| 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 with both eyes are compared with those made under conditions of monocular vision. The latter experiments were carried on in alternate series with those already described. The figures are given in the following tables:
TABLE VI.
JUDGMENTS MADE IN DIFFUSED LIGHT.
| Observer. | Constant Error. | Average Deviation. | Mean Variation. | |
|---|---|---|---|---|
| A | (50) | - 28.46 | 29.04 | 8.87 |
| C | " | + 7.54 | 14.86 | 8.01 |
| D | " | + 39.32 | 43.28 | 13.83 |
| E | " | + 50.46 | 65.26 | 9.86 |
| F | " | + 62.30 | 62.30 | 1.60 |
| G | " | 0.00 | 45.28 | 9.66 |
| H | " | + 22.92 | 79.12 | 5.07 |
| I | " | + 14.36 | 51.96 | 8.02 |
| K | " | + 9.26 | 38.10 | 9.55 |
| L | " | - 61.10 | 61.10 | 6.36 |
| Average: | + 11.66 | 49.03 | 8.18 |
TABLE VII.
JUDGMENTS IN ILLUMINATED POINT.
| Observer. | Constant Error. | Average Deviation. | Mean Variation. | |
|---|---|---|---|---|
| A | (50) | - 38.42 | 51.96 | 32.64 |
| C | (30) | - 29.03 | 41.23 | 35.75 |
| D | (20) | - 30.87 | 34.07 | 17.24 |
| E | (50) | + 65.30 | 75.86 | 29.98 |
| F | " | + 50.74 | 50.74 | 5.89 |
| G | " | + 66.38 | 88.10 | 44.98 |
| H | " | + 65.40 | 80.76 | 42.93 |
| I | " | - 0.02 | 80.22 | 47.53 |
| K | " | - 44.60 | 52.56 | 32.93 |
| L | " | - 71.06 | 73.30 | 31.86 |
| Average: | - 3.38 | 62.88 | 32.17 |