When one looks at a distant object, a near one in the same line of vision is indistinct. Now, if one focuses upon the nearer object, the distant one is not seen so well. This is because a clear image is obtained only of the object which is brought to focus upon the most sensitive portion of the retina. The other become indistinct as it goes out of focus. This shifting of focus is accomplished by a change in the thickness of the lens. In a normal eye the rays of light from a distant object come to focus upon the retina when the eye is at rest, that is, when the ciliary’s muscles are inactive and the lens is thin.
Farsightedness of Children
In this condition, called “hyperopia,” the eyeball is shorter than normal. This is a universal condition at birth but should disappear before there is much use of the eyes for near vision. If it persists, the rays of light with the eye at rest come to focus not on the retina but behind it.
Blurring of vision results
This is unsatisfactory, so the ciliary’s muscles correct the difficulty by contracting and making the lens thicker, thereby bringing the point of focus forward until it is on the retina. This gives clear vision but requires excessive work on the part of these muscles. For short periods of time this gives rise to no difficulties but if it continues muscular fatigue is inevitable. This in turn causes headache, pain in the eyes, nervousness, and general fatigue. This type of visual defect is the one which causes the most severe symptoms of eyestrain.
Important though farsightedness is as a cause of eyestrain, it is rarely discovered by the ordinary vision test, because during the test clear vision is secured by excessive use of the ciliary’s muscles. Hence, if a child brings a report from school that his vision test shows 20/20 in each eye this should not be accepted as conclusive evidence that the eyes are normal. If symptoms of eyestrain are present, a further examination is indicated.
The Use of Drops for Eye Examinations
There has been much misunderstanding and misinformation concerning the use of “Drops” for the examination of the eyes. The drops contain a drug, such as atropine or homatropine, which temporarily paralyzes the ciliary’s muscles. Unless this is done, the activity of these muscles in persons under about forty years of age makes the accurate measurement of certain visual defects impossible. Everyone who needs glasses should have a thorough eye examination, and for a young person this implies the use of drops.