The convergence to different foci, by a lens or mirror,
   of rays of light emanating from one and the same point, or the
   deviation of such rays from a single focus; called spherical
   aberration, when due to the spherical form of the lens or mirror, such
   form giving different foci for central and marginal rays; and chromatic
   aberration, when due to different refrangibilities of the colored rays
   of the spectrum, those of each color having a distinct focus.