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