Such an explanation involves many doubtful presuppositions, but if we were to grant to Lipps those, the following consideration would invalidate his account. Whether the feeling of innervation which he speaks of as being the underestimated factor is supposed to be a true innervation-feeling in the narrower sense, or a muscular sensation remembered from past movements, it would in the course of experience certainly come to be so closely associated with the corresponding objective distance as not to feel less than this. So far as an innervation-feeling might allow us to estimate distance, it could have no other meaning than to represent just that distance through which the innervation will move the organ in question. If OP is a distance and i is the feeling of such an innervation as will move the eye through that distance, it is inconceivable that i, if it represent any distance at all, should represent any other distance than just OP.
Cornelius[11] brought up the matter a year later than Lipps. Cornelius criticises the unwarranted presuppositions of Lipps, and himself suggests that the falsely localized streak is due to a slight rebound which the eye, having overshot its intended goal, may make in the opposite direction to regain the mark. This would undoubtedly explain the phenomenon if such movements of rebound actually took place. Cornelius himself does not adduce any experiments to corroborate this account.
The writer, therefore, undertook to find out if such movements actually are made. The observations were made by watching the eyes of several subjects, who looked repeatedly from one fixation-point to another. Although sometimes such backward movements seemed indeed to be made, they were very rare and always very slight. Inasmuch as the 'false' streak is often one third as long as the distance moved through, a movement of rebound, such as Cornelius means, would have to be one third of the arc intended, and could therefore easily have been noticed. Furthermore, the researches of Lamansky,[12] Guillery,[13] Huey,[14] Dodge and Cline,[15] which are particularly concerned with the movements of the eyes, make no mention of such rebounds. Schwarz[16] above all has made careful investigations on this very point, in which a screen was so placed between the observer and the luminous spot that it intervened between the pupil and the light, just before the end of the movement. Thus the retina was not stimulated during the latter part of its movement, just when Cornelius assumed the rebound to take place. This arrangement, however, did not in the least modify the appearance of the false streak.
This work of Schwarz certainly proves that the explanation of Cornelius is not correct. Schwarz found that the phenomenon takes place as well when the head moves and the eyes are fixed relatively to the head, as when the eyes alone move. He furthermore made this observation. Meaning by a the point of departure and by b the goal of either the eye-or the head-movement, movement, he says (ibid., S. 400-2): "While oftentimes the streak of the after-image extended uninterruptedly to the point b, or better seemed to proceed from this point,—as Lipps also reported—yet generally, under the experimental conditions which I have indicated, two streaks could be seen, separated by a dark space between; firstly the anomalous one" (the false streak) "rather brilliant, and secondly a fainter one of about equal or perhaps greater length, which began at the new fixation-point b and was manifestly an after-image correctly localized with regard to the situation of this point. This last after-image streak did not always appear; but it appeared regularly if the light at a was bright enough and the background dark.... It was impossible for this second after-image streak to originate in the point b, because it appeared equally when b was only an imaginary fixation-point.... This consideration makes it already conceivable that the two parts of the total after-image are two manifestations of the one identical retinal stimulation, which are differently localized.... Therefore we must probably picture to ourselves that the sensation from the strip of the retina stimulated during the quick eye-movement is, during the interval of movement or at least during the greater part of it, localized as if the axis of vision were still directed toward the original fixation-point. And when the new position of rest is reached and the disturbance on the retinal strip has not wholly died away, then the strip comes once more into consciousness, but this time correctly localized with reference to the new position of the axis of vision. By attending closely to the behavior as regards time of both after-image streaks, I can generally see the normal after-image develop a moment later than the anomalous one" (that is, the false streak). Schwarz finally suggests (S. 404) that probably between the first and second appearances of the streak an 'innervation-feeling' intervenes which affords the basis for localizing the second streak ('correctly') with reference to the new position of the eye.
After this digression we return to consider how this phenomenon is related to the hypothesis of anæsthesia during eye-movements. If we accept the interpretation of Schwarz, there is one retinal process which is perceived as two luminous streaks in space, localized differently and referred to different moments of time. It is surprising, then, that a continuous retinal process is subjectively interpreted as two quite different objects, that is, as something discontinuous. Where does the factor of discontinuity come in? If we suppose the retinal disturbance to produce a continuous sensation in consciousness, we should expect, according to every analogy, that this sensation would be referred to one continuously existing object. And if this object is to be localized in two places successively, we should expect it to appear to move continuously through all intervening positions. Such an interpretation is all the more to be expected, since, as the strobic phenomena show, even discontinuous retinal processes tend to be interpreted as continuously existing objects.
On the other hand, if there were a central anæsthesia during eye-movement, the continuous process in the retina could not produce a continuous sensation, and if the interval were long 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.
Fig. 1.