Projecting an image onto smoke with a mirror, from Nouvelles récréations physiques et mathématiques (1770)
Smoke and mirrors is a classic technique in magical illusions that makes an entity appear to hover in empty space. It was documented as early as 1770 and spread widely after its use by the charlatan Johann Georg Schröpfer, who claimed the apparitions to be conjured spirits. It subsequently became a fixture of 19th-century phantasmagoria shows. The illusion relies on a hidden projector (known then as a magic lantern) the beam of which reflects off a mirror into a cloud of smoke, which in turn scatters the beam to create an image.
The phrase "smoke and mirrors" has entered common English use to refer to any proposal that, when examined closely, proves to be an illusion.