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:
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