second, this underestimation of the filled space gradually decreases
until in the case of the filled distance of 18 cm. the judgments pass
over into pronounced overestimations; third, an increase in the number
of points of contact in the shorter distances increases the
underestimation, while an increase in the number of points in the
longer distance increases the overestimation; fourth, an increase of
pressure causes an invariable increase in the apparent length of
space. If a general average were made of the results given in Tables
IV., V., VI., VII. and VIII., there would be a preponderance of
evidence for the conclusion that the filled spaces are overestimated.
But we cannot ignore the marked tendencies in the opposite direction
for the long and the short distances. These anomalous results, which,
it will be remembered, were also found in our first series, call for
explanation. Several hypotheses were framed to explain these
fluctuations in the illusion, and then some shorter series of
experiments were made in different directions with as large a number
of variations in the conditions as possible, in the hope of
discovering the disturbing factors.


TABLE IV.¹

4 Centimeters.

A B D E
less = gr. less = gr. less = gr. less = gr.
R. (a) 7 2 1 8 1 1 6 2 2 5 1 4
(b) 7 3 0 7 1 2 6 2 2 6 1 3
F. (a) 6 3 1 7 1 2 7 0 3 6 0 4
(b) 7 0 3 9 1 0 6 1 3 5 2 3