llowing conditions. A
retinal stimulation must be given during an eye-movement. The moment
of excitation must be so brief and its intensity so low that the
process shall be finished before the eye comes to rest, that is, so
that no after-image shall be left to come into consciousness _after_
the movement is over. Yet, on the other hand, it must be positively
demonstrated that a stimulation of this _very same_ brief duration and
low intensity is amply strong enough to force its way into
consciousness if no eye-movement is taking place. If such a
stimulation, distinctly perceived when the eye is at rest, should not
be perceptible if given while the eye is moving, we should have a
valid proof that some central process has intervened during the
movement, to shut out the stimulation-image during that brief moment
when it might otherwise have been perceived.

Obviously enough, with the perimeter arrangement devised by Dodge,
where the eye moves past a narrow, illuminated slit, the light within
the slit can be reduced to any degree of faintness. But on the other
hand, it is clearly impossible to find out how long the moment of
excitation lasts, and therefore impossible to find out whether an
excitation of the same duration and intensity is yet sufficient to
affect consciousness if given when the eye is not moving. Unless the
stimulation is proved to be thus sufficient, a failure to see it when
given during an eye-movement would of course prove nothing at all.

Perhaps the most exact way to measure the duration of a light-stimulus
is to let it be controlled by the passing of a shutter which is
affixed to a pendulum. Furthermore, by means of a pendulum a
stimulation of exactly the same duration and intensity can be given to
the moving, as to the resting eye. Let us consider Fig. 4:1. If _P_ is
a pendulum bearing an opaque shield _SS_ pierced by the hole _tt_, and
_BB_ an opaque background pierced by the hole _i_ behind which is a
lamp, it is clear that if the eye is fixed on _i_, a swing of the
pendulum will allow _i_ to stimulate the retina during such a time as
it takes the opening _tt_ to move past _i_. The shape of _i_ will
determine the shape of the image on the retina, and the intensity of
the stimulation can be regulated by ground-or milk-glass interposed
between the hole _i_ and the lamp behind it. The duration of the
exposure can be regulated by the width of _tt_, by the length of the
pendulum, and by the arc through