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 between two collinear open
spaces for the particular region of the skin tested. Three points were
taken in a line, and one of the end points was moved until the two
adjacent open spaces were pronounced equal. Then one of the spaces was
filled, and the process of finding another open space equivalent to
this filled space was repeated as before. This finding of two
equivalent open spaces was repeated at frequent intervals. It was
found unsafe to determine an equivalent at the beginning of each
sitting to be used throughout the hour.

Two sets of experiments were made with the illusion in this form. In
one the contacts were made simultaneously; the results of this series