Do you see circles at the centre of each set of lines?

Do these circles vary in size?

The circles are illusory, and usually vary in size according to the length of the black lines surrounding them.

Click repeatedly on the image to show real circles where the illusory ones should appear (note that they are all the same size) and then return to the illusion.

Leave a few seconds for the afterimage of the circles to fade before tying to view the illusion again.