comparison of the two classes of illusions, with this question in
view, appears therefore in the present state of divergent opinion to
be a needed contribution to experimental psychology. Such an
experimental study, if it succeeds in finding the solution to this
debate, ought to throw some further light upon the question of the
origin of our idea of space, as well as upon the subject of illusions
of sense in general. For, on the one hand, if touch and sight function
alike in our judgment of space, we should expect that like peripheral
disturbances in the two senses would cause like central errors in
judgment, and every tactual analogue of an optical illusion should be
found to correspond both in the direction of the error and, to a
certain extent, quantitatively with the optical illusion. But if, on
the other hand, they are in their origin and in their developed state
really disparate senses, each guided by a different psychological
principle, the illusion in the one sense might well be the reverse of
the corresponding illusion in the other sense. Therefore, if the
results of an empirical study should furnish evidence that the
illusions are reversed in passing from one field to the other, we
should be obliged to conclude that we are here in the presence of what
psychologists have been content to call the 'unanalyzable fact' that
the two senses function differently under the same objective
conditions. But if, on the contrary, it should turn out that the
illusions are not reversed for the two senses, then the theory of the
ultimate uniformity of the psychical laws will have received an
important defence.

These experiments were carried on in the Harvard Psychological
Laboratory during the greater part of the years 1898-1901. In all,
fifteen subjects coöperated in the work at different times.