subjects were just reversed. For short distances the filled space was
underestimated, for long distances the filled space was overestimated.
A very plausible explanation for these anomalous results is again to
be found in the influence of the time factor. The open space seemed
longer while it was being traversed, but rapidly foreshortened after
it was left for the filled space. While on the other hand, if the
judgment was pronounced while the subject was still in the midst of
the filled space, it seemed shorter than it really was. The
combination of these two illusions is plainly again responsible for
the underestimation of the short filled spaces. The same double
illusion may be taken to explain the opposite tendency for the longer
distances.


IX.


The one generalization that I have thus far drawn from the
investigation--namely, that the optical illusions are not reversed in
passing from the field of touch, and that we therefore have a safe
warrant for the conclusion that sight and touch do function alike--has
contained no implicit or expressed assertion as to the origin of our
notion of space. I have now reached the point where I must venture an
explanation of the illusion itself.

The favorite hypothesis for the explanation of the geometrical optical
illusions is the movement theory. The most generally accepted
explanation of the illusion with whose tactual counterpart this paper
is concerned, is that given by Wundt.[15] Wundt's explanation rests on
variation in eye movements. When the eye passes over broken