Machinery can make big changes in social behavior--- the automobile completely changed patterns of courtship, even techniques and positions of sexual intercourse, in America. The development of the rail system, and of high speed printing, over the last half of the 19th Century, made possible the weekly magazine, and opened markets for fiction unprecedented in human history. Writers responded nobly to the demand for quantity and novelty. The type of literature we call ``science fiction'' today thus became a recognizable genre of popular fiction more than a century ago. Completely fictional themes and gimmicks developed during this heroic age have so deeply penetrated public consciousness, through use and reuse and retread and re-re-retread during the following ages of film and television, that it is often difficult to realize they have no counterpart whatsoever in reality.

The gimmick of invisibility was not invented by H. G. Wells, but he got there first with the best story. His inventor, Griffin, by swilling a magic potion, made himself totally transparent. (He would also have made himself totally blind, since the backs of his eyes would no longer absorb light, but Wells chose to ignore this complication.) The conceptual problem with this, as Wells was well aware but also chose to ignore, is that transparent objects are not invisible. Water, air and glass are fairly transparent, but quite visible. Another, physical problem is that transparent objects refract light--- change its direction slightly--- and also scatter or diffusely reflect light--- bounce it in all directions from their surfaces.A transparent drop of water flying through the air, or a transparent air bubble in water, are clearly visible, by the light they refract and reflect.

Furthermore, transparency and refractive index are intrinsic, inherent properties of a given material. They cannot be changed, being determined by the internal molecular structure, and the overall spacing of the molecules in the material. In order for substance A to be invisible in substance B, both materials must be transparent and both must have about the same index of refraction. No solid or liquid comes with an index anywhere close to the index of any gas. Thus, if you want to explore this kind of invisibility, you need a solid and a liquid. Try ordinary Wesson Vegetable Oil and laboratory pyrex glass. Fill a small pyrex container with Wesson oil and then submerge it in a large jar of Wesson oil and watch the pyrex container vanish! Wow, golly, Mr. Science!! But so long, Mr. Invisible Man.

Decades after Wells, writers tried other tactics. An object could be made invisible if you could ``bend light around it.'' The conceptual problem here is that, except in rare cases, we don't see objects as silhouettes! We see objects mainly because their surfaces scatter light diffusely. Since the light is scattered in all directions, no matter how it is bent, some gets to our eyes. A physical problem is that, in empty space or any given material, light travels as straight as it can travel. Only something that can change ``the shape of space itself,'' like gravity, can bend light ``around an object.'' For example, distant massive galaxies gravitationally bend the light from still more distant galaxies directly behind them, so that the more distant galaxies are visible as distorted arcs or in multiple images surrounding the central galaxy, in the beautiful ``gravitational lens'' phenomenon first predicted by Einstein. The Hubble Space Telescope has provided many wonderful examples of such lenses. Note that in every case the central galaxy that is bending the light is clearly visible--- indeed, it dominates the image! As it must.

With the real external world so inhospitable to invisibility, only the human mind is left. In the exciting world of pulp fiction, in the 1920s and '30s, various justice figures came up with a new way to be unseen. Like radio's Shadow, who could ``cloud men's minds so that they cannot see him,'' perhaps by ``guesturing hypnotically'' like the comic strips' Mandrake the Magician, these justice figures could tumble helpless bad guys around like so many ten-pins. And that's another story, for another time, plunging us into yet another myth, the myth of ``mass hypnosis.''

A test tube in Wesson oil... the part of the tube below the oil is invisible since the indices of refraction of pyrex glass and Wesson oil are close to the same.