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.

    44/168 (page 40)
    to the eye practically parallel. All rays in Nature, whose course has not been interfered with, are diver- gent, but beyond a distance of fifteen or twenty feet the divergence is so slight as to be inappreciable, and they are considered as parallel. When the object is brought nearer to the eye, the rays from it are then practically divergent, and require to be more strongly refracted to bring them to a focus. (Fig. XIII.) This Fig.XIII. —I. Focus of Parallel Rays. 2. Focus of Diver- gent Rays. 3. Focus of Divergent Rays brought forward by more Convex Lens. is a familiar fact in the use of any optical instrument, as an opera-glass; when directed from a distant to a near object, its focus must be adapted to the divergent rays by a mechanism provided for the purpose. It is easy to prove that the eye forms no exception to this rule. If we hold a veil between our eyes and