On the right there are many small upright bars which are vertically displaced following a sine-wave. The question is: do all bars look alike in height? Many would agree they don’t. You can use the control “compensate” on the top right, for me a value of 15–20 is about right, agreed?
As you will already have guessed, they are all identical in height, but don't look like it. If you press the button “amp Ø” on the left, the amplitude of the sine-wave is reduced to zero, revealing the veridical height of the vertical lines.
The original authors (Day & Stecher, 1991) write in their abstract: “The illusion is explained in terms of a perceptual compromise between the vertical extent and the greater overall dimensions of the section at the turn of the sine-wave figure and is thereby held to be the same in principle as the Müller-Lyer illusion.” Hmm… When I have obtained the original article (which is non-trivial for me) I will comment more.
The phenomenon has occurred in my laboratory a number of times, when we were looking at visual evoked responses in the EEG. Small structures that ride on a rising or falling edge are easily misjudged in size.
Day RH, Stecher EJ (1991) Sine of an illusion. Perception 20;49–55