of ten judgments was uniformly of one sign (four to six being counted as half) and the second half of the opposite sign.  The percentages of cases in which the series presented such a progression are as follows:  In diffused light, 7.6%; in darkness, point of regard illuminated, 18.3%; in complete darkness, 26.1%.  The element of constant error upon which such progressions depend is the tendency of the eye to come to rest under determinate mechanical conditions of equilibrium of muscular strain.

The relation of the successive judgments of a series to the reinstatement of specific eye-strains and to the presence of an error of constant tendency becomes clearer when the distribution of those series which show progression is analyzed simultaneously with reference to conditions of light and darkness and to binocular and monocular vision respectively.  Their quantitative relations are presented in the following table: 

TABLE IV.

  Illumination.  Per Cent.  Showing Progress.  Binocular.  Monocular.

In light.                 7.6 %                 50 %        50 %
In darkness.             18.3                   34.2        65.8

Among judgments made in daylight those series which present progression are equally distributed between binocular and monocular vision.  When, however, the determinations are of a luminous point in an otherwise dark field, the preponderance in monocular vision of the tendency to a progression becomes pronounced.  That this is not a progressive rectification of the judgment, is made evident by the distribution of the directions of change in the several experimental conditions shown in the following table: 

TABLE V.
                                Light.  Darkness. 
  Direction of Change.  Binocular.  Monocular.  Binocular.  Monocular. 
  Upward. 50 % 100 % 38.4 % 65.0 %
  Downward. 50 00.0 61.6 35.0
  Const.  Err. -7.70 +11.66 -36.62 -3.38

When the visual field is illuminated the occurrence of progression in binocular vision is accidental, the percentages being equally distributed between upward and downward directions.  In monocular vision, on the contrary, the movement is uniformly upward and involves a progressive increase in error.  When the illuminated point is exposed in an otherwise dark field the progression is preponderatingly downward in binocular vision and upward in vision with the single eye.  The relation of these changes to phenomena of convergence, and the tendency to upward rotation in the eyeball has already been stated.  There is indicated, then, in these figures the complication of the process of relocating the ideal horizon by reference to the sense of general body position with tendencies to reinstate simply the set of eye-muscle strains which accompanied the preceding judgment, and the progressive distortion of the latter by a factor of constant error due to the mechanical conditions of muscular equilibrium in the resting eye.