LENS, a piece of glass, or any other transparent
substance, the surfaces of which are so formed, that
the rays of light, by passing through it, are made to
change their direction, either tending to meet in a
point beyond the lens, or made to become parallel af-
ter converging or diverging; or lastly, proceeding as
if they had issued from a point before they fell upon
the lens. Some lenses are convex, or thicker in the
middle; some concave, or thinner in the middle; some
plano-convex, or plano-concave; that is with one side
flat, and the other convex or concave; and some are
called meniscuses, or convex on one side and concave on
the other. See DIOPTRICS.
Lenses are of two kinds, either blown or ground.