Farsightedness and Nearsightedness
A farsighted person is one who cannot see near objects so distinctly as far objects, and who in many cases cannot see near objects at all. The eyeball of a farsighted person is very short, and the retina is too close to the crystalline lens. Near objects are brought to a focus behind the retina instead of on it, and hence are not visible. Even though the muscles of accommodation do their best to bulge and thicken the lens, the rays of light are not bent sufficiently to focus sharply on the retina. In consequence objects look blurred. Farsightedness can be remedied by convex glasses, since they bend the light and bring it to a closer focus. Convex glasses, by bending the rays and bringing them to a nearer focus, overbalance a short eyeball with its tendency to focus objects behind the retina.
A nearsighted person is one who cannot see objects unless they are close to the eye. The eyeball of a nearsighted person is very wide, and the retina is too far away from the crystalline lens. Far objects are brought to a focus in front of the retina instead of on it, and hence are not visible. Even though the muscles of accommodation do their best to pull out and flatten the lens, the rays are not separated sufficiently to focus as far back as the retina. In consequence objects look blurred. Nearsightedness can be remedied by wearing concave glasses, since they separate the light and move the focus farther away. Concave glasses, by separating the rays and making the focus more distant, overbalance a wide eyeball with its tendency to focus objects in front of the retina.
FIG. - The farsighted eye.
FIG. - The defect is remedied by convex glasses.
FIG. - The nearsighted eye. The defect is remedied by concave glasses.