Stereograms

Sometimes the visual impact of a scene comes from the third dimension. The spaces between the leaves and branches in a tightly packed bush, the brittle petal of a flower only appear to us in three dimensions. Sometimes a two-dimensional photograph can simply look like an abstract muddle of colors. When seen in three-dimensions that separate the planes of color and form, the meaning of the scene pops out.

Unfortunately, presenting three dimensional images over distances is still a very clumsy affair. Images have to be stored and displayed, we still can't make the living image public.

Seeing a stereogram is cumbersome affair. Either the viewer has to use a stereoscope that focuses the eyes separately or the viewer has to learn how to relax his or her eyes in a way that lets the two sides of the stereogram merge together. The art may be diminished by the mechanics of moving the image further and nearer to get the third dimension to pop out, or maybe the effort enhances the art.
There is a lot written about the way to do it. There are also several kinds of stereograms. All use a similar form of relaxing discipline. Take a look at these instructions at http://www.eyetricks.com/stereograms/tips.htm

"1. Pick a spot on the picture (the middle seems to work best) and just stare at it.
2. Allow your eyes to relax, don't just stare AT the image, try to stare THROUGH it. You'll notice your eyes will go slightly out of focus. This is normal.
3. Keep staring, don't give up, once you begin to see the first image, it gets much easier."