Identifier: textbookofphysio1907howe (find matches)
Title: A text-book of physiology, for medical students and physicians
Year: 1907 (1900s)
Authors: Howell, William H. (William Henry), 1860-1945
Subjects: Physiology Physiology
Publisher: Philadelphia, London, W.B. Saunders Co.
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons

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asses 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 visionrecedes 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 ae:e will be less noticeable in the
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Fig. 131.—Diagram showing the difference be-tween normal (/I), myopic (£), and hypermetropic(C) eyes. In B and C the dotted lines represent thepath of the rays after correction by glasses.—(Bow-ditch.) 314 THE SPECIAL SENSES. myopic 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 or ideal eye the refractivesurfaces, cornea, anterior and posterior surfaces of the lens, aresections of true spheres, and, all the meridians being of equalcurvature, the refraction along these different meridians is equal.Such an eye will bring the cone of ray

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