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.


IV.


The influence of this factor is also exhibited when judgments made