"Pepper's Ghost"

I've received a few inquiries as to how we made (apparently) three dimensional objects appear and disappear on stage. Given my aversion to (and the expense of) cutting edge technology, we used a much simpler 19th century stage illusion known as "Pepper's Ghost." This involves lighting and reflections off a sheet of angled glass (or in this case, acrylic), much like seeing your own reflection in a window while riding in a train or car. The largest modern use that I know of is at the Disney parks in the Haunted Mansion variants. The ballroom dancing ghosts aren't, as popularly believed, to be holograms; they are lit reflections of hidden animatronic dancers off a gigantic piece of sheet glass.

There are several good descriptions of the illusion on the net (especially in articles on building Halloween haunted houses), but the most thorough explanation and history is in Jim Steinmeyer's book, Two Lectures on Theatrical Illusion, Hahne, 2001.