I've had one or two readers ask for more information on the "Pepper's Ghost" illusion, mentioned in my last entry on The Haunted Mansion.
The illusion was created by Henry Dircks and refined and made feasible for modern theaters by John Pepper in 1862.
The illusion is for transparent, ghost-like images of performers or objects to be seen onstage with live actors, and its a relatively simple process. A large sheet of glass is suspended at the front of the stage at an angle, between the audience and the performers. Below the audience at the front of the stage, a black backed secondary stage is created where the ghost actors will perform, facing the primary stage. The ghost actors are illuminated in a manner so that they cast a reflection onto the angled sheet of glass and appear transparent. They can be made to fade in and out by attenuating and intensifying the lighting.
In a variation, the ghost performers can be under or in front of the stage facing the audience. The illumination is projected through an oppositely angled two-way mirror onto the performers. The mirror then reflects the transparent images onto the sheet of glass.
In the Haunted Mansion at Disneyland, this effect is used for when the spirits appear next to you in the buggies as they're reflected in the mirror, and at the ghostly cotillion.
Variations on this effect have been used in illusions and stage craft since its invention. Click here for a link to the Pepper's Ghost entry at Wikipedia.