An test for the optical quality of a mirror that uses a layer of water placed atop a flat mirror. It produces an interference pattern between the water and the glass, which can be interpreted to give the flatness of the mirror.
Foucault Test, Mirror Tests, Rayleigh Water Test, Ronchi Test, Terebizh Null Test
References
Jones, E. "The Rayleigh Water Test." Sky & Telescope, 89-92, July 1990.
