which it swings.

If now the conditions are altered, as in Fig. 4:2, so that the opening
_tt_ (indicated by the dotted line) lies not in _SS_, but in the fixed
background _BB_, while the small hole _i_ now moves with the shield
_SS_, it necessarily follows that if the eye can move at just the rate
of the pendulum, it will receive a stimulation of exactly the same
size, shape, duration, and intensity as in the previous case where the
eye was at rest. Furthermore, it will always be possible to tell
whether the eye does move at the same rate as the pendulum, since if
it moves either more rapidly or more slowly, the image of _i_ on the
retina will be horizontally elongated, and this fact will be given by
a judgment as to the proportions of the image seen.

It may be said that since the eye does not rotate like the pendulum,
from a fulcrum above, the image of _i_ in the case of the moving eye
will be distorted as is indicated in Fig. 4, _a_. This is true, but
the distortion will be so minute as to be negligible if the pendulum
is rather long (say a meter and a half) and the opening _tt_ rather
narrow (say not more than ten degrees wide). A merely horizontal
movement of the eye will then give a practically exact superposition
of the image of _i_ at all moments of the exposure.

[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE PLATE II.
Fig. 4. Fig. 6.
HOLT ON EYE-MOVEMENT.]

Thus much of preliminary discussion to show how, by means of a
pendulum, identical stimulations can be given to the moving and to the
resting eye. We return to the problem. It is to find out whether a
stimulation given during an eye-movement can be perceived if its
after-image is so brief as wholly to elapse before the end of the
movement. If a period of anaesthesia is to be demonstrated, two
observations must be made. First, that the stimulation is bright
enough to be _unmistakably visible_ when given to the eye at rest;
second, that it is not visible when given to the moving eye. Hence, we
shall have three cases.

Case 1. A control, in which the stimulation is proved intense
enough to be seen by the eye at rest.

Case 2. In which the same stimulation is given to the eye
during movement.

Case 3. Another control, to make sure that no change in the
adaptation or fatigue of the eye has intervened during the
experiments to render the eye insensible to the stimulat