Lens

(Jenz). A regularly-shaped piece of gla or crystal for the refraction of rays of light. The crystalline lens of the eye. L., Achromatic. See Achromatic . L., Apochromatic. See Apochromatic . 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 (negative or minus lens), a thick edged lens having concave spheric surfaces upon its opposite sides; it is used in spectacles to correct myopia. L., Biconvex lens, a thin-edged lens; it has two convex surfaces, and is used to correct hyperopia. 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 (either minus or plus), one ground upon a cylindric tool, i. 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 convexoconcave 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- 12345 Different Forms of Spheric Lenses. 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 gla . L., Spheric, one 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, a ociated with astigmatism. See Spectacle lenses.