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.

    86/168 (page 82)
    As long-sight and short-sight depend upon the length of the axis of the eyeball, and not upon the curve of the cornea, it is hardly necessary to denounce the absurd pretensions of quacks, who have professed to cure them by increasing the convexity of the cor- nea with a suction apparatus, or diminishing it by pressure. Astigmatism is an optical condition of the eye in which, from a want of uniformity in the curvature of the cornea, different meridians have a different focus. The perfectly formed cornea is part of a sphere or globe, and has the same degree of convexity in all directions, and therefore the same focal distance for all meridians. The astigmatic cornea is not symmet- rical, but has different curvatures in directions at right angles to each other. Rays of light passing through the greatest curvature have the shortest focal distance, and those passing through the least curvature have the longest focal distance; and no sharply defined image can be formed by such a cornea. As a rough illus- tration, draw a line around an egg through the ends, and another around the middle, at right angles to the first; it will be evident that these lines have a very different curvature, and that if the egg were made of glass it could not form a sharp focus, as a glass sphere does — it would be astigmatic. Thus an astigmatic eye may be able to focus sharply either perpendicular or horizontal lines, but it cannot focus them both at i