The images on Theo Prins site are arranged for cross-eye viewing to create a steroscopic experience.

While looking at the center of the stereoscopic pair, place your finger about halfway between you and the screen. If you converge your eyes to focus on your finger you will notice the images on the screen begin to overlap.

Adjust your finger’s position until the images completely overlap. Because you are still focusing on your finger, the images on the screen will be blurred.

The trick is to remove your finger and allow yourself to focus on the screen without uncrossing your eyes. Done correctly, you will see three images on the screen. The center image will have depth and the two outer images will not.

Your brain translates the disparity between the images that have overlapped into the experience of depth.