Will O' Wisp, also called "ignis fatus", "corpse candle", "fairy light", etc., is a famous spirit that appears in the form of light in forests, swamps, bogs and marshs at twilight or night. They appear as a flickering lamp, and this misleads them as being mistaken for Jack'o'Lantern. It is also the name of a scientific phenomenon caused by the oxydation of Phosphine and Methane produced by organic decay that causes photon (light) emission. In the Irish folktale of Henry Glassie, Will O' Wisp is known as Willy the Whisp.