For this experiment you will need a sheet of paper, a flashlight, and the sort of unlighted room with a mirror that can be found in most modern homes.

Crumple up the paper, then straighten it enough to leave plenty of wrinkles. Take it into the bathroom, turn off the lights, hold the paper before the mirror and shine the flashlight along the edge of the paper. You should see an identical pattern of light and shadow on both the front of the paper and its mirror-image of the other side.

In my result shown at left, not only are the shadows repeated, but the wrinkles also match bulge for bulge. The bottom of the paper curls away from the viewer, and it also curls away in the mirror-image--the same illusion we saw in the Apollo 17 flags at the beginning of this section. Yet if you try this experiment, you probably won't get the same effect. Wrinkles that bulge out on the paper will appear to point in on the reflection, as we might expect. So why do we see matching, non-complementary wrinkles in the photographed Apollo flags and my demonstration?

The answer is simple: The camera has only one eye. Without benefit of stereoscopic vision our brain has fewer clues to work with and can make the wrong interpretation.

You can get the same effect if you photograph it. For my picture, I hung the paper from the ceiling with a length of Scotch tape; the paper is about 3" in front of the mirror (see 2^nd picture).

But you don't need a camera--let alone my sophisticated experimental rig--to get this illusion. All you have to do is close one eye. Then shine the light on the paper again. (You might have to close both eyes and shake your head to erase the stereo image from your memory.)

With a bit of trial and error you can get some impressive illusions. If you alternately shine the light toward the front and back of the paper the wrinkles seem to pop in and out on both sides of the mirror. It reminds me of that scene from Duck Soup in which Groucho Marx is trying to figure whether he's looking at a mirror or Chico disguised as him. So he whips up his right hand. And Chico lifts his right hand....

On the other hand, if you shine the light against an opaque medium such as tinfoil or a magazine, then you will get the sort of negative/positive images that David Percy expected a gauzy flag to show. If you don't believe me, try it yourself. I did.