The individual and general averages for this set of experiments are given in the following table:

TABLE III.

Observer. Constant Error. Average Deviation. Mean Variation.

_A_ (50) + 7.75 20.07 19.45 _C_ " + 14.41 25.05 2.94 _D_ " + 14.42 34.54 29.16 _E_ " +108.97 108.97 23.13 _F_ " - 5.12 23.00 2.02 _G_ " + 20.72 34.80 10.23 _H_ " + 35.07 53.60 33.95 _I_ " + 25.52 30.68 22.49 _K_ " - 8.50 40.65 21.07

Average: + 23.69 41.26 17.16

The point at which the eyes rest when seeking the plane of the horizon in total darkness is above its actual position, the positive displacement involved being of relatively large amount.

In addition to the removal of the whole diversified visual field there has now been eliminated the final point of regard toward which, in the preceding set of experiments, the sight was strained; and the factor of refined visual adjustment ceases longer to play a part in the phenomenon. The result of this release is manifested in a tendency of the eyes to turn unconsciously upward. This is their natural position when closed in sleep. But this upward roll is not an uncomplicated movement. There takes place at the same time a relaxation of binocular convergence, which in sleep may be replaced by a slight divergence.

This tendency of the axes of vision to diverge as the eyes are raised is undoubtedly connected biologically with the distribution of distances in the higher and lower parts of the field of vision, of which mention has already been made. Its persistence is taken advantage of in the artificial device of a.s.sisting the process of stereoscopic vision without instruments by holding the figures to be viewed slightly above the primary position, so that the eyes must be raised in order to look at them and their convergence thereby decreased. It is by the concomitance of these two variables that the phenomena of both this and the preceding series of experiments are to be explained. In the present case the elimination of a fixed point of regard is followed by a release of the mechanism of convergence, with a consequent approximation to parallelism in the axes of vision and its concomitant elevation of the line of sight.

The second fact to be noted is the reduction in amount of the mean variation. The series of values under the three sets of experimental conditions. .h.i.therto described is as follows: I. 7'.69; II. 31'.42; III. 17'.16. This increase of regularity I take to be due, as in the case of the lighted room, to the presence of a factor of constancy which is not strictly an element in the judgment of horizontality.

This is a system of sensory data, which in the former case were transient--the vision of familiar objects; and in the latter resident--the recognition of specific experiences of strain in the mechanism of the eye. The latter sensations exist under all three sets of conditions, but they are of secondary importance in those cases which include the presence of an objective point of regard, while in the case of judgments made in total darkness the observer depends solely upon resident experiences. Attention is thus directed specifically toward these immediate sensational elements of judgment, and there arises a tendency to reproduce the preceding set of eye-strains, instead of determining the horizon plane afresh at each act of judgment upon more general data of body position.

If the act of judgment be based chiefly upon sensory data connected with the reinstatement of the preceding set of strains, progressions should appear in these series of judgments, provided a constant factor of error be incorporated in the process. This deflection should be most marked under conditions of complete darkness, least in the midst of full illumination. Such a progression would be shown at once by the distribution of positive and negative values of the individual judgments about the indifference point of constant error. As instances of its occurrence all cases have been counted in which the first half of the series 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 a.n.a.lyzed simultaneously with reference to conditions of light and darkness and to binocular and monocular vision respectively. Their quant.i.tative 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 p.r.o.nounced. 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.

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

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 s.p.a.ce. 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 Munsterberg 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 p.r.o.nounced 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.

[1] Munsterberg, H., and Campbell, W.W.: PSYCHOLOGICAL REVIEW, I., 1894, p. 441.