In the second case (Fig. 3:2) the wall W is in position and the eye so adjusted in the eye-rest that the light L is not seen until the eye has moved about 10 deg. to the right, that is, until the axis of vision is at Ex. Clearly, then, the image of L falls at first a little to the right of the fovea, and continues in indirect vision to the end of the movement. The stimulated part of the retina is l-l’ (Fig. 3:2). Here, then, we have no stimulation of the eye during the first part of its movement. The corresponding appearance of the streak is also shown. Only the correctly localized streak is seen, extending from the light L toward the right but not quite reaching B’. Thus by cutting out that portion of the stimulation which was given during the first part of the movement, we have eliminated the whole of the false image, and the right-hand (foveal) part of the correct image.
Fig. 3:3 shows the reverse case, in which the stimulation is given only during the first part of the movement. The wall is fixed on the right of L, and the eye so adjusted that L remains in sight until the axis of vision reaches position Ex, that is, until it has moved about 10 deg.. A short strip of the retina next the fovea is here stimulated, just the part which in case 2 was not stimulated; and the part which in case 2 was, is here not stimulated. Now here the false streak is seen, together with just that portion of the correct streak which in the previous case was not seen. The latter is relatively dim.
Thus it looks indeed as if the streak given during the first part of an eye-movement is seen twice and differently localized. But one may say: The twice-seen portion was in both cases on the fovea; this may have been the conditioning circumstance, and not the fact of being given in the early part of the movement.
We must then consider Fig. 3, case 4. Here the eye moves from B to B’, through the same arc of 40 deg.. The wall W is placed so that L cannot be seen until the axis of vision has moved from EB to EL, but then L is seen in direct vision. Its image falls full on the fovea. But one streak, and that the correctly localized one, is seen. This is like case 2, except that here the streak extending from L to the right quite reaches the final fixation-point B’. It is therefore not the fact of a stimulation being foveal which conditions its being seen in two places.
It should be added that this experiment involves no particular difficulties of observation, except that in case 4 the eye tends to stop midway in its movement when the spot of light L comes in view. Otherwise no particular training of the subject is necessary beyond that needed for the observing of any after-image. Ten persons made the foregoing observations and were unanimous in their reports.