Coronas are luminous rings showing the spectral colors in the reverse order, that is, with the inner edge blue instead of red. They are usually of very small radius, scarcely two degrees, closely surrounding sun or moon and are produced--not by refraction--but by _diffraction_ or a bending aside of the rays as they pass between--without entering--very small drops of water in clouds. As in the case of refraction, the red rays are turned from their course the least and the violet rays the most.