This is a lost artform. Probably because it's so hard to visualize the imagery in its full splender. Yet if you can visualize stereo photographs, you'll see things that cannot be seen with standard single images.
Here's how to view them:
Note the image on the right is a picture of a wave cloud taken from a commercial jet from two angles. The idea is to cross your eyes so that you see the cloud on each side split into two.
You should see 4 wave clouds, 2 from each side.
The idea is to merge the two middle wave clouds until they lie on top of each other as you keep your eyes crossed.
Then the trick is to concentrate on focusing the two middle wave clouds while your eyes are crossed.
When you're successfull, you'll see a sharp picture in the middle that's in stereo. This takes practice so don't give up if your attempt fails on its first time.
If you like to see more, click on the gallery above.
If you cannot view the images above by crossing your eyes, and if you can get a hold of the red/blue glasses that you may have received from various add campaigns or NASA, you may try to view this image. Click on it to get the full effect.