Relative size in isolation as a stimulus for relative perceived distance.

2 experiments examined the perceived relative distance produced by relative size in isolation. The hypothesis was that S assumes identity of physical size and interprets the difference in apparent size as a difference in distance. Experiment I examined this hypothesis by observing the effect of experimentally trained size assumptions on subsequent relative distance judgments. Experiment II used pairs of familiar objects which had known, normally invariant physical sizes. By varying the actual physical sizes of the standards it was possible to study the hypothesized effect of size assumptions on relative perceived distance. The results of Experiment I did not agree with the hypothesis. Experiment II did confirm the hypothesis. Both experiments were discussed in the context of the assumptive hypothesis. Relative size is a stimulus correlate of perceived relative distance. The object which subtends the smaller visual angle appears more distant. This is the case when all other stimulus correlates, e.g., motion perspective, are eliminated, as well as under unrestricted conditions of viewing. One explanation of the effects of relative size under restricted conditions of observation is suggested in the following definition of the relative-size cue: For a given ratio of visual angles, assuming the ratio of physical sizes determines the ratio of distances. If the assumed sizes are equal, the object subtending the larger visual angle appears nearer and the one subtending the small appears farther [Ittelson, 1960, p. 69]. This definition proposes that the effect of relative size is mediated by the subjective assumption of identity. Ames (1946) has expressed this view succintly: "You presume identity of size... from similarity, and use the appearance of difference (in size) as an indication of distance [p. 46]. " Two experiments were conducted to examine this hypothesis.