In this version of the Poggendorf illusion, an intersecting line splits into into four colorful segments behind a gray rectangle.
    Drag the white line up or down. Click on the rectangle to make it disappear. Drag the rectangle to change its size.
     Try This: Move the white line segment until it is parallel. It aligns with the yellow line on the other side.
     Move the white line up. Which color does it line up with now? Move it down. Now which line is opposite? Does the rectangle make a difference?

Credits: Johann Poggendorf, 1860.
Java Applet: ©1997 Tony Azevedo