On the whole, then, I feel quite sure in concluding that the
overestimation of the filled cutaneous spaces is not traceable to the
influence of visualization. Parrish has explained all sporadic cases
of overestimation as due to the optical illusion carried over in
visualization. I have already shown that in my experiments
visualization has really the opposite effect. In Parrish's experiments
the overestimation occurred in the case of those collections of points
which were so arranged as to allow the greatest differentiation among
the points, and especially where the end-points were more or less
distinct from the rest. This, according to my theory, is precisely
what one would expect.

Those who have made quantitative studies in the optical illusion,
especially in this particular illusion for open and filled spaces,
have observed and commented on the instability of the illusion.
Auerbach[11] says, in his investigation of the quantitative variations
of the illusion, that concentration of attention diminishes the
illusion. In the Zöllner figure, for instance, I have been able to
notice the illusion fluctuate through a wide range, without
eye-movements and without definitely attending to any point, during
the fluctuation of the attention. My experiments with the tactual
illusion have led me to the conclusion that it fluctuates even more
than the optical illusion. Any deliberation in the judgment causes the
apparent size of the filled space to shrink. The judgments that are
given most rapidly and naïvely exhibit the strongest tendency to
overestimation; and yet these judgments are so consistent as to
exclude them from the category of guesses.

[11] Auerbach, F., _Zeitsch. f. Psych. u. Phys. d.
Sinnesorgane_, 1874, Bd. VII., S. 152.