These experiments were carried on in the presence of the definitely figured visual field of the lighted room, and the observers were conscious of taking these permanent features into account as 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. _C_ (50) - 8.02 11.82 9.47 - 48.14 48.14 9.52 _F_ (50) + 78.88 78.88 2.89 + 25.54 25.54 1.98 _G_ (50) - 22.56 24.64 6.58 -101.20 101.20 7.39 _H_ (50) - 83.84 83.84 11.78 -230.20 230.20 11.88 _J_ (50) +315.64 315.64 18.16 +120.12 120.12 9.01 Average: + 55.96 102.96 9.78 -44.98 104.84 7.96 Horizontal Planes. Observer. Const. Err. Av. Dev. Mean Var. _C_ (50) - 27.86 27.86 9.58 _G_ (50) - 73.84 73.84 7.59 _J_ (50) +243.72 243.72 18.52