The magic lantern was a tool  which  served  both to entertain and to educate. In many instances  a lecture was read to the audience as slides were projected. Special  lecturer's lamps  were employed that shone the light directly on to the text  yet allowed the rest of the room to remain in darkness. Often these lamps had some sort of signaling device (usually a bell or , more discretely , a tiny flash of coloured light). This signal indicated to the lanternist to project the next slide.