A nearsighted person is one who cannot see objects unless they are close to the eye.  The eyeball of a nearsighted person is very wide, and the retina is too far away from the crystalline lens.  Far objects are brought to a focus in front of the retina instead of on it, and hence are not visible.  Even though the muscles of accommodation do their best to pull out and flatten the lens, the rays are not separated sufficiently to focus as far back as the retina.  In consequence objects look blurred.  Nearsightedness can be remedied by wearing concave glasses, since they separate the light and move the focus farther away.  Concave glasses, by separating the rays and making the focus more distant, overbalance a wide eyeball with its tendency to focus objects in front of the retina.

[Illustration:  FIG. 81.—­The nearsighted eye.  The defect is remedied by concave glasses.]

118.  Headache and Eyes.  Ordinarily the muscles of accommodation adjust themselves easily and quickly; if, however, they do not, frequent and severe headaches occur as a result of too great muscular effort toward accommodation.  Among young people headaches are frequently caused by over-exertion of the crystalline muscles.  Glasses relieve the muscles of the extra adjustment, and hence are effective in eliminating this cause of headache.

An exact balance is required between glasses, crystalline lens, and muscular activity, and only those who have studied the subject carefully are competent to treat so sensitive and necessary a part of the body as the eye.  The least mistake in the curvature of the glasses, the least flaw in the type of glass (for example, the kind of glass used), means an improper focus, increased duty for the muscles, and gradual weakening of the entire eye, followed by headache and general physical discomfort.

119.  Eye Strain.  The extra work which is thrown upon the nervous system through seeing, reading, writing, and sewing with defective eyes is recognized by all physicians as an important cause of disease.  The tax made upon the nervous system by the defective eye lessens the supply of energy available for other bodily use, and the general health suffers.  The health is improved when proper glasses are prescribed.

Possibly the greatest danger of eye strain is among school children, who are not experienced enough to recognize defects in sight.  For this reason, many schools employ a physician who examines the pupils’ eyes at regular intervals.

The following general precautions are worth observing:—­

1.  Rest the eyes when they hurt, and as far as possible do close work, such as writing, reading, sewing, wood carving, etc., by daylight.

2.  Never read in a very bright or a very dim light.

3.  If the light is near, have it shaded.

4.  Do not rub the eyes with the fingers.

5.  If eyes are weak, bathe them in lukewarm water in which a pinch of borax has been dissolved.