farsightedness


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farsightedness

or

hyperopia,

condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eyeeye,
organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other
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 rather than upon it, either because the eyeball is too short or because the refractive power of the lens is too weak. Presbyopia, a similarly faulty vision, is attributable to physiological changes in the lens brought on by age. Corrective eyeglasseseyeglasses
or spectacles,
instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses mounted in a frame to hold them in position before the eyes.
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 with convex lenses compensate for the refractive errors.

Farsightedness

 

(hyperopia), a deviation from normal refraction in the eye wherein parallel light rays, after refraction in the eye, are brought to a focus behind the retina and the retinal images as a result are diffuse and unclear.

Farsightedness is caused either by the fact that the refractive mediums of the eye (the cornea and the crystalline lens) refract light poorly (index hyperopia) or because the antero-posterior axis of the eye is too short (axial hyperopia); most often the two causes are found in combination. Farsightedness is found in the majority of newborns, but as the infant grows, the eyeball enlarges somewhat and the farsightedness usually disappears. The hyperopic eye, which is poorly suited for bringing parallel rays together at the retina, is even less capable of focusing divergent rays (that is, those from close objects). Thus, vision is poor both at distances and from up close, so that essentially the term “farsightedness” is not altogether accurate. With slight degrees of farsightedness, young people continually strain the so-called ciliary muscle, increasing the curvature of the crystalline lens; this increases the refractive capacity of the lens (accommodation), and clear vision may be attained. In the middle-aged and elderly this capacity for accommodation is weakened; so-called old-age farsightedness (presbyopia) develops, manifested in the efforts of the person to hold observed objects (for example, a book or newspaper) somewhat further from the eyes. Farsightedness may cause headaches; in children it may lead to the development of convergent strabismus. Correction of vision (compensation) for farsightedness is achieved by wearing eyeglasses with convex lenses; the strongest lens is chosen that will ensure the clearest vision. By increasing the refractive power of the eye, such a lens transfers the focus of the light rays to the retina.

S. I. TAL’KOVSKII

farsightedness

[′fär¦sīd·əd·nəs]
(medicine)
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