The origin of such compensatory reactions is connected with the permanent relations of the whole bodily organism to the important objects which surround it. The relations of the body to the landscape are fairly fixed. The objects which it is important to watch lie in a belt which is roughly on a horizontal plane with the observing eye.

They move or are moved about over the surface of the ground and do not undergo any large vertical displacement. It is of high importance, therefore, that the eye should be capable of continuous observation of such objects through facile response to the stimulus of their visual appearance and movements, in independence of the orientation of the head. There are no such determinate spatial relations between body position and the world of important visual objects in the case of those animals which are immersed in a free medium; and in the organization of the fish and the bird, therefore, one should not expect the development of such free sensory reflexes of the eye in independence of head movements as we know to be characteristic of the higher land vertebrates. In both of the former types the eye is fixed in its socket, movements of the whole head or body becoming the mechanism of adjustment to new objects of observation. In the adjustment of the human eye the reflex determination through sensory stimuli is so facile as to counteract all ordinary movements of the head, the gaze remaining fixed upon the object through a series of minute and rapidly repeated sensory reflexes. When the eyes are closed and no such visual stimuli are presented, similar reflexes take place in response to the movements of the head, mediated possibly by sensations connected with changes in position of the planes of the semicircular ca.n.a.ls.

VII.

If eye-strain be a significant element in the process of determining the subjective horizon, the induction of a new center of muscular equilibrium by training the eyes to become accustomed to unusual positions should result in the appearance of characteristic errors of displacement. In the case of two observers, _A_ and _H_, the eyes were sharply raised or lowered for eight seconds before giving judgment as to the position of the illuminated spot, which was exposed at the moment when the eyes were brought back to the primary position. The effect of any such vertical rotation is to stretch the antagonistic set of muscles. It follows that when the eye is rotated in the contrary direction the condition of equilibrium appears sooner than in normal vision. In the case of both observers the subjective horizon was located higher when judgment was made after keeping the eyes raised, and lower when the line of sight had been depressed. In the case of only one observer was a quant.i.tative estimation of the error made, as follows: With preliminary raising of the eyes the location was +36'.4; with preliminary lowering, -11'.4.

When the illuminated b.u.t.ton is exposed in a darkened room and is fixated by the observer, it undergoes a variety of changes in apparent position due to unconscious shifting of the point of regard, the change in local relations of the retinal stimulation being erroneously attributed to movements in the object. These movements were not of frequent enough occurrence to form the basis of conclusions as to the position at which the eyes tended to come to a state of rest. The number reported was forty-two, and the movement observed was rather a wandering than an approximation toward a definite position of equilibrium. The spot very rarely presented the appearance of sidewise floating, but this may have been the result of a preconception on the part of the observer rather than an indication of a lessened liability to movements in a horizontal plane. Objective movements in the latter direction the observer knew to be impossible, while vertical displacements were expected. Any violent movement of the head or eyes dispelled the impression of floating at once. The phenomenon appeared only when the illuminated spot had been fixated for an appreciable period of time. Its occurrence appears to be due to a fatigue process in consequence of which the mechanism becomes insensible to slight changes resulting from releases among the tensions upon which constant fixation depends. When the insensitiveness of fatigue is avoided by a slow continuous change in the position of the illuminated spot, no such wandering of the eye from its original point of regard occurs, and the spot does not float. The rate at which such objective movements may take place without awareness on the part of the observer is surprisingly great. Here the fatigue due to sustained fixation is obviated by the series of rapid and slight sensory reflexes which take place; these have the effect of keeping unchanged the retinal relations of the image cast by the illuminated spot, and being undiscriminated in the consciousness of the observer the position of the point of regard is apprehended by him as stationary. The biological importance of such facile and unconscious adjustment of the mechanism of vision to the moving object needs no emphasis; but the relation of these obscure movements of the eyes to the process of determining the plane of the subjective horizon should be pointed out.

The sense of horizontality in the axes of vision is a transient experience, inner conviction being at its highest in the first moments of perception and declining so characteristically from this maximum that in almost every case the individual judgment long dwelt upon is unsatisfactory to the observer. This change I conceive to be a secondary phenomenon due to the appearance of the visual wanderings already described.

VIII.

The influence of sensory reflexes in the eye upon the process of visual orientation was next taken up in connection with two specific types of stimulation. At top and bottom of the vertical screen were arranged dark lanterns consisting of electric bulbs enclosed in blackened boxes, the fronts of which were covered with a series of sheets of white tissue-paper, by which the light was decentralized and reduced in intensity, and of blue gla.s.s, by which the yellow quality of the light was neutralized. Either of these lanterns could be illuminated at will by the pressure of a b.u.t.ton. All other experimental conditions remained unchanged. The observers were directed to pay no special regard to these lights, and the reports show that in almost every case they had no conscious relation to the judgment. The results are presented in the following table:

TABLE X.

Light Below. Light Above.

Observer. Const.Err. Av.Dev. M.Var. Const.Err. Av.Dev. M.Var.

_C_ (40) +156.37 156.37 19.67 +169.85 169.85 19.22 _D_ (20) + 39.30 43.30 17.95 + 46.65 47.35 15.41 _F_ (30) + 19.47 19.47 8.83 + 58.37 58.37 7.83 _G_ (50) + 66.11 112.76 14.65 +117.86 117.86 13.10 _H_ (30) -147.63 147.63 21.07 -105.30 105.30 30.31 _J_ (20) + 1.90 31.95 22.33 + 44.40 44.40 20.55 Average: + 22.59 85.28 17.42 + 55.30 90.52 17.74

The eye is uniformly attracted toward the light and the location of the disk correspondingly elevated or depressed. The amount of displacement which appears is relatively large. It will be found to vary with the intensity, extent and distance of the illuminated surfaces introduced. There can be little doubt that the practical judgments of life are likewise affected by the distribution of light intensities, and possibly also of significant objects, above and below the horizon belt. Every brilliant object attracts the eye toward itself; and the horizon beneath a low sun or moon will be found to be located higher than in a clouded sky. The upper half of the ordinary field of view--the clear sky--is undiversified and unimportant; the lower half is full of objects and has significance. We should probably be right in attributing to these characteristic differences a share in the production of the negative error of judgment which appears in judgments made in daylight. The introduction of such supplementary stimuli appears to have little effect upon the regularity of the series of judgments, the values of the mean variations being relatively low: 17'.42 with light below, 17'.74 with it above.

IX.

In the final series of experiments the influence of limiting visual planes upon the determination of the subjective horizon was taken up.

It had been noticed by Dr. Munsterberg in the course of travel in hill country that a curious negative displacement of the subjective horizon took place when one looked across a downward slope to a distant cliff, the alt.i.tude (in relation to the observer's own standpoint) of specific points on the wall of rock being largely overestimated.

Attributing the illusion to a reconstruction of the sensory data upon an erroneous interpretation of the objective relations of the temporary plane of the landscape, Dr. Munsterberg later made a series of rough experiments by stretching an inclined cord from the eye downward to a lower point on an opposite wall and estimating the height above its termination of that point which appeared to be on a level with the observing eye. He found an illusion present similar to the case of an extended slope of country.

The first experiments of this group repeated those just described. The previous mechanical conditions were varied only by the introduction of a slender cord which was stretched from just below the eyes to the bottom of the vertical screen. Full results were obtained from only two observers, which are given in the following table:

TABLE XI.

Observer. Const. Err. Av. Dev. Mean Var. Exp. Conds.

_C_ (30) +123.92 123.92 11.94 Cord present and _G_ (30) +66.47 66.47 15.56 consciously referred to.

_C_ (30) +126.90 126.90 6.31 Cord not present.

_G_ (30) +83.20 83.20 6.31 _C_ (30) +126.93 126.93 6.39 Cord present but not _G_ (30) +86.63 86.63 9.40 consciously referred to.

Averages. I +95.19 95.19 13.75 " II +105.05 105.05 6.31 " III +106.78 106.78 7.89

The effect of introducing such an objective plane of reference is twofold: the mean variation is increased, and the plane of the subjective horizon is displaced downwards. First, then, it acts as a simple factor of disturbance; it distracts from those habitual adjustments upon which the accuracy of the judgment depends. Secondly, it enters as a source of constant error into the determination of the subjective horizon, which is attracted toward this new objective plane. In the third section of the table are given the results of judgments made in the presence of such a plane but without conscious reference to it.[2] The figures here are of intermediate value in the case of the mean variation and of slightly greater value than the first in that of the constant error. In other words, the introduction of such a plane cannot be wholly overlooked, though it may be greatly abstracted from.

[2] In the preceding experiments the cord was definitely to be taken into account in making the judgment. The method of so doing was by running the eye back and forth over the cord preliminary to determining the location of the point.

The single cord was next replaced by a plane of blackened wood six inches wide and extending from the observer to the vertical screen.

This strip was arranged in two ways: first, from the observer's chin to the bottom of the screen, and secondly, from the feet of the observer to a point on the screen a short distance below the plane of the objective horizon. The individual and average results are given in the following table:

TABLE XII.

Observer. Descending Plane. Ascending Plane.

_A._ (10) +18.80 18.80 5.24 +35.10 35.10 8.27 _E._ (20) +79.30 79.30 11.56 +131.67 131.67 12.07 _H._ (10) -37.50 37.50 16.80 -46.90 46.90 7.90 _K._ (30) +71.40 71.40 12.85 +48.05 48.05 5.11 Average: +33.00 51.75 11.61 +41.95 65.43 8.34

The introduction of a descending plane lowers the apparent horizon; that of an ascending plane elevates it. The general disturbance of judgment appears distinctly greater in the case of a downward than in that of an upward incline.

The results of a third variation of the experimental conditions may be presented at once. In it the location of the subjective horizon under normal conditions was compared with the results of adjustments made when the screen bearing the white disc was rotated backward from the observer through an angle of varying magnitude. The averages for each of the two subjects are as follows:

TABLE XIII.

Observer Const. Err. Av. Dev. Mean Var. Rotation.

_F_ (20) +130.50 130.50 3.20 20 " " +115.50 115.50 1.10 50 _J_ (20) +443.10 443.10 9.47 45

These experiments were carried on in the presence of the definitely figured visual field of the lighted room, and the observers were conscious of taking these permanent features into account as correctives in making their judgments. Before proceeding, this defect was remedied as far as possible by enclosing the apparatus of experimentation, including the observer, between two walls of black fabric. Nothing was to be seen but these two walls, and the inclined plane which terminated the observer's view. The position of the screen remained constant at an inclination of 45. The upper bounding lines of the enclosing walls, on the contrary, were adjusted in three different relations to the plane of the gravity horizon. In the first arrangement these lines were horizontal; in the second the ends next to the observer were depressed five degrees; while in the final arrangement these ends were elevated through a like angular distance.

The inclined position of the screen was of course observed by every reactor, but of the changes in the enclosing walls no subject was informed, and none discerned them on any occasion. Each observer was questioned as to alterations in the experimental conditions after the use of each arrangement, and at the close of the whole series inquiry was made of each as to the planes of the upper boundaries of the walls. On various occasions, but not customarily, the observer was aware of a change of some kind in the whole set of conditions, but the particular feature altered was not suspected. The results for all three arrangements are given in the following table; of the sections of this table the third is incomplete, full results having been reached in the cases of only three observers:

TABLE XV.

Ascending Planes. Descending Planes.

Observer Const. Err. Av. Dev. M. Var. Const. Err. Av. Dev. M. V.

_C_ (50) - 8.02 11.82 9.47 - 48.14 48.14 9.52 _F_ (50) + 78.88 78.88 2.89 + 25.54 25.54 1.98 _G_ (50) - 22.56 24.64 6.58 -101.20 101.20 7.39 _H_ (50) - 83.84 83.84 11.78 -230.20 230.20 11.88 _J_ (50) +315.64 315.64 18.16 +120.12 120.12 9.01 Average: + 55.96 102.96 9.78 -44.98 104.84 7.96

Horizontal Planes.

Observer. Const. Err. Av. Dev. Mean Var.

_C_ (50) - 27.86 27.86 9.58 _G_ (50) - 73.84 73.84 7.59 _J_ (50) +243.72 243.72 18.52

For every individual observer, the position of the disc on the screen has been affected by each change in the direction of these visible lines. In every case, also, its location when these boundaries lay in a horizontal plane was intermediate between the other two. The importance of such relations in the objects of the visual field as factors in our ordinary determination of the subjective horizon is made evident by these experimental results. They become construction lines having a.s.sumed permanence in the world of visual-motor experience. The conception of unchanging spatial relations in the fundamental lines of perspective vision receives constant reinforcement from the facts of daily experience. The influence of the above-described changes in experimental conditions is mediated through their effect upon the location of the focus of the limiting and perspective lines of vision. As the plane of the upper boundaries of the enclosing walls was elevated and depressed the intersection of the two systems of lines was correspondingly raised and lowered, and in dependence upon the location of this imaginary point the determination of the position of the white disc was made, and the plane of perspective positively or negatively rotated.

Why such perspective lines should enter into the process of judgment it is not difficult to infer. The plane of perspective for human beings is characteristically horizontal, in consequence of the distribution of important objects within the field of visual perception. Roughly, the belt of the earth's horizon contains the loci of all human perspective planes. Both natural and artificial arrangements of lines converge there. The systems of visual objects on the earth and in the sky are there broken sharply off in virtue of their practically vast differences in quality and significance for the observer. The latter perspective probably never extends downward illusorily to points on the earth's surface; and the former system of objects is carried continuously upward to skyey points only on relatively rare occasions, as when one mistakes clouds for mountains or the upper edge of a fog-belt on the horizon for the rim of sea and sky. The point of convergence of the fundamental lines of perspective thus becomes a.s.similated with the idea of the visual horizon, as that concept has fused with the notion of a subjective horizon. There can be little doubt that the disposition of such lines enters constantly into our bodily orientation in s.p.a.ce along with sensations arising from the general body position and from those organs more specially concerned with the static sense.

Upon the misinterpretation of such objective planes depends the illusion of underestimation of the height or incline of a hill one is breasting, and of the converse overestimation of one seen across a descending slope or intervening valley. The latter illusion is especially striking, and in driving over forest roads (in which case the correction of a wider range of view is excluded) the stretch of level ground at the foot of a hill one is descending is constantly mistaken for an opposing rise. This illusion is put into picturesque words by Stevenson when he describes the world, seen from the summit of a mountain upon which one stands, as rising about him on every side as toward the rim of a great cup. The fitness of the image may be proved by climbing the nearest hill. In all such cases a reconstruction of the sensory data of judgment takes place, in which the most significant factor is the plane determined by the positions of the observing eye and the perspective focus. In these judgments of spatial relationship, as they follow one another from moment to moment, this plane becomes a temporary subjective horizon, and according as it is positively or negatively rotated do corresponding illusions of perception appear.