Achromatic lens

Different kinds of glass produce spectra of different widths. Flint glass (contains lead) produced wider spectrum than crown (regular) glass. By combining a convex lens out of crown, and a concave lens out of flint glass, he made a lens that magnified without producing colors.
 Chester Moor Hall England 1733 Achromatic lens

Different kinds of glass produce spectra of different widths. Flint glass (contains lead) produced wider spectrum than crown (regular) glass. By combining a convex lens out of crown, and a concave lens out of flint glass, he made a lens that magnified without producing colors.