Illusion
Pepper's Ghost is a clever, although simple illusion which was used to great effect in stage plays of the 19th century and probably a considerable time before that. It is sometimes known as The Blue Room illusion.
It was often used to allow the presence of a mysterious and ethereal figure to "appear" on stage and have people be able to see through the form, or if they were at the right angle, wave say a sword or arm quickly through it without ruining the whole illusion. Variations on this theme gave a range of strange effects.
If you've ever seen those corny (but marvellous) fairground shows where a girl transforms into a gorilla, then you have seen the concept at work.
It works on the principle of it being all done with mirrors. A large sheet of glass is placed at 45 degrees between the viewer and the original image or subject, who is then reflected in such a fashion as to appear directly in front of the viewer. The trick is that directly behind this piece of glass at the same distance is a second person standing in the darkness. This person is not visible for the same reasons that you can't see who's creeping around in your garden at night when you have blazing lights throwing back your own reflection at you.
By slowly increasing the illumination of the hidden figure or object is slowly becomes more solid and apparent. With the invention of dimmer controls and electric lighting it's easy to give this trick a new lease of life.
Making your own Pepper's Ghost
Here's a quick and fun way of demonstrating the idea in a practical format. Take a tall bottle, filled with water, a lit candle and a mirror. Place all of the above in a large box placed a good distance away from the viewer. Behind the mirror is a lit candle, whilst hidden from direct sight
is the bottle of water. This gives the image of a burning candle suspended in a sealed vessel of water - dramatic and impressive! Keep people well back though or they will soon see the trick and take care not to let the box catch fire!
Needless to say, in today's world of CGI and other technological methods of making film-goers see things that aren't really there, it seems primitive by comparison, but it has a certain charm of its own and an ingenuity to fool the senses with some very simple equipment.
Sources
http://www.hauntworld.com/Uncle_Timmy_Zombie/peppers.html
Last updated : 28th January, 2003