A spiral design superimposed over a radiating pattern...Wait! A spiral? Follow any "spiral" around the design and see for yourself. It's really a series of concentric circles.

The spiral effect is caused by an additional set of contrasting lines which make the circles look "twisted."

The Poggendorf Illusion 

This is an illusion of direction. It is based on the fact that most people do not see the red and green lines as being part of the same line. In order to make it easier to see the illusion, this diagram was made with three possible lines to match with the red line. When asked, the majority of subjects would say that the red line matched either the blue, or orange line. Even after holding up a straight edge to this image to prove that the red and green lines are really part of one continuous line, the illusion will persist.

The Zöllner Illusion

Are the black diagonal lines parallel ? You bet they are! Measure 'em!

The pattern of lines in the background are perfectly straight and parallel. There is a subtle distortion in the 'rightness' of the background lines where they go under the overlay?

The Café Wall Illusion

When you look at the Café Wall, you should see the 'mortar lines' (the horizontal lines between the tiles) slope alternately upward and downward to the right. This gives the impression that the tiles are wedge-shaped. The lines are actually parallel, and the tiles are all perfectly square and of the same size

The Ponzo Illusion

When placed between oblique, symmetrically converging inducing lines, a pair of parallel horizontal line segments will seem to have different lengths when they are measurably equal.

Most people observe that box A seems larger than B, but a quick measurement shows they are the same size. Box A appears larger than B because the background, railroad tracks disappearing into the distance, gives us the illusion that box A and B are sitting on the tracks at different distances from our eyes. Our mind knows that if object A is far away but is the same size as a nearby object B, object A must be larger than B

The Muller-Lyer Illusion

It consists of two lines which are equal and bordered by fins. The one with the outgoing brackets is seen as larger than the one with the inward brackets but both the lines are of the same length.You can measure them to verify.