Above: A scene in Metropolis involving miniatures is composed using the process. Image: Wikipedia
Wikipedia on the schufftan process:
The process was refined and popularized by the German cinematographer Eugen Schüfftan while he was working on the movie Metropolis (1927), although there is evidence that other film-makers were using similar techniques earlier than this. The movie’s director, Fritz Lang, wanted to insert the actors into shots of miniatures of skyscrapers and other buildings, so Schüfftan used a specially made mirror to create the illusion of actors interacting with huge, realistic-looking sets.
Videomaker explains the effect better than the explanation on the wikipedia page:
This special effect involves strategically scraping some of the silver off a mirror, then setting the mirror in front of the camera at an angle. While the mirror reflects a miniature off to the side, the live action takes place in front of the camera, seen through the scraped area of the mirror. This works quite well. Since the mirror itself is out of focus, the edge between the miniature and the live action is blurred.
I love the invention and resourcefulness involved in filmmaking.