An Interactive Poggendorf Illusion Demonstration

Can you connect the two white line segments? Use your left mouse button to independently move the two tilted white lines up and down along either side of the gray rectangle.  When you think that the two lines are positioned such that they lie along the same line, press the "Connect Me" button.  If you have positioned the two lines so that they are connected, a green line will be drawn between the two white lines.  If you did not, a red line will be drawn.

Hold or click these buttons to change the width of the gray rectangle

Hold or click these buttons to change the angle of the two white line segments

This button will connect the two line segments

If the applet is blank, press this button


As you can see, there are many visual factors that affect the strength of this illusion.  However, by using such casual observation, we cannot mathematically measure how much an effect these factors have; moreover, it is difficult to understand precisely why they have an effect.  Experimental psychological techniques are needed to address these questions.  We therefore now invite you to participate in a short 5-10 minute experiment.