disappear in a similar manner during movement. Exner offers another
and a highly suggestive explanation. He says of the phenomenon (_op.
citat._, S. 47), "This is obviously related to the following fact,
that objective and subjective impressions are not to be distinguished
as such, so long as the eye is at rest, but that they are immediately
distinguished if an eye-movement is executed; for then the subjective
phenomena move with the eye, whereas the objective phenomena are not
displaced.... This neglect of the subjective phenomena is effected,
however, not by means of an act of will, but rather by some central
mechanism which, perhaps in the manner of a reflex inhibition,
withholds the stimulation in question from consciousness, without our
assistance and indeed without our knowledge." The suggestion of a
central mechanism which brings about a reflex inhibition is the
significant point.

[4] Fick, Eug., and Gürber, A., _Berichte d. ophthalmologischen
Gesellschaft in Heidelberg_, 1889.

It is furthermore worth noting that movements of the eyelid and
changes in the accommodation also cause the after-images to disappear
(Fick and Gürber), whereas artificial displacement of the eye, as by
means of pressure from the finger, does not interfere with the images
(Exner).

Another motive for suspecting anæsthesia during eye-movement is found
by Dodge,[5] in the fact that, "One may watch one's eyes as closely as
possible, even with the aid of a concave reflector, whether one looks
from one eye to the other, or from some more distant object to one's
own eyes, the eyes may be seen now in one position and now in another,
but never in motion." This phenomenon was described by Graefe,[6] who