The single cord was next replaced by a plane of blackened wood six inches wide and extending from the observer to the vertical screen. This strip was arranged in two ways: first, from the observer’s chin to the bottom of the screen, and secondly, from the feet of the observer to a point on the screen a short distance below the plane of the objective horizon. The individual and average results are given in the following table:
Observer. Descending Plane. Ascending Plane.
A. (10) +18.80 18.80 5.24 +35.10 35.10 8.27 E. (20) +79.30 79.30 11.56 +131.67 131.67 12.07 H. (10) -37.50 37.50 16.80 -46.90 46.90 7.90 K. (30) +71.40 71.40 12.85 +48.05 48.05 5.11 Average: +33.00 51.75 11.61 +41.95 65.43 8.34
The introduction of a descending plane lowers the apparent horizon; that of an ascending plane elevates it. The general disturbance of judgment appears distinctly greater in the case of a downward than in that of an upward incline.
The results of a third variation of the experimental conditions may be presented at once. In it the location of the subjective horizon under normal conditions was compared with the results of adjustments made when the screen bearing the white disc was rotated backward from the observer through an angle of varying magnitude. The averages for each of the two subjects are as follows:
Observer Const. Err. Av. Dev. Mean Var. Rotation. F (20) +130.50 130.50 3.20 20 deg. " " +115.50 115.50 1.10 50 deg. J (20) +443.10 443.10 9.47 45 deg.
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 deg.. 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.