bringing out new facts, apparatus was
arranged as in Fig. 1, which is a horizontal section.

_DD_ is a disc which revolves in a vertical plane, 56 cm. in diameter
and bearing near its periphery one-centimeter holes punched 3 cm.
apart. _E_ is an eye-rest, and _L_ an electric lamp. _SS_ is a screen
pierced at _H_ by a one-centimeter hole. The distance _EH_ is 34 cm.
The disc _DD_ is so pivoted that the highest point of the circle of
holes lies in a straight line between the eye _E_ and the lamp _L_.
The hole _H_ lies also in this straight line. A piece of milk-glass
_M_ intervenes between _L_ and _H_, to temper the illumination. The
disc _DD_ is geared to a wheel _W_, which can be turned by the hand of
the observer at _E_, or by a second person. As the disc revolves, each
hole in turn crosses the line _EL_. Thus the luminous hole _H_ is
successively covered and uncovered to the eye _E_; and if the eye
moves, a succession of points on the retina is stimulated by the
successive uncovering of the luminous spot. No fixation-points are
provided for the eye, since such points, if bright enough to be of use
in the otherwise dark room, might themselves produce confusing
streaks, and also since an exact determination of the arc of
eye-movement would be superfluous.

[Illustration: Fig. 1.]

The eye was first fixated on the light-spot, and then moved
horizontally away toward either the right or the left. In the first
few trials (with eye-sweeps of medium length), the observations did
not agree, for some subjects saw both the false and the correct
streaks, while others saw only the latter. It was found later that
all the subjects saw both streaks if the arc of movement was large,
say 40 deg., and all saw only the correctly localized streak if the arc
was small, say 5 deg.. Arcs of medium length revealed individual
differences between the persons, and these differences, though
modified, persisted throughout the experiments. After the subjects had
become somewhat trained in observation, the falsely localized streak
never appeared without the correctly localized one as well. For the
sake of brevity the word 'streak' is retained, although the appearance
now referred to is that of a series of separate spots of light
arranged in a nearly straight line.

The phenomena are as follows.--(1) If the arc of movement is small, a
short, correctly localized streak is seen extending from the final
fixation-point to the light-spot. It is brig