Letter | Published:

Inappropriate Constancy Explantion of Spatial Distortions

Naturevolume 207page893 (1965) | Download Citation



PROF. DAY omits the major feature of my theory. The omission is evident in his reference to the Moon illusion. What is interesting about the Moon illusion is that its apparent size is not a simple function of its apparent distance. On the horizon it appears large and near. Ptolemy was not correct in attributing its apparent size simply to its apparent distance, and the effect is not a straightforward example of Emmert's law. For this and other reasons I suggested that there is more to constancy than apparent distance: that constancy can be set directly by depth cues which are not always appropriate. Prof. Day disregards what I have called “primary constancy scaling1” without which I believe we cannot hope to develop a consistent theory of these distortions in terms of depth perception.

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  1. 1

    Gregory, R. L., Nature, 199, 678 (1963).

  2. 3

    Gregory, R. L., Nature, 204, 302 (1964).

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  1. Department of Psychology, University of Cambridge

    • R. L. GREGORY


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