The Foucault knife-edge test is an optical method used to evaluate the surface quality of mirrors, particularly in telescopes. It involves shining a light source onto the mirror and observing the reflection through a knife-edge or razor blade. By moving the knife-edge across the reflected light, one can detect imperfections in the mirror's surface based on the shadows created. This test was developed by the French philosopher and historian Michel Foucault in the 19th century. It is a simple yet effective technique that allows for precise measurements of the mirror's shape, helping to ensure optimal performance in optical systems.