If you want to project images on a screen, you need two things: A bright light source, and an optical apparatus to direct and focus the light through the image. Three things, if you count the semi-transparent slide bearing the source image.
All three may be seen in today's demonstration from the Cabinet of Physics in Florence. We begin with the Duboscq arc lamp, introduced in 1851 (many years before Edison's incandescent light bulbs appeared). An electric arc across the gap between two carbon rods can produce an astonishingly bright light. But it can be erratic and diminish in brightness as the carbon is consumed. Léon Foucault and Jules Duboscq devised a feedback mechanism to adjust the gap and keep the lamp's brightness steady.
This reliable and intense light source is the heart of a "lantern" whose lenses can project an image of the arc on a white screen some distance away. A glass slide can be inserted into the focal plane of the optical system. Suddenly the tiny picture on the slide appears, much magnified, on the screen.
Such projectors became a boon to education and entertainment. Artwork or photographs could be shared witha roomful of people. A lecturer could transport an audience to the Piazza San Marco in Venice, as we see here.
Though the less-cumbersome incandescent light came along later in the 19th century, arc lamps remained useful for a long time afterward in specialized applications such as projectors or theatrical lighting. When I began lecturing, Kodak-style slide projectors with very bright incandescent bulbs were the standard tool. Eventually video projectors became common, and transparent film was replaced by video signals generated inside a laptop computer. Microsoft's PowerPoint software became as ubiquitous as Kodak projectors once had been.
The ability to show pictures to an audience remains a great blessing, and speakers—not least those who lecture about astronomy—should be thankful for it. The greatest challenge of my speaking career came shortly after the Cassini spacecraft delivered the Huygens probe to Saturn's largest moon. In the room where I was to speak, the projector's bulb had failed, and no replacement was to be found. Yet The Show Must Go On. I found myself describing the surface of Titan orally for the next hour. I got through it, with the help of adrenaline. But the experience made me more grateful to Messrs. Foucault and Duboscq and all of their fellow developers of display technologies.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.