WE LOOK AT THE WORLD from two slightly different vantage points, which correspond to the positions of our two eyes. These dual vantage points create tiny differences between the two eyes’ images that are proportional to the relative depths of objects in the field of view. The brain can measure those differences, and when it does so the result is stereovision, or stereopsis.
To get an idea of this effect, extend one arm to point at a distant object. While keeping your arm extended, alternately open and close each eye. Notice how your finger shifts in relation to the object, illustrating the horizontal disparity between the eyes.
Viewing devices that took advantage of stereopsis to create illusions of depth in images of natural scenes, architectural monuments and even pornography became immensely popular in Victorian drawing rooms. View-Master and Magic Eye are their familiar descendants available today.