4. In the 1960's, the term " Op Art " was coined to describe the work of a growing group of abstract painters. This movement was led by Victor Vasarely and Bridget Riley . Blaze 1964
12. " A bulge" The floor appears to bulge out, though this image consists of only squares.
13. "Primrose's field" This checkered background consists of squares but appears to wave. In addition, this figure also shows a waving motion illusion.