Found Object : Antique Stereo View Cards
Antique Stereo View Cards for Cross-Eyed Viewing.
Stereo view cards had two pictures mounted for parallel viewing, on 7 x 3.5 inch heavy cards, usually curved slightly with axis along the long length. The pictures were taken with a two-lensed camera, recording the subject from two points of view separated by about 2.5 inches, duplicating what our two eyes see.
Below is a Greeting Card made with an Antique Stereo View Card…cool, I think:)