McDonnell Aircraft Corporation3 experiments were performed on the accuracy of distance estimation in space as a function of stereopsis alone, stereopsis plus motion parallax, and motion parallax alone using a motion-picture stereoscope. The motion parallax was induced by a simulated head motion. During training, Ss received 10 sets of 20 discrete distance presentations each and were asked to identify them as to distance as they were presented and were immediately informed of the correct answer. The identifying code used was a rank order. Procedure was repeated during testing except knowledge of results was omitted. Equations, expressing the median and the dispersion of judged distance were given as a function of actual distance. With proper training, a cyclical head motion can add significantly to the accuracy of distance and size estimation. The equipment and techniques are described. The moon illusion is discussed relative to an informal observation made during the experiment.
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