flint glass(n = noun.substance) optical flint - optical glass of high dispersion and high refractive index;
is a kind of optical glass

   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]

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