. Public School Physiology and Temperance. deviation. Of those which come fromthe different angles, some are too near the outer border of thecornea and are reflected back, while others pass in, and aremore or less refracted, or brought nearer together. Thehumors of the eye also bring the rays nearer, but the lensis the chief instrument of refraction. By reference to Fig. 138 PHYSIOLOGY AND TEMPERANCE. 43, the direction of the rays from the several points on theirway to the retina will be clearly seen. There is a delicate little muscle in the interior of the eye-ball, attached to the membrane,
. Public School Physiology and Temperance. deviation. Of those which come fromthe different angles, some are too near the outer border of thecornea and are reflected back, while others pass in, and aremore or less refracted, or brought nearer together. Thehumors of the eye also bring the rays nearer, but the lensis the chief instrument of refraction. By reference to Fig. 138 PHYSIOLOGY AND TEMPERANCE. 43, the direction of the rays from the several points on theirway to the retina will be clearly seen. There is a delicate little muscle in the interior of the eye-ball, attached to the membrane, which encloses and holds thelens in place. Its use is important. It finds the focus forvarying distances, just as a boy pulls his spy-glass in or outto get a correct focus according to the distance. Our eyesare arranged naturally for seeing distant objects. When welook at near objects, this little muscle is brought into draws upon the membrane and slackens it, allowing thelens to become more rounded. The lens is rapidly adjusted. Fia. 43.—Diagram showing how the image of an object is formed uponthe retina. in this way for varying distances, and brings the rays of lightto a focus exactly on the retina. If the lens be too flat, therays will not come to a focus before reaching the retina. Ifit be too rounded, the rays come to a focus in front of theretina. In either case the eyesight is defective. In the onecase the person will be far-sighted, and in the other near-sighted. The far-sighted person finds relief in convex glasses,the near-sighted person in concave glasses. It may not be the fault of the lens that the rays are notbrought to a focus on the retina. The eye-ball itself may THE SPECIAL SENSES. 139 be too shallow or too deep, that is, the retina may be toonear or too far away from the lens. 16. Hygiene of the Eye-—After middle life, and as ageadvances, most persons find their eyesight defective. This iscommonly due to a loss of power in the little muscle, and
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