The Science of Human Nature - A Psychology for Beginners by William Henry Pyle
page 49 of 245 (20%)
page 49 of 245 (20%)
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vision. However, in about one third of school children the defect is
great enough to need to be corrected by glasses. The principle of the correction of sight by means of glasses is merely this:[1] When the focusing apparatus of the eye is not perfect, it can be made so by putting in front of the eye the proper kind of lens. There is nothing strange or mysterious about it. In some cases, the eye focuses the light before it reaches the retina. Such cases are known as nearsightedness and are corrected by having placed in front of the eyes concave lenses of the proper strength. These lenses diverge the rays and make them focus on the retina. In other cases, the eye is not able to focus the rays by the time they reach the retina. In these cases, the eyes need the help of convex lenses of the proper strength to make the focus fall exactly on the retina. [1] The teacher should explain these principles and illustrate by drawings. Consult a good text in physiology. Noyes' University of Missouri Extension Bulletin on eye and ear defects will be found most useful. Another defect of the eye, known as astigmatism, is due to the fact that the eye does not always have a perfectly spherical front (cornea). The curvature in one direction is different from that in others. For example, the vertical curvature may be more convex than the horizontal. Such a condition produces a serious defect of vision. It can be corrected by means of cylindrical lenses of the proper strength so placed before the eye as to correct the defect in curvature. Still another defect of vision is known as presbyopia or farsightedness due to old age. It has the following explanation: In early life, when we |
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