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The Poggendorf Illusion I

A straight line passes behind an obstruction. The line starts out white and splits into four lines. This effect was designed to test our ability to match line segments across a gap. At any given point only one of the four colored lines will most closely appear to align with the white line segment.

Drag the white line upwards to tilt it to the highest angle. It now looks as if the white line most closely aligns with the Red line. Now drag the white line downward to its lowest angle. It now appears as if the white line matches up with the Green line. Move the white line to the middle so that the line segments are level. Which color does the white segment align with? Which color line is the true match for the white line? Click on the gray rectangle to make it disappear.

Java Applet: Copyright 1997 Tony Azevedo

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