If the finger be held vertically in front of the eyes and a distant object be looked at, it is well known that two images of the finger will be seen, quite transparent if the two images do not overlap, and opaque only at overlapping parts. In place of the finger, use a thin rod at a distance of some 6 or more feet, and focus on another vertical rod at a distance of 30 feet or more, so that the second rod is seen between the two images of the first. The apparent position of the second rod will be found to be distinctly nearer than its real position.