unable, to adjust the lens of the eye for near vision and
requires the artificial lens for its correction when young in order
to use near vision comfortably and effectively. In case of a very
considerable degree of myopia, the person may need glasses in
childhood to enable him to see clearly even at short distances.
With a mild degree of short-sightedness one may need glasses
for distant vision but may see well at short distances without
them. The slighter grades of these difficulties usually pass unno-
ticed until some trouble with the eyes calls for an examination.
Few persons are absolutely free from imperfections in the sym-
metry of the curvature of the cornea, known as astigmatism.
This distorts the vision. It is capable of correction by suitably
fitted glasses. This should be done wherever it is found to pre-
vent eye strain and its attending consequences, even though the
vision may be regarded as perfect.
The first aid necessary to the person with normal vision is
usually in the focusing of the eye. This is done by placing an
additional lens before the eye to do a portion of the focusing,
relieving the muscles. Only partial relief can be given so long
as the muscles focusing the lens retain their activity, for the
reason that the muscles bringing the eyeballs in position for
short vision are coordinate in their action with the muscles focus-
ing the lens. One can not converge the eyes for short vision with-
out setting the focusing muscles in action also. This adjustment
of the artificial lens must be such as to balance the difficulties
between these two sets of muscles. Therefore, for continued
relief as the person grows older, a continued increase in the