THE STEREOSCOPE 49
the man be moved (without changing his posture) so that the
parallax is diminished by 50 percent, he will appear to be 40 feet
away and to be about 12 feet tall. If the motion is in a direction
to increase the parallax by 50 percent he will appear to be 15 feet
distant and to have a height of approximately four and a half feet.
This principle was once widely used in making fantastic stereo-
grams such as a dancing girl inside a wine bottle, and the like;
and is again becoming popular among those who like to experi-
ment with this so-called Space Control.
Size Indefinite.-Our perception of absolute size is most un-
reliable. The reason the controlled parallax just described is op-
erable is that the parallax definitely places the image at a fixed
distance, and we estimate the size by comparison with that of
known objects at the same distance. Thus when the image plane
is artificially moved nearer, by increased parallax we still have the
image size as originally diminished by perspective, so the object
Fig. 3-12. One of the most notable of stereo illustrated books, with
231 three-dimensional pictures in full color.