Lens

Portion of a transparent medium, bounded in general by curved surfaces: in particular cases one of these surfaces may be plane. Lenses may be either spherical, cylindrical, elliptical, or parabolic, according to the character of their curved faces. Lenses spherical or approximately so are chiefly used in optical instruments. They are divided into two cla es, according to their action upon a luminous beam of parallel rays; viz., converging or convex, which are thicker in the middle than at the Lenses. edges, and diverging or concave, which are thinner in the middle. Of each cla there are three forms, the double-convex, plano-convex, and converging meniscus, double-concave, planoconcave, and diverging meniscus. Achromatic lenses refract light, without dispersion, which produces colored fringes. They are formed by a double convex lens of crown gla with a plano-concave lens of flint gla , first devised 1757 by J. DOLL AND (q.v.).