Astigmatism

(as-tig’ ’-mat-izm) [d priv.; arly/ja, a point, because rays of light from a point are never brought to a point by the refractive media of the eye]. That conditio of the eye in which rays of light do not converge to a point on the retina. It is usually due to inequality of curvature of the different meridians of the cornea (corneal A.), but may be caused by imperfections of the lens (lenticular A.), unequal contraction of the ciliary muscle, or may perhaps be due to retinal imperfection. It may be acquired or congenital, and may complicate hypermetropia or myopia, producing either simple hypermetropic A., in” which one principal meridian is emmetropic, the other hypermetropic;or compound hypermetropic A., in which both meridians are hypermetropic, but one more so than the other. Complicating myopia, we may in the same way have simple myopic or compound myopic A. In mixed A., one principal meridian is myopic, the other hypermetropic. Regular A. is when the two principal meridians are at right angles to each other. Irregular A., when different parts of a meridian have different refracting powers. 96