When a lens curves inward, it is an example of a concave lens.
Aristotle makes [Fire] to move to the concave of the Moon. - Thomas Salusbury (1661).
Concave Lenses are used in the treatment of myopia or shortsight.
Rayleigh points out that this clinging of the sound to the surface of a concave wall does not depend on the exactness of the spherical form.
He refers to Maurolycus' work with concave specula.