Slope of regard as a distance cue

It is proposed that some distance cues are learned when a perceptual parameter that varies with observation distance is regularly associated with objects whose distances are perceived because another distance cue operates. If that is the way distance cues can come into existence, it may be possible to identify a parameter that varies with distance but is not a known distance cue and to show that it functions as one. The slope of regard with which an object on the ground is viewed is such a potential distance cue. Its angle varies approximately with the reciprocal of distance. An experiment was done that showed that this slope angle functions as a distance cue. Subjects who looked through a device that altered slope angles gave estimates of the dimensions of an object on the ground. Perceived sizes, which vary inversely with distance, were found to be altered in accordance with the altered slope angle.