Refraction of the eye : its diagnosis and the correction of its errors with a chapter on keratoscopy / by A. Stanford Morton.

  • Morton, A. Stanford.
Date:
1882
Catalogue details

Licence: Public Domain Mark

Credit: Refraction of the eye : its diagnosis and the correction of its errors with a chapter on keratoscopy / by A. Stanford Morton. Source: Wellcome Collection.

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    c. Diverging rays (Fig-, i., Ma, Mb) meeting- such a lens unite at a point, K, which is behind the principal focus. Conversely, rays from a point further from the lens than its principal focus emerge as convergent rays. d. Converging rays (Fig. i., Ca, Cb) falling on a convex lens meet at a point H, nearer the lens than its principal focus. Conversely, rays from a point wilhin the principal focus are, on emerging from the lens, siill divergent, though less so than before meeting it. Fig. 2. c c e. Parallel rays (Fig. 2., Aa, B3) falling on a concave spherical lens acquire a a'zVergence (aC, ^C) as if proceed- ing from a point, F, on the same side of the lens as that from which the parallel rays proceed. The distance of this point from the lens gives the negative focal length of the lens. Conversely, rays with a convergence as if pro- ceeding to this point are rendered parallel. A cylindrical lens acts according to the same rules' as a spherical, but only in a direction at righi angles to its axis.