Some illusions are developmental - children don't see them, but adults do. Some illusions are based on violations of perspective. Still other illusions are the result of the way in which our brains process information. If you are interested in doing a study with illusions, we also have computer based illusions and there are several sites on the Internet. In the mirror tracer task the subject has to trace the pattern of a star as they look at it in a mirror.
Some subjects succeed easily, while other struggle to move their hand.
This electronic version of the mirror-tracer counts the number of times that the subject goes out of the boundaries. Further, you might consider dividing the pattern into sectors to see if the behavior differs from one area to the next. This task is the same as the one above, but the subject traces the pattern on a piece of paper with a pen. While there is no counter, the paper record can be analyzed later to compare subjects. In both tasks, remember to carefully position the hand of each subject in the same way.