\Flint" glass`\ (Chem.)
A soft, heavy, brilliant glass, consisting essentially of a
silicate of lead and potassium. It is used for tableware, and
for optical instruments, as prisms, its density giving a high
degree of dispersive power; -- so called, because formerly
the silica was obtained from pulverized flints. Called also
{crystal glass}. Cf. {Glass}.

Note: The concave or diverging half on an achromatic lens is
      usually made of flint glass.