A regularly-shaped piece of glass or crystal for the refraction of rays of light. The crystalline lens of the eye. L., Achromatic. See Achromatic. L., Apochromatic. See Apo- chromatic. L., Astigmatic. See L., Cylindric. L., Astigmatic, of Stokes, an apparatus consisting of two plano-cylindric lenses, one concave, the other convex, the two of equal focal distance; it is used in the diagnosis of astigmatism. L., Biconcave L., Biconvex L., Bifocal. See Bifocal. L., Cicatricular, a lenticular body in the ovum formed by the junction of the germinal vesicle and the non-germinal layer of the ovum. L., Convergent or Converging, a double convex or planoconvex lens that focuses rays of light. L., Crystalline, the lens of the eye, situated immediately behind the pupil. L., Cylindric e., one with a plane surface in one axis and a concave or convex surface in the axis at right angles to the first. L., Decentered, one in which the optic center is not opposite the pupil of ‘the eye. L., Dispersing, a name for a concave lens. L.-paper, a soft absorbent paper much used for cleaning lenses; it is also known as Japanese filter-paper. L., Periscopic, one with concavo-convex or convexo- concave surfaces, the opposite sides being of different curvatures; it is used to avoid spheric aberration, and to gain a greater field of clear vision; such lenses are called meniscus lenses. L., Plano-concave, Plano-1. Biconvex lens. 2. Plano-convex. 3. Concavo-convex, or convergent meniscus. 4. Biconcave 5. Plano-concave. 6. Convexo-concave, or divergent meniscus. convex, Plano-cylindric, have a plane surface upon one side and a curved surface upon the reverse side. L., Prismatic, a triangular bar of glass. L., Spheric, on whose curved surface, either concave or convex, is a segment of a sphere, in contradistinction to a cylindric lens. L., Sphero-cylindric, one with a spheric surface upon one side, and a cylindric surface upon the reverse, used for the correction of either myopia or hyperopia, associated with astigmatism. See Spectacle- lenses. L.-star, the stellate figure seen on the posterior surface of the crystalline lens. L., Test or Trial, one of a set of lenses employed in determining refractive errors.
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