fectly
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
question, Are the end-circles horizontally elongated? has not to be
answered with mathematical accuracy. It is enough if the end-circles
are approximately round, or indeed are narrower than 9 cm.
horizontally, for at even that low degree of concentration the handle
was still visible to the resting eye. Again, in the experiment with
the color-phases, only two questions are essential to identify the
appearance 5: Does the horizontal yellow band extend quite to both
edges of the image? and, Is there certainly no trace of red or orange
to be seen? The first question does not require a quantitative
judgment, but merely one as to whether there is any green visible to
the right or left of the yellow strip. Both are therefore strictly
questions of quality. And the two are sufficient to identify
appearance 5, for if no red or orange is visible, images 1, 2, and 3
are excluded; and if no green lies to the right or left of the yellow
band, image 4 is excluded. Thus if one is to make the somewhat
superficial distinction between qualitative and quantitative
judgments, the judgments here required are qualitative. Moreover, the
subjects make these judgments unhesitatingly.

Finally, the method of making judgments on after-images is not new in
psychology. Lamansky's well-known determination of the rate of
eye-movements[22] depends on the possibility of counting accurately
the number of dots in a row of after-images. A very much bolder
assumption is made by Guillery[23] in another measurement of the rate
of eye-movements. A trapezoidal image