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.

87/168 (page 83)

the same time, or with the same accommodative effort at a given distance. One meridian may be correct and the other myopic or hypermetropic, which is simple astigmatism; they may have different degrees of myopia or hypermetropia—compound as- tigmatism ; or one meridian may be myopic and the other hypermetropic — iftixed astigmatism. The statement that the natural cornea is a section of a perfect sphere requires some modification, as, in point of fact, such a cornea is never met with. More or less astigmatism always exists; but, in the typical eye, the degree is so slight that it gives no inconven- ience, and it can be detected only by delicate tests. If Figure XVIII. (one of Dr. John Green's test dia- grams) be brought gradually towards the eye, it will be found that one set of lines remains distinct after those at right angles to it become blurred. Usually, the hori- zontal lines can be seen closer to the eye than the vertical; at the ordinary reading distance, however, they should all appear alike. Fig. XVIII. — Test for Astigmatism.