This experiment leaves it impossible to doubt that the conjecture of Schwarz, that the correct image is only the false one seen over again, is perfectly true. It would be interesting to enquire what it is that conditions the length of the false streak. It is never more than one third that of the correct streak (Fig. 3:1; except of course under the artificial conditions of Fig. 3:3) and may be less. The false streak seems originally to dart out from the light, as described by Lipps, visibly growing in length for a certain distance, and then to be suddenly eclipsed or blotted out simultaneously in all its parts. Whereas the fainter, correct streak flashes into consciousness all parts at once, but disappears by fading gradually from one end, the end which lies farther from the light.
Certain it is that when the false streak stops growing and is eclipsed, some new central process has intervened. One has next to ask, Is the image continuously conscious, suffering only an instantaneous relocalization, or is there a moment of central anaesthesia between the disappearance of the false streak and the appearance of the other? The relative dimness of the second streak in the first moment of its appearance speaks for such a brief period of anaesthesia, during which the retinal process may have partly subsided.
We have now to seek some experimental test which shall demonstrate definitely either the presence or the absence of a central anaesthesia during eye-movements. The question of head-movements will be deferred, although, as we have seen above, these afford equally the phenomenon of twice-localized after-images.
A. Apparatus must be devised to fulfil the following conditions. A retinal stimulation must be given during an eye-movement. The moment of excitation must be so brief and its intensity so low that the process shall be finished before the eye comes to rest, that is, so that no after-image shall be left to come into consciousness after the movement is over. Yet, on the other hand, it must be positively demonstrated that a stimulation of this very same brief duration and low intensity is amply strong enough to force its way into consciousness if no eye-movement is taking place. If such a stimulation, distinctly perceived when the eye is at rest, should not be perceptible if given while the eye is moving, we should have a valid proof that some central process has intervened during the movement, to shut out the stimulation-image during that brief moment when it might otherwise have been perceived.
Obviously enough, with the perimeter arrangement devised by Dodge, where the eye moves past a narrow, illuminated slit, the light within the slit can be reduced to any degree of faintness. But on the other hand, it is clearly impossible to find out how long the moment of excitation lasts, and therefore impossible to find out whether an excitation of the same duration and intensity is yet sufficient to affect consciousness if given when the eye is not moving. Unless the stimulation is proved to be thus sufficient, a failure to see it when given during an eye-movement would of course prove nothing at all.