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. [ 1913 Webster ]

☞ The concave or diverging half on an achromatic lens is usually made of flint glass. [ 1913 Webster ]