In the following discussion, based upon the experience of a scientist who has specialized in the field of ophthalmology, are presented facts of the greatest practical importance, not only to all those who desire to have perfect sight, but to those whose safety depends upon the sight of others. Revolutionary as these statements are, they are supported by such wealth of evidence that they cannot well be questioned.
IT is generally believed that the normal eye has perfect sight all the time.
It has been compared to a perfect machine which is always in good working order. No matter what the object regarded may be, whether new, strange or familiar, whether the light is good or imperfect, whether the surroundings are pleasant or disagreeable, orthodoxy teaches that the normal eye is always normal and its sight always perfect. Even under conditions of nerve strain and bodily disease, the normal eye is expected to have perfect sight always.
This idea is very far from the truth. A careful study of the refraction of the eye extending over a period of many years has proven that no eye has perfect sight continuously. It is unusual, in fact, to find persons who can maintain perfect sight longer than a few minutes, even under the most favorable conditions. Of twenty thousand school children more than half had normal sight, but not one had perfect sight in each eye every day. The sight of many of them might be good in the morning and imperfect in the afternoon, while many with imperfect sight in the morning would have perfect sight in the afternoon.
Many children could read some letters of the alphabet perfectly, but were unable to distinguish others of the same size under similar conditions.
The degree of the imperfect sight varied within wide limits from one-third of the normal to one-tenth or less; duration was also variable. Under some conditions the imperfect sight might continue for only a few minutes or less. Under other conditions, however, a small number of students (sometimes all with normal eyes) would have sufficient loss of sight to prevent them from seeing writing on the blackboard for days, weeks, or even longer.
The sight of adults of all ages varies in a similar manner. Persons over seventy years of age with normal eyes have had attacks of loss of sight variable in degree and duration. A man aged eighty with normal eyes had periods of imperfect sight which would last from a few minutes to several hours or longer.
Both adults and children with normal eyes may have attacks of color blindness, and all persons, when their sight for form is lowered, are less able to distinguish colors than at other times. One patient, with normal eyes, perfect sight and perfect color perception in the daytime, had always been color-blind at night; he had no perception of colors after sunset.
There can be no doubt that accidents on railroads, at sea and on the streets often occur because the normal eyes of the responsible persons, for a time, had imperfect sight.
Unfamiliar objects almost always produce eye strain and are imperfectly seen. School children with normal eyes who can read small letters one-quarter of an inch high, at a distance of ten feet, always have trouble in reading strange writing on the blackboard, although the letters may be as much as two inches high. Strange maps always produce imperfect sight in the normal eyes of school children because they cause a strain to see. Temporary myopia, or myopic astigmatism, is always produced under these conditions, and if the strain is frequently repeated it may become continuous.
The strain may be conscious or unconscious, and may or may not produce pain, discomfort or fatigue.
Unfamiliar objects seen at the near-points are also a cause of eye strain. For this reason school children or adults learning to read, write, draw, or sew, suffer from defective vision, although they have normal eyes. In such cases temporary hypermetropia, or hypermetropic astigmatism, is produced, and with frequent repetition of the strain it becomes permanent.
This matter is of such great practical importance in the education of children that the attention of teachers should be called to the facts.1
Many children lose interest in their school work and become truants and incorrigibles from this one cause.
Light has a very important effect on vision of the normal eye; an unexpected strong light always produces defective vision. The vision of all persons is imperfect when the eyes are first exposed to the strong light of the sun, or to any strong artificial light. Rapid or sudden changes in the intensity of the light always produce defective vision, not sufficiently great to be manifest to the individual, but always to be demonstrated by careful test of the vision and by use of the retinoscope. The defective vision produced by strong light may be temporary, but it has been observed to continue in many cases for a number of weeks, frequently running into months, although it is never, probably, a permanent disability. If the eyes are gradually accustomed to strong lights, however, they will be benefited, and one may even become able to look directly at the strong light of the sun without any loss of vision whatever.
Noise is a frequent cause of defective vision in the normal eye. All persons see imperfectly when they hear any unexpected loud sound. Familiar noises do not usually lower the visual power, but unfamiliar, new or strange noises, which cause shock, always do, with the production of a temporary error of refraction. Country children from quiet schools, after they move to a noisy city, often suffer from defective vision for long periods of time. In the classroom they rarely do well with their work because of impaired sight. It is a gross injustice for teachers and others to criticize, scold or humiliate such children.
The reading of small distant familiar letters, for a few minutes at least every day, is very successful in preventing these fluctuations of sight, as it tends to prevent strain in looking at unfamiliar objects. Not only the Snellen eye chart, but a calendar, a sign with small letters, or even a single small letter, may be used for such practice. The good results of this simple system of eye training justify its use in schools, in the Army and Navy, in the Merchant Marine and on railroads, as well as by every one who desires or needs continuous perfect sight.
1. See New York Medical Journal, August 30, 1913, Myopia Prevention by Teachers. [link]
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