The Wundt-Hering Illusion

This illusion has two variants.  In the Wundt variant, the two horizontal lines appear to bend inward in the center, although they are straight and parallel.  In the Hering variant, the lines appear to bow outward in the center.  You can select one or the other variant with the Wundt and Hering menu commands.  You can change the number of vertical lines with the More Lines and Fewer Lines commands, and you can change the distance between the horizontal lines with the Increase Line Distance and Decrease Line Distance commands.  For the Wundt variant, you can change the distance between the two points to which the vertical lines converge with the Increase Focus Distance and Decrease Focus Distance commands.

References

Coren, S. & Girgus, J.S. (1978).  Seeing is Deceiving: The Psychology of Visual Illusions.  Hillsdale, New Jersey: Lawrence Erlbaum Associates.

Luckiesh, M. (1965).  Visual Illusions, Their Causes, Characteristics and Applications.  New York: Dover.