English: The optical explanation of the defect of astigmatism will be understood with the aid of this figure. Thus, suppose the vertical meridian C A D to be more strongly curved than the horizontal F A E, the rays which fall on C A D will be brought to a focus G, and those falling on F A E at B. If we divide the pencil of rays at successive points, G, H, I, K, B, by a section perpendicular to A B, the various forms it would present at these points are seen in the figures underneath, so that if the eye were placed at G, it would see a horizontal line a a′; if at H, an ellipse with the long axis a a′ parallel to A B; if at I, a circle; if at K, an ellipse, with the long axis, b c, at right angles to A B; and if at B, a vertical line b c. The degree of astigmatism is ascertained by measuring the difference of refraction in the two chief meridians; and the defect is corrected by the use of cylindrical glasses, the curvature of which, added to that of the minimum meridian, makes its focal length equal to that of the maximum meridian.