Averages. I +95.19 95.19 13.75 " II +105.05 105.05 6.31 " III +106.78 106.78 7.89 The effect of introducing such an objective plane of reference is twofold: the mean variation is increased, and the plane of the subjective horizon is displaced downwards. First, then, it acts as a simple factor of disturbance; it distracts from those habitual adjustments upon which the accuracy of the judgment depends. Secondly, it enters as a source of constant error into the determination of the subjective horizon, which is attracted toward this new objective plane. In the third section of the table are given the results of judgments made in the presence of such a plane but without conscious reference to it.[2] The figures here are of intermediate value in the case of the mean variation and of slightly greater value than the first in that of the constant error. In other words, the introduction of such a plane cannot be wholly overlooked, though it may be greatly abstracted from. [2] In the preceding experiments the cord was definitely to be taken into account in making the judgment. The method of so doing was by running the eye back and forth over the cord preliminary to determining the location of the point. 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: TABLE XII. 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: TABLE XIII. Observer Const. Err. Av. Dev. Mean Var. Rotation. _F_ (20) +130.50 130.50 3.20 20° " " +115.50 115.50 1.10 50° _J_ (20) +443.10 443.10 9.47 45°