can be observed also on after-images of considerable intensity; these,
however, flash back instantly into view, so that the observation is
somewhat more difficult. Exner,[2] in speaking of this phenomenon,
adds that in general "subjective visual phenomena whose origin lies in
the retina, as for instance after-images, Purkinje's vessel-figure,
or the phenomena of circulation under discussion, are almost
exclusively to be seen when the eye is rigidly fixed on a certain
spot: as soon as a movement of the eye is made, the subjective
phenomena disappear."

[2] Exner, Sigmund, _Zeitschrift f. Psychologie u. Physiologie
der Sinnesorgane_, 1890, I., S. 46.

The facts here mentioned in no wise contradict a phenomenon recently
discussed by McDougall,[3] wherein eye-movements revive sensations
which had already faded. Thus an eye-movement will bring back an
after-image which was no longer visible. This return to vividness
takes place after the movement has been completed, and there is no
contention that the image is seen just during the movement.

[3] McDougall, W., _Mind_, N.S., X., 1901, p. 52.

The disappearance of after-images during eye-movements is mentioned by
Fick and Gürber,[4] who seek to explain the phenomenon by ascribing it
to a momentary period of recovery which the retina perhaps undergoes,
and which would for the moment prevent further stimulations from being
transmitted to the optic nerve. Exner observes that this explanation
would not, however, apply to the disappearance of the vessel-figure,
the circulation phenomenon, the foveal figure, the polarization-sheaf
of Haidinger, Maxwell's spot, or the ring of Löwe; for these phenomena