attempt to prove in this part of my paper, first, that the former
experiments have not been made with the real counterpart of the
optical illusion; second, that the optical illusion can be quite
exactly reproduced on the skin; third, that where the objective
conditions are the same, the filled cutaneous space is overestimated,
and the illusion thus exists in the same sense for both sight and
touch.

Let me first call attention to some obvious criticisms on Parrish's
experiments. They were all made with one distance, namely, 6.4
centimeters; and on only one region, the forearm. Furthermore, in
these experiments no attempt was made to control the factor of
pressure by any mechanical device. The experimenter relied entirely on
the facility acquired by practice to give a uniform pressure to the
stimuli. The number of judgments is also relatively small. Again, the
open and filled spaces were always given successively. This, of
course, involves the comparison of a present impression with the
memory of a somewhat remote past impression, which difficulty can not
be completely obviated by simply reversing the order of presentation.
In the optical illusion, the two spaces are presented simultaneously,
and they lie adjacent to each other. It is still a debated question
whether this illusion would exist at all if the two spaces were not
given simultaneously and adjacent. Münsterberg[5] says of the optical
illusion for the open and filled spaces, "I have the decided
impression that the illusion does not arise from the fact of our
comparing one half with the other, but from the fact that we grasp the
line as a whole. As soon as an interval is inserted, so that the
perception of the whole line as constituted of two halves vanishes,
the illusion also disappears." This is an important consideration, to
which I shall return again.