the opposite side of the screen from the observer was mounted a
vertical scale graduated in millimeters, over which passed a light
index-point attached to the silk cord, by means of which the position
of the cardboard disc in front was read off. The observer was seated
in an adjustable chair with chin and head rests, and a lateral
sighting-tube by which the position of the eyeball could be vertically
and horizontally aligned. The distance from the center of the eyeball
to the surface of the screen opposite was so arranged that, neglecting
the radial deflection, a displacement of 1 mm. in either direction was
equal to a departure of one minute of arc from the plane of the eyes'
horizon.

The observer sat with the light at his back, and by manipulation of
the cords adjusted the position of the white disc freely up and down
the screen until its center was judged to be on a level with the eye.
Its position was then read off the vertical scale by the conductor
(who sat hidden by an interposed screen), and the error of judgment
was recorded in degrees and fractions as a positive (upward) or
negative (downward) displacement. The disc was then displaced
alternately upward and downward, and the judgment repeated. From the
time of signalling that the point had been located until this
displacement the observer sat with closed eyes. These determinations
were made in series of ten, and the individual averages are in general
based upon five such series, which included regularly the results of
sittings on different days. In some cases twice this number of
judgments were taken, and on a few occasions less. The number of
judgments is attached to each series of figures in the tables. In that
which follows the individual values and their general averages are
given as minutes of arc for (_a_) the constant error or position of
the subjective horizon, (_b_) the average deviation from the objective