Harvard Psychological Studies, Volume 1 | Page 8

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 streaks being separate localizations of the same retinal stimulation was an extremely shrewd conjecture. The facts speak strongly in its favor; first, that when the arc of movement is rather long, there is a distinct feeling of succession between the appearances of the falsely and the correctly localized images; second, that when both streaks are seen, the correct streak is always noticeably dimmer than the false streak.
It is of course perfectly conceivable that the feeling of succession is an illusion (which will itself then need to be explained), and that the streak is