In 1862, inventor Henry Dircks developed the Dircksian Phantasmagoria, a technique used to make a ghost appear onstage. He tried unsuccessfully to sell his idea to theaters. His method would require theaters to be completely rebuilt just to support the effect. Later in the year, Dircks set up a booth at the Royal Polytechnic, where it was seen by John Pepper.
Pepper realized that the method could be modified to make it easy to incorporate into existing theaters. Pepper first showed the effect during a scene of Charles Dickens's The Haunted Man, to great success. Pepper's implementation of the effect tied his name to it permanently. Though he tried many times to give credit to Dircks, the title "Pepper's ghost" stuck.
In order for the illusion to work, the viewer must be able to see into the main room, but not into the hidden mirror room. The edge of the glass may be hidden by a cleverly designed pattern in the floor. Both rooms may be identical mirror-images; this approach is useful in making objects seem to appear or disappear. This effect can also be used to make an actor reflected in the mirror appear to turn into an actor behind the mirror (or vice versa).
The mirror room may instead be painted black, with only light-coloured objects in it. When light is cast on the objects, they reflect strongly in the glass, making them appear as ghostly images superimposed in the visible room. The world's largest implementation of this illusion can be found at the Haunted Mansion and Phantom Manor attractions at several Walt Disney Parks and Resorts theme parks. Here, a 90-foot (27 m)-long scene features a single Pepper's ghost effect. Guests travel along an elevated mezzanine, looking through a 30-foot (9.1 m)-tall pane of glass into an empty ballroom. Animatronic ghosts move in hidden black rooms beneath and above the mezzanine.The reflections in the glass, which is vertical rather than angled, create the appearance of three-dimensional, translucent ghosts. These swarm through the ballroom, seeming to interact with props in the physical ballroom, disappearing when the lights on the animatronics are turned off.