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 Accommodation.—In order to focus the light upon the retina, the lens must change shape for every change in the distance of the object looked at (see Fig. 125). The shape of the lens can be readily changed, for it is elastic and has muscular fibers around its edges (Exp. 17).

—Change of lens in accommodation. (Jegi.)

Defects in the Eye.—Some eyeballs are too long, and the lens brings the rays to a focus before they reach the retina. Such eyes are near-*sighted (Fig. 126) and require glasses that round inward (concave). Some eyeballs are too flat, and the rays are not brought to a focus soon enough. Such eyes are farsighted and require glasses that round outward (convex). See Fig. 127. (Repeat Exp. 15.)

(ball too long), which only focuses rays for near objects (2) when concave glasses are used (3).

(ball too short) which needs convex lens to focus rays upon retina.

Care of the Eyes.—Because the eyes can do a large amount of work without giving pain, they are often abused. When reading or doing intricate work, turn the eyes from the work occasionally and look at some distant object; stop work before the eyes are tired. Twilight of early evening has ruined many good eyes. You should always stop work before the twilight begins, for the