Harvard Psychological Studies, Volume 1 - Containing Sixteen Experimental Investigations from the Harvard Psychological Laboratory. by Various
page 35 of 880 (03%)
page 35 of 880 (03%)
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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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