III.

In the next set of experiments the room was made completely dark. The method of experimentation was adapted to these new conditions by substituting for the wooden screen one of black-surfaced cardboard, which was perforated at vertical distances of five millimeters by narrow horizontal slits and circular holes alternately, making a scale which was distinctly readable at the distance of the observer. Opposite the end of one of these slits an additional hole was punched, constituting a fixed point from which distances were reckoned on the scale. As the whole screen was movable vertically and the observer knew that displacements were made from time to time, the succession of judgments afforded no objective criterion of the range of variation in the series of determinations, nor of the relation of any individual reaction to the preceding. The method of experimentation was as follows: The observer sat as before facing the screen, the direction of which was given at the beginning of each series by a momentary illumination of the scale. In the darkness which followed the observer brought the direction of sight, with open eyes, as satisfactorily as might be into the plane of the horizontal, when, upon a simple signal, the perforated scale was instantly and noiselessly illuminated by the pressure of an electrical button, and the location of the point of regard was read off the vertical scale by the observer himself, in terms of its distance from the fixed point of origin described above. 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 assisting 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 hitherto 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 analyzed simultaneously with reference to conditions of light and darkness and to binocular and monocular vision respectively. Their quantitative 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 pronounced. 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.