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}.