The plane of vision in judgments made with the right eye alone is deflected upward from the true horizon to a greater degree than it is depressed below it in those made with binocular vision, the respective values of the constant errors being -7'.70 and +11'.66, a difference of 19'.36. When the field of vision is darkened except for the single illuminated disc, a similar reversion of sign takes place in the constant error. With binocular vision the plane of the subjective horizon is deflected downward through 36'.62 of arc; with monocular vision it is elevated 3'.38, a difference of 40'.00, or greater than in the case of judgments made in the lighted room by 20'.64. This increase is to be expected in consequence of the elimination of those corrective criteria which the figured visual field presents. The two eyes do not, of course, function separately in such a case, and the difference in the two sets of results is undoubtedly due to the influence of movements in the closed eye upon that which is open; or rather, to the difference in binocular functioning caused by shutting off the visual field from one eye. The former expression is justified in so far as we conceive that the tendency of the closed eye to turn slightly upward in its socket affects also the direction of regard in the open eye by attracting toward itself its plane of vision. But if, as has been pointed out, this elevation of the line of sight in the closed eye is accompanied by a characteristic change in the process of binocular convergence, the result cannot be interpreted as a simple sympathetic response in the open eye to changes taking place in that which is closed, but is the consequence of a release of convergence strain secondarily due to this act of closing the eye.
Several points of comparison between judgments made with binocular and with monocular vision remain to be stated. In general, the process of location is more uncertain when one eye only is used than when both are employed, but this loss in accuracy is very slight and in many cases disappears. The loss in accuracy is perhaps also indicated by the range of variation in the two cases, its limits being for binocular vision +46'.29 to -56'.70, and for monocular +62'.30 to -61'.10, an increase of 20'.41. In the darkened room similar relations are presented. The mean variations are as follows: binocular vision, 31'.42; monocular, 32'.17. Its limits in individual judgments are: binocular, -1'.62 to -128'.70, monocular, +66'.38 to -71'.06, an increase of 10'.36. In all ways, then, the difference in accuracy between the two forms of judgment is extremely small, and the conclusion may be drawn that those significant factors of judgment which are independent of the figuration of the visual field are not connected with the stereoscopic functioning of the two eyes, but such as are afforded by adjustment in the single eye and its results.
VI.
The experimental conditions were next complicated by the introduction of abnormal positions of the eyes, head and whole body. The results of tipping the chin sharply upward or downward and keeping it so fixed during the process of location are given in the following table, which is complete for only three observers:
TABLE VIII.
| Observer. | Upward Rotation. | Downward Rotation. | |||||
|---|---|---|---|---|---|---|---|
| C.E. | A.D. | M.V. | C.E. | A.D. | M.V. | ||
| L | (50) | +43.98 | 43.98 | 5.62 | +28.32 | 28.32 | 5.02 |
| K | (50) | -33.72 | 33.72 | 71.33 | +19.49 | 19.49 | 55.22 |
| L | (20) | -39.10 | 45.90 | 33.60 | -68.65 | 69.25 | 25.20 |
| Average: | - 9.61 | 41.20 | 36.85 | -19.94 | 39.02 | 28.48 | |
| Normal: | -64.14 | 67.08 | 33.51 | ||||
The results of rotating the whole body backward through forty-five and ninety degrees are given in the following table:
TABLE IX.
| Observer. | Rotation of 45°. | Rotation of 90°. | |||||
|---|---|---|---|---|---|---|---|
| C.E. | A.D. | M.V. | C.E. | A.D. | M.V. | ||
| B | (30) | + 4.10 | 24.57 | 18.56 | |||
| D | (30) | +291.03 | 291.03 | 61.86 | |||
| G | (50) | +266.78 | 266.78 | 22.83 | +200.16 | 200.16 | 11.00 |
| F | (60) | +116.45 | 116.45 | 17.14 | - 36.06 | 36.30 | 6.29 |
| J | (20) | +174.30 | 174.63 | 30.94 | |||
| Average: | +170.53 | 174.69 | 30.66 | ||||
The errors which appear in these tables are not consistently of the type presented in the well-known rotation of visual planes subjectively determined under conditions of abnormal relations of the head or body in space. When the head is rotated upward on its lateral horizontal axis the average location of the subjective horizon, though still depressed below the true objective, is higher than when rotation takes place in the opposite direction. When the whole body is rotated backward through 45° a positive displacement of large amount takes place in the case of all observers. When the rotation extends to 90°, the body now reclining horizontally but with the head supported in a raised position to allow of free vision, an upward displacement occurs in the case of one of the two observers, and in that of the other a displacement in the opposite direction. When change of position takes place in the head only, the mean variation is decidedly greater if the rotation be upward than if it be downward, its value in the former case being above, in the latter below that of the normal. When the whole body is rotated backward through 45° the mean variation is but slightly greater than under normal conditions; when the rotation is through 90° it is much less. A part of this reduction is probably due to training. In general, it may be said that the disturbance of the normal body relations affects the location of the subjective horizon, but the specific nature and extent of this influence is left obscure by these experiments. The ordinary movements of eyes and head are largely independent of one another, and even when closed the movements of the eyes do not always symmetrically follow those of the head. The variations in the two processes have been measured by Münsterberg and Campbell[1] in reference to a single condition, namely, the relation of attention to and interest in the objects observed to the direction of sight in the closed eyes after movement of the head. But apart from the influence of such secondary elements of ideational origin, there is reason to believe that the mere movement of the head from its normal position on the shoulders up or down, to one side or the other, is accompanied by compensatory motion of the eyes in an opposite direction, which tends to keep the axis of vision nearer to the primary position. When the chin is elevated or depressed, this negative reflex adjustment is more pronounced and constant than when the movement is from side to side. In the majority of cases the retrograde movement of the eyes does not equal the head movement in extent, especially if the latter be extreme.