Before passing to the experiments in which the open spaces were
presented first, I wish to offer an explanation for the divergent
tendencies that were exhibited through all the experiments of the last
two sections, namely, that the short filled spaces are overestimated
and the long spaces underestimated. Let us take two typical judgments,
one in which a filled space of 3 cm. is judged equal to an open space
of 4.2 cm., and then one in which the filled space is 9 cm., and is
judged equal to an open space of 7.4 cm. In the case of the shorter
distance, because of its shortness, after the finger leaves it, it is
held in a present state of consciousness for some moments, and does
not suffer the foreshortening that comes from pastness. This is,
however, only a part of the reason for its overestimation. After the
finger-tip has left the filled space, and while it is traversing the
first part of the open space, there is a dearth of sensations. The
tactual sensations are meager and faint, and muscular tensions have
not yet had time to arise. It is not until the finger has passed over
several centimeters of the distance, that the surprise of its
barrenness sets up the organic sensations of muscular strain. One
subject remarked naïvely at the end of some experiments of this kind,
that the process of judging was an easy and comfortable affair so long
as he was passing over the filled space, but when he set out upon the
open space he had to pay far more strict attention to the experiment.

By a careful introspection of the processes in my own case, I came to
the conclusion that it is certainly a combination of these two
illusions that causes the overestimation of the short filled
distances. In the case of the long distances, the underestimation of
the filled space is, I think, again due to a combination of two
illusions. When the finger-tip leaves the filled space, part of it,