displacement involved being of relatively large amount.

In addition to the removal of the whole diversified visual field there
has now been eliminated the final point of regard toward which, in the
preceding set of experiments, the sight was strained; and the factor
of refined visual adjustment ceases longer to play a part in the
phenomenon. The result of this release is manifested in a tendency of
the eyes to turn unconsciously upward. This is their natural position
when closed in sleep. But this upward roll is not an uncomplicated
movement. There takes place at the same time a relaxation of binocular
convergence, which in sleep may be replaced by a slight divergence.
This tendency of the axes of vision to diverge as the eyes are raised
is undoubtedly connected biologically with the distribution of
distances in the higher and lower parts of the field of vision, of
which mention has already been made. Its persistence is taken
advantage of in the artificial device of assisting the process of
stereoscopic vision without instruments by holding the figures to be
viewed slightly above the primary position, so that the eyes must be
raised in order to look at them and their convergence thereby
decreased. It is by the concomitance of these two variables that the
phenomena of both this and the preceding series of experiments are to
be explained. In the present case the elimination of a fixed point of
regard is followed by a release of the mechanism of convergence, with
a consequent approximation to parallelism in the axes of vision and
its concomitant elevation of the line of sight.

The second fact to be noted is the reduction in amount of the mean
variation. The series of values under the three sets of experimental
conditions hitherto described is as follows: I. 7'.69; II. 31'.42;
III. 17'.16. This increase of regularity I take to be due, as in the