This is a home projector from the 1930s, but the ones in the Fallon Theatre operated on the same principle, though they were significantly scaled up. There is a light in the back that the film passes in front of. The lens then focuses the image as the film runs.
Skilled Projectionist Wanted
Every screening relied on a skilled projectionist. They would watch for little markings on the screen, warning them when it was time to switch the rolls of film so the show could go on.