. A text-book of physiology : for medical students and physicians . aneye looks at a distant ob-ject a clear image may beobtained only by using theciliary muscle, and to pre-vent this constant strainupon the muscle of accom-modation convex glassesmust be worn. Glasses ofthis kind converge therays and if properly chosenwill bring parallel rays toa focus without the con-stant aid of accommoda-tion. It is obvious thatin the hypermetropic eyethere is no far point ofdistinct vision when theeye is at rest, since someaccommodation must beused to bring even parallel rays to a focus. The near point ofd
. A text-book of physiology : for medical students and physicians . aneye looks at a distant ob-ject a clear image may beobtained only by using theciliary muscle, and to pre-vent this constant strainupon the muscle of accom-modation convex glassesmust be worn. Glasses ofthis kind converge therays and if properly chosenwill bring parallel rays toa focus without the con-stant aid of accommoda-tion. It is obvious thatin the hypermetropic eyethere is no far point ofdistinct vision when theeye is at rest, since someaccommodation must beused to bring even parallel rays to a focus. The near point ofdistinct vision will be farther away than in the normal eye, sinceaccommodation begins when the rays are parallel and its limitsare reached with a less degree of divergence; hence the name offar-sightedness. Presbyopia or old-sightedness has been referred to above. Itis due to a gradual failure in the effectiveness of accommodationwith increasing age, and is attributed usually to a progressive in-crease of rigidity in the lens. The near point of distinct vision21. Fig. 132.—Diagram showing the difference be-tween normal (A), myopic (B), and hypermetropic(C) eyes. In B and C the dotted lines represent thepath of the rays after correction by glasses.—{Bow-ditch.) 322 THE SPECIAL SENSES. recedes farther and farther from the eye, and consequently in closework convex glasses must be worn to aid the accommodation. Itis obvious that this effect of old age will be less noticeable in themyopic than in the emmetropic eye, since in the former the greaterlength of the eyeball requires less accommodation in near vision andthe failure of the lens to refract is therefore not felt so soon Whatis known as second-sight in the old may be brought about bythe late development of a myopic condition,—that is, by a changein the length of the eyeball or by a swelling of the crystallinelens,—and in such a case convex glasses for near work may bedispensed with. Astigmatism.—In a perfectly normal
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