File:A text-book of physiology for medical students and physicians (1911) (14592183549).jpg

Identifier: textbookofphysio1911howe (find matches)
Title: A text-book of physiology for medical students and physicians
Year: 1911 (1910s)
Authors: Howell, William H. (William Henry), 1860-1945
Subjects: Physiology
Publisher: Philadelphia, London, W.B. Saunders company
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons

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ibuted to adefect in the curvature of the cornea. If the astigmatism is suchthat the vertical meridian has the greatest curvature it is termedwith the rule. since usuallv this meridian is slightly more DIOPTRICS OF THE EYE. 317 curved than the horizontal one. If, on the contrary, the cur-vature along the horizontal meridian is greater, the astigmatismis against the rule. The meridians of greatest and least curva-ture may not lie in the vertical and horizontal planes, but in someof the oblique planes; but so long as they are at right angles theastigmatism is regular. It is evident that such a condition maybe corrected by the use of cylindrical lenses, so chosen as to in-crease the refraction along the meridian in which the corneahas the least curvature, in which case a convex or plus cylinder isused, or, on the other hand, to diminish appropriately the refractionalong the meridian of greatest curvature, in which case a concaveor minus cylinder is used. An eye that suffers from a marked
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Fig. 133.—Schema to illustrate the paths of the rays of light in a cornea showingregular astigmatism.—(McKendrick.) The lower line of figures represents the section ofthe cone of light, or the images obtained at different distances. The image varies from ahorizontal to a vertical line, but at no place can a point be obtained at which rays alongall meridians are focused. degree of astigmatism cannot focus distinctly at the same timelines that are at right angles to each other; hence the use of aseries of lines whose images are formed along the different meridiansof the eye, as shown in Fig. 134, will reveal this defect if it exists.If the eye is directed to the center of intersection of the lines someof the lines appear distinct while those at right angles to themare blurred. A normal eye can be thrown into an astigmatic con-dition by approximating the eyelids closely. In this position thetears make a concave cylindrical lens, which alters the curvaturealong the vertical meridian.

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