Abstract
I WOULD disagree with Mr. Wallace that Figs. 1a and 1b have the same perspective sense. On my view there are gradients of expansion along each of the converging lines of 1a. This expansion is due to the converging perspective of these lines, so that any feature near the middle of this figure should be, and is, expanded. This gives the outward bowing of the vertical line, since the expansion is greater where these lines are near the centre of the figure. Fig. 1b is less obvious. If the arrangement of equally spaced circles is viewed as a luminous figure, with a single eye, to prevent the competing information that it is in fact flat, it appears flat. If, however, the spacing of the circles is non-uniform, then it will be seen as a tunnel or a cone, depending on whether the spacing increases or decreases outward from the centre. In other words, the depth of the background figure depends on the spacing of the circles, not the circles themselves.
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GREGORY, R. Optical Illusions. Nature 209, 328 (1966). https://doi.org/10.1038/209328a0
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DOI: https://doi.org/10.1038/209328a0
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