Mike Simmons wrote:The reason we don't see two more images is that they're outside the field of view. If the mirrors in the box were bigger, then we would see them. But the places we would see them are outside the box, so we can't.
Alternately, if the cup were moved closer to the box, you'd see all three images. But since the cup is far from the box, the left and right images are far from the center, off to the far left and far right respectively, where they can't be seen.