Place
your mouse pointer over the image in order to envision the illusion!Size Constancy | Ponzo's Optical Illusions
For an explanation of the constancy phenomena, see Perceptual Constancy.
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Which creature appears bigger? If you measure them you'll probably be surprised to find out that they are both the same size. The creature appears to increase in size as it gets further away due to what is called 'perspective'. Perspective is evoked in this image by the pattern, and the lines on the wall which tend to converge on to a common point in the distance.
Source: Roger Shepard (1990). Mind Sights: Original Visual Illusions, Ambiguities, and Other Anomalies. San Francisco: W. H. Freeman.
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The two slanted lines arranged like an inverted V evoke the impression of perspective. Thus, the upper objects (line, circle, barrel), which are framed by the diagonally converging line segments, are perceived to differ in length from the twin objects beneath.
