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Harvard Psychological Studies, Volume 1 - Containing Sixteen Experimental Investigations from the Harvard Psychological Laboratory. by Various
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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
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