The Ebbinghaus Illusion consists of two circles surrounded by larger and smaller circles, respectively. To the human eye, it seems that the left circle is smaller than the right circle. In fact, the two circle are the same size. The presence of the outer circles tricks the eye into seeing two different sizes.
The first slider changes the opacity of the outer circles. See at what percentage the inner circles start looking the same size. Alternatively, move the second slider to move the inner circles towards one another, and compare sizes. You can even swap their places!