especially striking, and in driving over forest roads (in which case
the correction of a wider range of view is excluded) the stretch of
level ground at the foot of a hill one is descending is constantly
mistaken for an opposing rise. This illusion is put into picturesque
words by Stevenson when he describes the world, seen from the summit
of a mountain upon which one stands, as rising about him on every side
as toward the rim of a great cup. The fitness of the image may be
proved by climbing the nearest hill. In all such cases a
reconstruction of the sensory data of judgment takes place, in which
the most significant factor is the plane determined by the positions
of the observing eye and the perspective focus. In these judgments of
spatial relationship, as they follow one another from moment to
moment, this plane becomes a temporary subjective horizon, and
according as it is positively or negatively rotated do corresponding
illusions of perception appear.

* * * * *




THE ILLUSION OF RESOLUTION-STRIPES ON THE COLOR-WHEEL.

BY EDWIN B. HOLT.


If a small rod is passed slowly before a rotating disc composed of two
differently colored sectors, the rod appears to leave behind it on the
disc a number of parallel bands of about the width of the rod and of
about the colors, alternately arranged, of the two sectors. These