A text-book of physiology, for medical students and physicians . ht, 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. 133, will reveal this defect if it the eye is directed to the center of intersection of the lines someof the l
A text-book of physiology, for medical students and physicians . ht, 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. 133, will reveal this defect if it 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. What is known as irregular astig-matism is due to the fact that the meridians of greatest and leastcurvature are not at approximately right angles, or, as is more 316 THE SPECIAL commonly the case, it is due to an irregularity in the curvaturealong some one meridian, such as ma} be produced by a scar uponthe cornea. This condition may be produced from a variety ofcauses affecting either the cornea or the lens, and practically itcan not be corrected by the use of lenses. As Helmholtz has shown, a small degree of irregularastigmatism is present nor-mally, owing to a certainasymmetry in the curvatureof the lens. This defect ismade apparent in the visualsensations caused by a pointof light, such as is furnished,for instance, by a fixed retinal image in thesecases, instead of being a sym-metrical point, is a radiatefigure the exact form ofwhich may vary in differenteyes. For this reason thefixed stars give us the well-known star-shaped imageinstead of a clearly and Physiological Control of the Ciliary Muscleand the Muscles of the Iris.—From an optical point of view
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