Since I still have trouble with File Transfer Protocol, I will marvel in the wonders of Wikipedia's explanation of the Stereoscope.
Evidently most people can see the 3D image without a stereoscope but it tends to cause eye strain and fatigue.
L.E. on 21st December 2018 @ 8:02am
I've gotten pretty good at seeing these images in 3D without a viewer, though it definitely does cause eye strain. This one looks particularly fake. The cat, child, and the wall behind the child look like they are cutouts spaced too far from each other. Card makers would exaggerate the 3D effect so viewers wouldn't be disappointed. It's just like boosting the bass in an audio system so listeners think they are getting their money's worth.
Arthur on 21st December 2018 @ 1:34pm
The technique, as I learned in Geomorphology class at the University of Alaska Fairbanks, is to put your two index fingers together as if each tip were pointing at each other, and center these at the division between the two images. Then slowly pull the fingers apart while trying to keep an eye on each fingertip simultaneously. This will cause you to cross your eyes (this is where the eye strain starts), and you'll soon see two sets of stereo images. The goal at that point is to get it so that you put together the two sets of dual images so that you only see three images total. The center image will then appear to be in 3-D. (Geologists, surveyors, and engineers, etc., have traditionally used this technique to study stereoscopic aerial photography on the fly, when there wasn't a stereoscope around, to view the physical features.)
This works even when your eyes get bad enough that you need glasses to see just about anything with any degree of clarity... :)
Ben M. Angel on 29th December 2018 @ 4:58pm