quantitative studies in this particular illusion. I made a number of
tests of the optical illusion, with these results: that the illusion
is strongest when the attention is fixed at about the middle of the
open space, that there is scarcely any illusion left when the
attention is fixed on the middle of the filled space. It is stronger
when the outer end-point of the open space is fixated than when the
outer end of the filled space is fixated. For the moving eye, I find
the illusion to be much stronger when the eye passes over the filled
space first, and then over the open space, than when the process is
reversed.

Now, the movement hypothesis does not, it seems to me, sufficiently
explain all the fluctuations in the illusion. My experiments with the
tactual illusion justify the belief that the movement theory is even
less adequate to explain all of the variations there, unless the
movement hypothesis is given a wider and richer interpretation than is
ordinarily given to it. In the explanation of the tactual illusion
which I have here been studying two other important factors must be
taken into consideration. These I shall call, for the sake of
convenience, the æsthetic factor and the time factor. These factors
should not, however, be regarded as independent of the factor of
movement. That term should be made wide enough to include these within
its meaning. The importance of the time factor in the illusion for
passive touch I have already briefly mentioned. I have also, in
several places in the course of my experiments, called attention to
the importance of the æsthetic element in our space judgments. I wish
now to consider these two factors more in detail.

The foregoing discussion has pointed to the view that the
space-perceiving and the localizing functions of the skin have a