correctives in making their judgments. Before proceeding, this defect
was remedied as far as possible by enclosing the apparatus of
experimentation, including the observer, between two walls of black
fabric. Nothing was to be seen but these two walls, and the inclined
plane which terminated the observer's view. The position of the screen
remained constant at an inclination of 45°. The upper bounding lines
of the enclosing walls, on the contrary, were adjusted in three
different relations to the plane of the gravity horizon. In the first
arrangement these lines were horizontal; in the second the ends next
to the observer were depressed five degrees; while in the final
arrangement these ends were elevated through a like angular distance.

The inclined position of the screen was of course observed by every
reactor, but of the changes in the enclosing walls no subject was
informed, and none discerned them on any occasion. Each observer was
questioned as to alterations in the experimental conditions after the
use of each arrangement, and at the close of the whole series inquiry
was made of each as to the planes of the upper boundaries of the
walls. On various occasions, but not customarily, the observer was
aware of a change of some kind in the whole set of conditions, but the
particular feature altered was not suspected. The results for all
three arrangements are given in the following table; of the sections
of this table the third is incomplete, full results having been
reached in the cases of only three observers:


TABLE XV.

Ascending Planes. Descending Planes.
Observer Const. Err. Av. Dev. M. Var. Const. Err. Av. Dev. M. V.