Originally Posted by LostNYC
So we have to add one more factor to all this, and that is illusion!
I have a number of stereoviews, and there is a technique of free-viewing them without a stereo viewer but it takes some practice, once you get the hang of how to do it the images seem to pop out of the cardboard into 3D almost like magic.
You have to relax the eyes and not try to focus on the image as they want to do, and when you get the right distance and relaxation of the eyes' focus, all of sudden the 3D image appears and you can actually start to carefully focus on THAT image. So the eyes can play tricks on the brain with an illusion.
That technique also is common knowledge in the field of crystallography. Many journals publish these stereographic pairs of images, showing the arrangement of atoms within a molecule or crystal, and commonly also the typical range of motion of each atom. These “thermal ellipsoids" as they are called, are stylized odd-shaped footballs, with each of the three axes showing a sort of average motion along that axis. It’s possible to view these stereo pairs with the usual viewer, but most workers in the field acquire the ability to defocus the eyes and see the depth without viewers.