Physiology and Hygiene for Secondary Schools

by Francis M. Walters

Defects in Focusing.—The delicacy and complexity of the sense organs of sight render them liable to a number of imperfections, or defects, the most frequent and important being those of focusing. Such defects not only result[pg 384] in the imperfect vision of objects, but they throw an extra strain upon the nervous system and may render the process of seeing exceedingly painful.

A normal eye is able, when relaxed, to focus light accurately from objects which are twenty feet or more away and to accommodate itself to objects as near as five inches. An eye is said to be myopic, or short-sighted, when it is unable to focus light waves from distant objects, but can only distinguish the objects which are near at hand. In such an eye the ball is too long for the converging power of the lenses, and the image is formed in front of the retina (C, Fig. 164).

Fig. 164—Diagrams illustrating long-sightedness and short-sightedness, and method of remedying these defects by lenses. A. Normal eye. B. Long-sighted eye. C. Short-sighted eye.

A long-sighted, or hypermetropic, eye is one which can focus light from distant objects, but not from near objects. In such an eye the ball is too short for the converging power of the lenses and the image tends to form back of the retina (B, Fig. 164). These defects in focusing are remedied by wearing glasses with lenses so shaped as to counteract them. Short-sightedness is corrected by concave lenses and long-sightedness by convex lenses, as shown in diagrams above.

Astigmatism is another defect in the focusing power of the eye. In astigmatism the parts of the eye fail to form the image in the same plane, so that all portions of the object do not appear equally distinct. Certain parts of it are indistinct, or blurred. The cause is found in some[pg 385] difference in curvature of the surfaces of the cornea or crystalline lens. It is corrected by lenses so ground as to correct the particular defects present in a given eye.

Whenever defects in focusing are present, particularly in astigmatism, extra work is thrown on the ciliary muscle as well as the muscles that move the eyeballs. The result is frequently to induce a condition, known as muscle weakness, which renders it difficult to use the eyes. Even after the defect in focusing has been remedied, the muscles recover slowly and must be used with care. For this reason glasses should be fitted by a competent oculist[126] as soon as a defect is known to exist. When one is unduly nervous, or suffers from headache, the eyes should be examined for defects in focusing (page 326).

Eye Strain and Disease.—The extra work thrown upon the nervous system through seeing with defective eyes, especially in reading and other close work, is now recognized as an important cause of disease. Through the tax made upon the nervous system by the eyes, there may be left an insufficient amount of nervous energy for the proper running of the vital processes. As a result there is a decline of the health. Ample proof that eye strain interferes with the vital processes and causes ill health, is found in the improvements that result when, by means of glasses, this is relieved.

The Eyes of School Children.—School children often suffer from defects of vision which render close work burdensome, and cause headache, general nervousness, and disease. Furthermore, the visual defects may be unknown both to themselves and to their parents. Pupils showing indications of eye-strain should be examined by an oculist,[pg 386] and fitted with glasses should defects be discovered.[127] The precaution, adopted by many schools, of having the eyes of all children examined by a competent physician employed for the purpose, is most excellent and worthy of imitation.

Reading Glasses.—Many people whose eyes are weak, because slightly defective, find great relief in the use of special glasses for reading and other close work. By using such glasses they may postpone the time when they are compelled to wear glasses constantly. It is in the close work that the extra strain comes upon the eyes, and if this is relieved, one can much better withstand the work of distant vision. The reading glasses should be fitted by a competent oculist, and used only for the purpose for which they are intended.