The home medical adviser, a popular work on the treatment of disease . rvature of theeyeball in the astigmatic eye is greater in one meridian than in theopposite; in other words, the front of the eyeball is not regularlyspherical, but bulges along a certain line or meridian, while thecurvature is flattened or normal in the other meridian. Eor instance,if two imaginary lines were drawn, one vertically and the other hori-zontally across the front of the eyeball, intersecting in the center ofthe pupil, they would represent the principal meridians—the verticaland the horizontal. As a rule the meri
The home medical adviser, a popular work on the treatment of disease . rvature of theeyeball in the astigmatic eye is greater in one meridian than in theopposite; in other words, the front of the eyeball is not regularlyspherical, but bulges along a certain line or meridian, while thecurvature is flattened or normal in the other meridian. Eor instance,if two imaginary lines were drawn, one vertically and the other hori-zontally across the front of the eyeball, intersecting in the center ofthe pupil, they would represent the principal meridians—the verticaland the horizontal. As a rule the meridian of greatest curvature isapproximately vertical, and that of least curvature is at right anglesto it, or horizontal. Rays of light in passing through the different meridians of theastigmatic eye are differently bent, so that in one of the principalmeridians rays may focus perfectly on the retina, while in the otherthe rays may focus on a point behind the retinal field. In thiscase the eye is made farsighted or hyperopic in one meridian, and 698 THE EYE AND EAR. Fig. 71.—Simple Hyperopic Rays h. h. Pass through the Vertical orHyperopic Meridian and are Focused Behindthe Retina at h. is normal in the other. Or again, the rays may be focused in frontof the retina in one meridian, and directly on the retina in the other;this would be an example of nearsighted or myopic astigmatism. Farsightedness and nearsightedness are then both caused by astig-matism, although in this case not by the length of the eyeball, but byinequality in the curvature of the front part (cornea) of the eyeball. For example, in simple astig-matism one of the principalmeridians is hyperopic (turn-ing the rays so that theyfocus behind the retina) ormyopic (bending the rays sothat they focus in front ofthe retina, while the othermeridian is normal). In mixed astigmatism, one of the principalmeridians is myopic, the other hyperopic; in compound astigmatismthe principal meridians are bo
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