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 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.