Eyesight, and how to care for it / edited by George Black.

Date:
[1888]
Catalogue details

Licence: Public Domain Mark

Credit: Eyesight, and how to care for it / edited by George Black. Source: Wellcome Collection.

    88/168 (page 84)
    Astigmatic persons, in reading, require different degrees of focusing for the horizontal and the vertical lines of the letters, and the distress that results from this continued and irregular strain of the muscle of accommodation may be readily understood. They usually attribute their defective vision to short-sight, but are disappointed in their efforts to correct it by short-sighted glasses, or by any glasses to be found in the shops. The surfaces of these glasses are spherical, and if they correct one meridian of an astigmatic eye, they cannot correct the other, but may make it worse. What is needed, in simple astigmatism, is a glass that will correct one meridian without affecting the other; and in compound or mixed astigmatism, a glass with two curved surfaces, one adapted to each meridian. Such a glass is found in the section of a cylinder in- stead of the section of a sphere. Fig. XIX. represents a cylindrical glass, and it will be seen that rays of light passing through its axis, a b, meet with no curve, and are therefore not refracted; while in the direction Fig. XIX. at right angles to the axis, c d. Cylindrical Lens. jf. jg strongly convex. This glass is cut into the usual oval form, and is set in the spec- tacle-frame with its axis corresponding to the direc- tion of the correct curvature of the cornea. In this way we are able to place the proper convex or con-