Ocular refraction and the shadow test . ndisturbed. Atright angles to the axis, it causes the parallel rays to diverge to thedegree necessary for the eye to bring them to a focus also upon theretina. Third:—In one of the principal meridians of the eye parallel raysof light are brought to a focus before the retina, in the other princi-pal meridian, behind the retina. To this condition of refraction the term compound astigmatism isgiven. In one of the principal meridians a hypermetropic cimduion exists while in the other a myopic condition obtains. The correctionfor compound astigmatism is there
Ocular refraction and the shadow test . ndisturbed. Atright angles to the axis, it causes the parallel rays to diverge to thedegree necessary for the eye to bring them to a focus also upon theretina. Third:—In one of the principal meridians of the eye parallel raysof light are brought to a focus before the retina, in the other princi-pal meridian, behind the retina. To this condition of refraction the term compound astigmatism isgiven. In one of the principal meridians a hypermetropic cimduion exists while in the other a myopic condition obtains. The correctionfor compound astigmatism is therefore a piano convex cylinder com-bined with a piano concave cylinder, their axes being at right anglesto each othei-,and located in the required positions as described in thecorrection of simple hypermetropic and simple myopic astigmatic will be remembered that contra-generic cross cylinders may betransposed into equivalent spherocylinder lenses, therefore, com-pound astigmatic corrections may be made by contra-generic cross. Figure 113. Utigmatism illustrated. The concave cylinder gives a divergence to tlieparallel rays in the horizontal while in the vertical meridian they pass convex spheric il lens then changes the parallel rays in the vertical meridian to con-vergent ray>, and reduces the divergency ot those in the horizontal meridian sufficiently forthe eye to focus them all upon the retin i. cylinders, axes at right angles; by a convex spherical, combined witha concave cylinder; or, by a concave spherical combined with a con-vex cylinder Figure 112 represents a sectional view of an eye In which a com-pound astigmatic condition exists. Parallel rays in the vertical mer-idian are brought to a focus behind the retina, showing that in thismeridian the eye does not possess sufficient refraction. In the hori-zontal meridian, the parallel rays focus before the retina, too muchrefraction existing through this meridian. Figure 113 illustra
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