II.  THE PHENOMENON OF ‘FALSELY LOCALIZED AFTER-IMAGES.’

A further fact suggestive of anaesthesia during movement comes from an unexpected source.  While walking in the street of an evening, if one fixates for a moment some bright light and then quickly turns the eye away, one will observe that a luminous streak seems to dart out from the light and to shoot away in either of two directions, either in the same direction as that in which the eye moved, or in just the opposite.  If the eye makes only a slight movement, say of 5 deg., the streak jumps with the eye; but if the eye sweeps through a rather large arc, say of 40 deg., the luminous streak darts away in the opposite direction.  In the latter case, moreover, a faint streak of light appears later, lying in the direction of the eye-movement.

This phenomenon was probably first described by Mach, in 1886.[8] His view is essentially as follows:  It is clear that in whatever direction the eye moves, away from its luminous fixation point, the streak described on the retina by the luminous image will lie on the same part of the retina as it would have lain on had the eye remained at rest but the object moved in the opposite direction.  Thus, if the eye moves to the right, we should expect the streak to appear to dart to the left.  If, however, the streak has not faded by the time the eye has come to rest on a new fixation point (by supposition to the right of the old), we should expect the streak to be localized to the left of this, that is, to the right of the former fixation-point.  In order to be projected, a retinal image has to be localized with reference to some point, generally the fixation-point of the eyes; and it is therefore clear that when two such fixation-points are involved, the localization will be ambiguous if for any reason the central apparatus does not clearly determine which shall be the point of reference.  With regard to the oppositely moving streak Mach says:[9] “The streak is, of course, an after-image, which comes to consciousness only on, or shortly before, the completion of the eye-movement, nevertheless with positional values which correspond, remarkably enough, not to the later but to the earlier position and innervation of the eyes.”  Mach does not further attempt to explain the phenomenon.

   [8] Mach, Ernst, ‘Beitraege zur Analyze der Empfindungen,’ Jena,
   1886.

   [9] Mach, op. citat., 2te Aufl., Jena, 1900, S. 96.

It is brought up again by Lipps,[10] who assumes that the streak ought to dart with the eyes and calls therefore the oppositely moving streak the ‘falsely localized image.’  For sake of brevity we may call this the ‘false image.’  The explanation of Lipps can be pieced together as follows (ibid., S. 64):  “The explanation presupposes that sensations of eye-movements have nothing to do with the projection of retinal impressions into the visual