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### An Interactive Ponzo Illusion Demonstration

Between the tilted lines are two horizontal lines. The second, smaller line is called the comparison line. Your task is to adjust the comparison line so that it is the same length as the larger horizontal line above it. You can change the length of the line by pressing the "Increase line" and "Decrease line" buttons. When you think the two lines are equal in length, press the "Equal?" button. If the two horizontal lines are equal, the comparison line will turn green. If not, it will turn red.

1st) adjust the length of the comparison line by using the "Increase line" and "Decrease line" buttons.

2nd) when you think the two horizontal lines are equal, press the Equal?" button.

3rd) if you fail, hit the "Remove lines" button to remove all the non-horizontal lines and try again.

 If you don't see anything but black in the area below, press this button:
 Hold or click these buttons to change the length of the comparison line These buttons will add and remove tilted background lines This button will test if the standard & comparison lines are equal. If they are, the comparison line will turn green. If not, it will turn red. This button will reset the comparision line to new state

NOW EXPERIMENT WITH THE ILLUSION!
• Add and remove non-horizontal lines using the "Add lines" and "Remove lines" buttons. Hit the "Reset" to reset the length and position of the comparison line.

Use your mouse button to grab the comparison line and move it closer to the top horizontal line. Now adjust the comparison line.

• Track the comparison line!

Adjust the comparison line until you think it is about equal in length to the top horizontal line. Now use your mouse button to grab the comparison line and move it up and down tracking the motion with your eyes. What do you see?

As you can see, many factors affect the strength of this illusion. By playing with this demonstration, we can see that the illusion distorts our perception of the length of the comparison line. But if we only rely on such simple, casual observation, it is very difficult to know exactly how much our perception is distorted, and how the strength of the distortion will vary under different conditions. To answer these questions, we need to use experimental psychological techniques. We now invite you to participate in a short 10-15 minute on-line experiment.