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 seen continuously, its spacial reference only undergoing an instantaneous substitution. If this is the case, it is singular that the correctly seen streak seems to enter consciousness so much reduced as to intensity below that of the false streak when it was eclipsed. Whereas, if a momentary anæsthesia could be demonstrated, both the feeling of succession and the discontinuity of the intensities would be explained (since during the anæsthesia the after-image on the retina would have faded). This last interpretation would be entirely in accordance with the observations of McDougall,[17] who reports some cases in which after-images are intermittently present to consciousness, and fade during their eclipse, so that they reappear always noticeably dimmer than when they disappeared.
Now if the event of such an anæsthesia could be established, we should know at once that it is not a retinal but a central phenomenon. We should strongly suspect, moreover, that the anæsthesia is not present during the very first part of the movement. This must be so if the interpretation of Schwarz is correct, for certainly no part of the streak could be made before the eye had begun to move; and yet approximately the first third was seen at once in its original intensity, before indeed the 'innervation-feelings' had reached consciousness. Apparently the anæsthesia commences, it at all, after the eye has accomplished about the first third of its sweep. And finally, we shall expect to find that movements of the head no less than movements of the eyes condition the anæsthesia, since neither by Schwarz nor by the present writer was any difference observed in the phenomena of falsely localized after-images, between the cases when the head, and those when the eyes moved.
III. THE PERIMETER-TEST OF DODGE, AND THE LAW OF THE LOCALIZATION OF AFTER-IMAGES.
We have seen (above, [p. 8]) how the evidence which Dodge adduces to disprove the hypothesis of anæsthesia is not conclusive, since, although an image imprinted on the retina during its movement was seen, yet nothing showed that it was seen before the eye had come to rest.
Having convinced himself that there is after all no anæsthesia, Dodge devised a very ingenious attachment for a perimeter 'to determine just what is seen during the eye-movement.'[18] The eye was made to move through a known arc, and during its movement to pass by a very narrow slit. Behind this slit was an illuminated field which stimulated the retina. And since only during its movement was the pupil opposite the slit, so only during the movement could the stimulation be given. In the first experiments nothing at all of the illuminated field was seen, and Dodge admits (ibid., p. 461) that this fact 'is certainly suggestive of a central explanation for the absence of bands of fusion under ordinary conditions.' But "these failures suggested an increase of the illumination of the field of exposure.... Under these conditions a long band of light was immediately evident at each movement of the eye." This and similar observations were believed 'to show experimentally that when a complex field of vision is perceived during eye-movement it is seen fused' (p. 462).
Between the 'failures' and the cases when a band of light was seen, no change in the conditions had been introduced except 'an increase of the illumination.' Suppose now this change made just the difference between a stimulation which left no appreciable after-image, and one which left a distinct one. And is it even possible, in view of the extreme rapidity of eye-movements, that a retinal stimulation of any considerable intensity should not endure after the movement, to be then perceived, whether or not it had been first 'perceived during the movement'?
Both of Dodge's experiments are open to the same objection. They do not admit of distinguishing between consciousness of a retinal process during the moment of stimulation, and consciousness of the same process just afterward. In both his cases the stimulation was given during the eye-movement, but there was nothing to prove that it was perceived at just the same moment. Whatever the difficulties of demonstrating an anæsthesia during movement, an experiment which does not observe the mentioned distinction can never disprove the hypothesis.