Now you see it, now you don't. But the question is, was it ever really there at all?
The new Liquid Vision exhibit at the Museum of Science in Boston explores the world of optical illusions and imaging, from modern virtual reality and computer-aided design to fiber-optic displays.
Step inside the hall, and you immediately get a feel for what's in store as a 5-foot skull of a Tyrannosaurus Rex menacingly jumps into your path - courtesy of a giant hologram. On the other side of the gallery, a three-foot tarantula looms ominously out of the glass display.
Holograms feature prominently in Liquid Vision because they constitute the information-storage medium of tomorrow, says John Shane, vice president of programs for the Museum of Science. Because they are three-dimensional, holograms contain exponentially more data than any flat-storage medium of today, such as compact discs.
Holograms' progenitors, lasers, appear only in Liquid Vision's centerpiece, a fiber-optic fireworks display that visitors set off by aiming laser guns at targets on a cityscape.
The exhibit also shows older optical-illusion techniques for comparison. At one station, for example, a mini-Elvis appears on stage at the press of a button. Wait a few seconds and he fades away.
Look closer, and it's clear that while the wooden stage set is real, Elvis Presley is only smoke and mirrors.
The trick uses an Elvis Doll tucked away inside the exhibit's box. When a viewer presses the button, a light shines on the doll, and its image is reflected on a piece of glass, making it look as if it's on the stage.
Liquid Vision uses such tricks to pique curiosity about the capabilities of light. The show includes a variety of three-dimensional imaging techniques, from a gallery of holograms to the King's reflection to the traditional 3-D ''stereovision'' eyeglasses.
The hit of the show is virtual volleyball. As two or more players step into a projection booth, their shadows appear against a wall along with the shadow of a net. Suddenly, the ball's shadow appears and the players have to make their silhouettes turn, run, and jump to hit it.
Visitors can also hang-glide off skyscrapers in a virtual city or fly a plane over the landscape in the Virtual Navigator.
While all three virtual-reality displays seem dated by today's standards (the exhibit was assembled in 1993, in Columbus, Ohio), they will give travelers a taste for the future.
Already, virtual-reality stations are being used to train pilots, astronauts, and heavy-equipment operators - without risk of injury. That's good news, of course, when you've never hang-glided off a building before.
On the practical side, computer workstations throughout the hall also allow users to explore visual possibilities.
One takes a picture of your face and allows you to watch it ''morph'' into a wolf, a car, or a caricature.
Another lets kids finger-paint on a computer screen - and leaves no mess to clean up.
A third lets would-be designers redecorate a room and adjust everything from paint colors to ceiling lights to the brightness of the sun before viewing the final results.
A popular attraction for kids is the video production studio, where hams can play air guitars and sing while their friends record and edit the action. (For an extra price, you can take your own tape home.)
Liquid Vision will be shown in Boston through April 28 and then will continue its tour to St. Paul, Minn. before it returns to its Columbus, Ohio, home for permanent display.
Even for a generation raised on high-tech computer graphics, this show beats any video arcade.