The easiest way to see an image in stereo is to look at the double image through a viewer, in some cases a pair of glasses with +3.0 lenses will give you the effect, hold them away from your face close to the image and move them towards you until the image comes in to 3D focus. (Keep your own glasses on if you wear them!)
To see the image without a viewer… Hold your flat hand fingers pointing up with your index finger against your nose (as if you’re trying to stop someone poking you in the eye). This acts as a blinker between your eyes and helps you to see the separate images. Stare at the picture as though you’re looking through it at something behind the screen, the two images will merge to create one 3D picture. Don’t try to focus in too quickly as they merge, stay relaxed and it will slowly come in to focus. For me it works best at about 2′ back and keeping the image size fairly small.
The 3D effect is caused by each eye viewing a slightly different picture. Your brain tries to put them together to create an image and turns it in to 3D…. magic!
Image available on Opensea – GhostExMachina