Source : Webster's Revised Unabridged Dictionary (1913)

Lens \Lens\ (l[e^]nz), n.; pl. {Lenses} (-[e^]z). [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. Lenses

   Note: 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.

   {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 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}.