Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 118

Astigmatism is a state of unequal refraction of light along the different meridians of the eye. Normally, a given refractive structure of the eye (e.g., the cornea or lens) has a constant curvature radius in all its meridians (though the cornea may flatten toward the limbus). Astigmatism occurs when the horizontal and vertical meridians of the cornea or lens have different curvature radii. Because of these differing curvatures, light entering the eye through the vertical meridian may be refracted more (i.e., direct, or with‐the‐rule, astigmatism) or less (i.e., indirect, or against‐the‐rule, astigmatism) than light entering through the horizontal meridian. Astigmatism is diagnosed by refracting the eye in both the horizontal and vertical meridians. A difference of 0.5 D or more in the refractive power of the horizontal and vertical meridians in the same eye is defined as astigmatism.