The two pictures are ever so slightly different, with the left picture showing more landscape to the left and vice-versa. When viewed through a stereoscope, our eyes are tricked into believing that this card is one three dimensional image.
We have a set of Orcadian scenes printed in this way that are very delicate, each wrapped in a piece of archival tissue paper.
"Well thanks a lot!" you cry "but I don't have a stereoscope and I really really like looking at cool 3D images, you're just teasing us!"
Don't worry, faithful readers. Merely print out the image and look at it cross-eyed. You can do this whilst looking at it on the computer screen if you're lazy but it is probably really bad for your eyes.
If you are in a room with other people, don't worry about looking stupid; we've been doing it all morning and we looked awesome.