The intersections of the lines produce some perplexing visual effects. Light grey dots appear at the intersections of black lines whereas dark grey dots can be seen where the white lines intersect. Moreover, they have a fleeting existence — look closely at an intersection and the dot will disappear!

The effect is referred to as the Hermann-Hering grid illusion, and it has been studied extensively in visual science. However, the interest in this simple pattern is not restricted to the individual dots because they can be perceptually grouped to see a set of contours, rather like those of the back descending a staircase. The ‘backs’ are made up from the illusory dots. Here we have an instance of a second order illusion. The circles do not exist physically but they are the perceptual completion of the individual dots, which also do not exist physically.

This is a relatively low level visual illusion that is used for artistic and scientific effect.