HOW TO VIEW 3DStereo drawings and photographs require two pictures, one for each eye. Then the pictures must be presented to your eyes so that each eye sees only the picture intended for it. Generally two methods are used: (1) parallel viewing and (2) cross-eyed viewing. Both methods require one to learn a new visual skill.
Normally when we look at the "real" world, our eyes converge on an object in the scene and they also focus on that same object. We habitually do this, and our brains have become accustomed to a one-to-one correspondence between focus and convergence.
We can learn to "unlock" focus and convergence, enabling us to view stereo pairs without optical aid. In this document we use a display method which can be adapted to either parallel-eye viewing or crossed-eye viewing. Three pictures will be shown side by side. The middle one is to be viewed with the right eye. The other two are identical and one is to be viewed with the left eye. Here's how to view them.
(1) Parallel viewing. This is sometimes called wall-eyed viewing. Use the left and middle pictures only. Look at a distant object then bring your eyes down to the paper trying not to converge or focus on the paper. You'll see a blurred double image. Consciously try to bring the double image into one. Now try to focus your eyes on it without allowing the two images to drift apart.
For practice, try this illustration from Sir Charles Wheatstone's book The Stereoscope.
Don't expect to succeed the first time. This skill takes conscious effort and concentration. When you do succeed, you'll see the pictures snap into full three-dimensional depth. The picture will look like a wire-frame box. You'll actually see two 3-D images, one with normal depth, one with inverted (pseudoscopic depth). On either side of these you'll see fainter, phantom images with no depth. Ignore them.
Here's some more practice examples:
If you use cross eyed viewing on the pair intended for parallel, or vice versa, you will see a "pseudoscopic" depth, in which near and far are reversed. In the first picture, the pseudoscopic view appears as if you are looking down onto a truncated pyramid. In the second picture, the cone seems upright in the normal view, but tilted back and viewed from its base in the pseudoscopic view. Wire-frame stereo drawings often look interesting either way.
Here's another example for practice.
Now for some practice with actual stereo photograps. This one is for parallel viewing. It is small because most people cannot diverge their eyes, so pictures for parallel viewing should not be more than about 2.5 inches apart. Old-fashioned Holmes stereoscopes had prismatic lenses to allow viewing pictures 3 inches apart.
Now the same picture, larger, for cross-eyed viewing.
Stereos for cross-eyed viewing in 3d Gallery One.