We have two eyes, and because our eyes are separated by a couple inches, we see a slightly different image with each eye. The brain melds these images into something we can understand. This separation of our eyes helps us to judge spacial relationships of different objects to each other, because we can see slightly around the objects.

Sometimes things in the foreground are so close we have to almost cross our eyes so as to bring both images from each eye into focus. Things in the middle ground are easier to see, and we can tell what items are in front of others. Objects that are a great distance away appear flat because both eyes are virtually seeing the same image, and there is no depth perception.

Stereographic photography takes advantage of the way our eyes see two slightly different images. Two photographs are taken, one slightly to the right or left of the other. These images are mounted and viewed with a special viewer that allows the left eye to see one picture and the right eye to see the other. Your brain fuses these images together, just as it would if you were viewing the images live, and you get the effect of seeing the image as if you were really there!