Popular in the 18th century, the Claude glass was a mirror that took the scene behind you and transformed it into something different, much like the filters in Instagram or Hipstamatic promise to do.
The Claude glass was a sort of early pocket lens without the camera and it was held aloft to observe a vista over one's shoulder. The technology was simple: A blackened mirror reduced the tonal values of its reflected landscape, and a slightly convex shape pushed more scenery into a single focal point, reducing a larger vista into a tidy snapshot.