Astereoscope provides the illusion of depth in 2-d images by

A stereoscope uses two pictures from slightly different viewpoints in order to provide the illusion in 2-d images. The two slightly different pictures of the same object or scene will be seen by the left eye and the other picture will be seen by the right eye, and then when they are viewed in stereoscope, the two pictures were fused perceptually into single image containing depth.