Harvard Psychological Studies, Volume 1 | Page 8

enough the image
might well be referred to two objects; since also, in the strobic

appearances, the stimulations must succeed at a certain minimal rate in
order to produce the illusion of continuous existence and movement.
This consideration seemed to make it worth while to perform some
experiments with the falsely localized after-images. The phenomenon
had also by chance been noted in the case of the eye moving past a
luminous dot which was being regularly covered and uncovered. The
appearance is of a row of luminous spots side by side in space, which
under conditions may be either falsely or correctly localized. Since
these dots seemed likely to afford every phenomenon exhibited by the
streaks, with the bare chance of 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°, and all saw only the
correctly localized streak if the arc was small, say 5°. 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 brightest at the end nearer the light.
(2) If the eye-movement is 40° or more, a streak having a length of
about one third the distance moved through is seen on the other side of
the light from the final fixation-point; while another streak is seen of
the length of the distance moved through, and extending from the final
fixation-point to the light. The first is the falsely, the second the
correctly localized streak. The second, which is paler than the first,
feels as if it appeared a moment later than this. The brighter end of each
streak is the end which adjoins the luminous spot. (3) Owing to this last
fact, it sometimes happens, when the eye-movement is 40° or a trifle
less, that both streaks are seen, but that the feeling of succession is
absent, so that the two streaks look like one streak which lies
(unequally parted) on both sides of the spot of light. It was observed, in
agreement with Schwarz, that the phenomenon was the same whether
the head or the eyes moved. Only one other point need be noted. It is
that the false streak, which appears in the beginning to dart from the
luminous hole, does not fade, but seems to suffer a sudden and total
eclipse; whereas the second streak flashes out suddenly in situ, but at a
lesser brilliancy than the other, and very slowly fades away.
These observations thoroughly confirmed those of Schwarz. And one
could not avoid the conviction that Schwarz's suggestion of the two