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.
Above a lecturer's candle reading lamp, the small ring at the left of the shade uncovered a window of red glass at the rear of the lamp thus signaling to the lanternist that the next slide should be projected.