If one of the radii becomes infinite, the lens is plano-concave (fig. 9, 5).
plano-concave and double concave lenses produced similar effects.
The achromatic lens consists of a double convex lens of crown glass combined with a plano-concave lens of flint glass.
The chromatic combination consists of a convex lens of crown backed by a concave, or plano-concave, lens of flint.