whatever direction it takes, is certainly due to some sort of
qualitative differences in the two kinds of touch sensations, those
from the punctured, and those from the smooth half, it seemed
especially desirable to introduce as many changes into the nature of
the filling as possible. The punctured cards I found very
unsatisfactory, because they rapidly wear off, and thus change the
quality of the sensations, even from judgment to judgment.

[Illustration: FIG. 7.]

The first piece of apparatus that I used in the investigation of the
illusion for open and filled space with active touch is shown in Fig.
7. A thimble _A_, in which the finger was carried, moved freely along
the rod _B_. The filled spaces were produced by rows of tacks on the
roller _C_. By turning the roller, different kinds of fillings were
brought into contact with the finger-tip. The paper _D_, on which the
judgments were recorded by the subject, could be slowly advanced under
the roller _E_. Underneath the thimble carrier there was a pin so
arranged that, by a slight depression of the finger, a mark was made
on the record paper beneath. A typical judgment was made as follows;
the subject inserted his finger in the thimble, slightly depressed the
carrier to record the starting points, then brought his finger-tip
into contact with the first point in the filled space. The subject
was, of course, all the while ignorant of the length or character of
the filling over which he was about to pass. The finger-tip was then
drawn along the points, and out over the smooth surface of the roller,
until the open space passed over was judged equal to the filled space.
Another slight depression of the finger registered the judgment on the
paper below. The paper was then moved forward by turning the roller
_E_, and, if desired, a different row of pins was put in place for