The Collaborative International Dictionary of English v.0.48:
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
[1913 Webster] 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
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.