ong distances are practically impossible. Secondly, the
apparatus enabled me to control accurately the pressure of each point.
Thirdly, the contacts could be made simultaneously or successively
with much precision. This apparatus (Fig. 1) was planned and made in
the Harvard Laboratory, and was employed not only in our study of this
particular illusion, but also for the investigation of a number of
allied problems.

[Illustration: FIG. 1.]

Two aesthesiometers, A and B, were arranged in a framework, so that
uniform stimulations could be given on both arms. The aesthesiometers
were raised or lowered by means of the crank, C, and the cams, D and
E. The contacts were made either simultaneously or successively, with
any interval between them according to the position of the cams on the
crank. The height of the aesthesiometer could be conveniently adjusted
by the pins F and H. The shape of the cams was such that the descent
of the aesthesiometer was as uniform as the ascent, so that the
contacts were not made by a drop motion unless that was desired. The
sliding rules, of which there were several forms and lengths, could be
easily detached from the upright rods at _K_ and _L_. Each of the
points by which the contacts were made moved easily along the sliding
rule, and could be also raised or lowered for accommodation to the
unevenness of the surface of the skin. These latter were the most
valuable two features of the apparatus. There were two sets of points,
one of hard rubber, the other of metal. This enabled me to take into
account, to a certain extent, the factor of temperature. A wide range
of apparent differences in temperature was secured by employing these
two stimuli of such widely different conductivity. Then, as each point
was independent of the rest in its movements, its weight could also be
changed without affecting the rest.

In the first series of experiments I endeavored to reproduce for touch
the optical illusion in its exact form. There the open and the filled
spaces are adjacent to each other, and are presented simultaneously
for passive functioning of the eye, which is what concerns us here in
our search for the analogue of passive touch. This was by no means an
easy task, for obviously the open and the filled spaces in this
position on the skin could not be compared directly, owing to the lack
of uniformity in the sensibility of different portions of the skin. At
first, equivalents had to be established