A manual of ophthalmic practice / by Charles Higgens.

  • Higgens, C. (Charles), 1846-1920.
Date:
1903
Catalogue details

Licence: Public Domain Mark

Credit: A manual of ophthalmic practice / by Charles Higgens. Source: Wellcome Collection.

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    normal, because it passes into a more highly refracting medium (see p. 2). At K it experiences a second refrac- tion, but it then deviates from the normal, for it passes into air which is less refractive than glass (see also p. 2). The light is thus refracted twice in the same direction, and the eye which receives the emergent ray KH, sees the object O at O'; that is objects seen through a prism appeay displaced towards its edge ov summit. Prisms are numbered by the degree of inclination of their surfaces ; we speak of a prism of so many degrees. Lenses.—We next come to the consideration of Fra.- 4, lenses. These are transparent media which from the curvature of their surfaces, have the power of refracting luminous rays which traverse them so that they either converge or diverge. According to their curvature, they are either spherical, cylindrical, elliptical or para- bolic. We have to consider only spherical and cylin- drical lenses. These are usually made of crown or flint glass, of which the latter is the more highly refractive ; spectacle lenses are frequently made of what is known among opticians as ‘‘ pebble” or rock crystal which is harder than glass and does not easily scratch.
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