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Abstract: Studies were carried out with 48 Ss on size perception. The standard stimulus was a balloon (diameter: 38.2 cm.) placed at distances of 50, 100, 150, 200 and 250 metres from S, the comparison stimulus being another balloon placed by the side of S, and adjustable in size by S operating a valve, Ss were instructed to make size judgments with a naive attitude.It was found that when the observer sat erect, size constancY broke down strikingly at 200 and 250 metres, but only a few Ss made over-estimations. When the observer was in a prone position (raising his head and eyes) and in a supine position (lowering his head and eyes), the perceptual size shrank in comparison with the results obtained in the erect observations. The prone position caused a more marked decrease in perceptual size, and observations made in a fully prone position yielded smaller perceptual sizes than those made in a semi-prone position, When the supine observer bent his head backward to see the stimulus in an inverse visual field, the perceptual size decreased to a minimum, and he reported at the same time an increase in perceptual distance. The results of this study were discussed in relation to the moon illusion. It is suggested that a normal relation between the postural position of the observer and the environment plays an important role in size perception, the distortion of which also causes a breakdown of the constancy phenomena.

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