Lens

n.; pl. Lenses [ L. lens a lentil. So named from the resemblance in shape of a double convex lens to the seed of a lentil. Cf. Lentil. ] (Opt.) A piece of glass, or other transparent substance, ground with two opposite regular surfaces, either both curved, or one curved and the other plane, and commonly used, either singly or combined, in optical instruments, for changing the direction of rays of light, and thus magnifying objects, or otherwise modifying vision. In practice, the curved surfaces are usually spherical, though rarely cylindrical, or of some other figure. [ 1913 Webster ]

☞ Of spherical lenses, there are six varieties, as shown in section in the figures herewith given: viz., a plano-concave; b double-concave; c plano-convex; d double-convex; e converging concavo-convex, or converging meniscus; f diverging concavo-convex, or diverging meniscus. [ 1913 Webster ]

Crossed lens (Opt.), a double-convex lens with one radius equal to six times the other. --
Crystalline lens. (Anat.) See Eye. --
Fresnel lens (Opt.), a compound lens formed by placing around a central convex lens rings of glass so curved as to have the same focus; used, especially in lighthouses, for concentrating light in a particular direction; -- so called from the inventor. --
Multiplying lens or
Multiplying glass (Opt.), a lens one side of which is plane and the other convex, but made up of a number of plane faces inclined to one another, each of which presents a separate image of the object viewed through it, so that the object is, as it were, multiplied. --
Polyzonal lens. See Polyzonal. [ 1913 Webster ]