you can view stereo image pairs without a viewing device by crossing your eyes until the images overlap then adjusting focus until things become sharp. it's a neat skill you can learn with a few hours' practice

https://youtu.be/dG4s6WoREpg
https://youtu.be/nD-T0T9J-p8
https://youtu.be/4S5pZEnIbrQ?t=232