well, there is a moment in which stimulations are not transmitted from
the retina to the cerebral cortex, that is, a moment of central
anæsthesia_. The reason for saying 'and _probably_ of the head as
well,' is that although the phenomena described are gotten equally
well from movements of the head, yet it is not perfectly certain that
when the head moves the eyes do not also move slightly within the
head, even when the attempt is made to keep them fixed.

Most of the criticisms which apply to this last experiment apply to
that with the dumb-bell and have already been answered. There is one
however which, while applying to that other, more particularly applies
here. It would be, that these after-images are too brief and
indistinct to be carefully observed, so that judgments as to their
shape, size, and color are not valid evidence. This is a perfectly
sensible criticism, and a person thoroughly convinced of its force
should repeat the experiments and decide for himself what reliance he
will place on the judgments he is able to make. The writer and those
of the subjects who are most trained in optical experiments find the
judgments so simple and easily made as not to be open to doubt.

In the first place, it should be remembered that only those cases are
counted in which the movement was so timed that the image was seen in
direct vision, that is, was given on or very near the fovea. In such
cases a nice discrimination of the shape and color of the images is
easily possible.

Secondly, the judgments are in no case quantitative, that is, they in
no case depend on an estimate of the absolute size of any part of the
image. At most the proportions are estimated. In the case of the
dumb-bell the question is, Has the figure a handle? The other