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.

    77/168 (page 73)
    curately correspond, the focus will not fall upon the retina, but in front of it or behind it, and this is what is meant by an error of refraction. This is easily illustrated by holding a convex lens, or com- mon magnifying-glass, in such a position in front of a piece of white paper that the image of some object, as a candle-flame or window-sash, will be formed upon the paper. When the outlines of the image are sharp and the picture is clear, the distance between the lens and the paper corresponds to the focal distance of the lens, just as in the emmetropic eye the distance between the cornea and the retina corresponds to the focal distance of the refracting surfaces of the eye. If the paper is moved ever so little, towards or away from the lens, the distance is no longer according to measure, and the image becomes indistinct; pre- cisely in the same way, the retinal image becomes in- distinct if, from an imperfection in the form of the eyeball, the retina is too near or too far from the cornea. In hyperme- tropia {hyper- meiros, beyond the measure), the axis of the eyeball is too short, and the focus falls beyond the retina, Fig. XVI.— Hypermetropia.