dependence upon the location of this imaginary point the determination
of the position of the white disc was made, and the plane of
perspective positively or negatively rotated.

Why such perspective lines should enter into the process of judgment
it is not difficult to infer. The plane of perspective for human
beings is characteristically horizontal, in consequence of the
distribution of important objects within the field of visual
perception. Roughly, the belt of the earth's horizon contains the loci
of all human perspective planes. Both natural and artificial
arrangements of lines converge there. The systems of visual objects on
the earth and in the sky are there broken sharply off in virtue of
their practically vast differences in quality and significance for the
observer. The latter perspective probably never extends downward
illusorily to points on the earth's surface; and the former system of
objects is carried continuously upward to skyey points only on
relatively rare occasions, as when one mistakes clouds for mountains
or the upper edge of a fog-belt on the horizon for the rim of sea and
sky. The point of convergence of the fundamental lines of perspective
thus becomes assimilated with the idea of the visual horizon, as that
concept has fused with the notion of a subjective horizon. There can
be little doubt that the disposition of such lines enters constantly
into our bodily orientation in space along with sensations arising
from the general body position and from those organs more specially
concerned with the static sense.

Upon the misinterpretation of such objective planes depends the
illusion of underestimation of the height or incline of a hill one is
breasting, and of the converse overestimation of one seen across a
descending slope or intervening valley. The latter illusion is