The 3-D image is produced by simultaneously looking at two 2-D images printed side by side. The images are
not duplicates, but almost so. The observer looks at or through the
left-hand picture with the left-hand eye and the same with the right-hand
picture and eye. With sufficient time and effort , the observer fuses the
two images into one by properly diverging or converging and focusing the eyes. The
observer will then "see" the image in 3-D. Its a very satisfying skill, and
once learned, it is easy to put into practice.