The nineteenth century saw the publication of several books explaining how magical effects and spectral appearances could be performed using the science of optics. It started in 1831, when Sir David Brewster (famed for his discovery of Brewster polarization and inventing the kaleidoscope) published Letters on Natural Magic. In this book, Brewster showed how to produce images of ghosts using partially silvered mirrors and by using a magic lantern to project images onto screens or onto clouds of vapor.
Brewster’s work inspired generations of stage magicians including Henry Dircks and John Henry Pepper, who eventually patented what came to be called the “Pepper’s Ghost” illusion. Pepper’s Ghost, impressive as it was, barely scratched the surface of what was possible using optical engineering. It’s a little surprising that more sophisticated optical tricks did not appear until the twentieth century, considering that the rules of imaging had been derived well over two hundred years earlier. But in 1909 something new and impressive burst forth in its fully formed glory. Antoine Francois Sallé, an engineer living in Paris, was granted a United States patent on a “Means for Producing Theatrical Effects” (US 922,722).
Sallé’s holographic invention
Sallé’s invention used the Newtonian rules of imaging, which hold both for lenses and for curved mirrors. Under the correct circumstances, one can create a “real” image (through which light rays pass, as opposed to a “virtual” image, which light rays do not pass through; if you place a screen where a real image is located, it will show up on the screen—a virtual image will not) that is the same size as the object, or magnified, or reduced, depending upon the relative positions of the lens or mirror, the object, and the focal length of the optic. The image will, however, be upside-down, but this can be corrected with an image inverter.
Using mirrors has advantages over using lenses to work this trick. There is no separation of colors as with a simple lens. In addition, a curved mirror can be used to cover a larger angular region, letting the image be viewed farther off-axis than can be seen with a lens, unless a very large and expensive lens is used. The result is that an image produced using a curved mirror can look much more convincingly real, and over a larger range of angles, than an image produced by a lens. With a subject that is brightly illuminated, the result was that a viewer on the other side of the apparatus saw a perfect reduced image of the subject. If the subject was a person, it could walk around and interact with things. Moreover, the image appeared to be three-dimensional, since each eye of the observer saw the image from a slightly different angle that provided a stereoscopic effect. The word wasn’t yet being used in this context, but a later generation would describe such images as holographic—perfect miniature 3D images of a moving person.