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 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 are given in Table I. In the second set of experiments the central point which divided the open from the filled space touched the skin first, and then the others in various orders.  The object of this was to prevent fusion of the points, and, therefore, to enable the subject to pronounce his judgments more rapidly and confidently.  A record of these judgments is given in Table II.  In both of these series the filled space was always taken near the wrist and the open space in a straight line toward the elbow, on the volar side of the arm.  At present, I shall not undertake to give a complete interpretation of the results of these two tables, but simply call attention to two manifest tendencies in the figures.  First, it will be seen that the short filled distance of four centimeters is underestimated, but that the long filled distance is overestimated.  Second, in Table II., which represents the judgments when the contacts were made successively, the tendency to underestimate the short distance is less, and at the same time we notice a more pronounced overestimation of the longer filled distances.  I shall give a further explanation of these results in connection with later tables.