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 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.