Concentrate on the left image, where you can see four circles with quarter of their constructional lines painted blue. If you blur your eyes a little, you should perceive a faint bluish color within the square region. As you presumed, it’s only an illusion – the square region is completely white. To prove this, let us move to the next image on the right. When additional small circles are placed at the edges of the illusory square, the neon color spreading disappears. Now how is that for an illusion! For more simmilar illusions, you can open the section of this site, called Color Adapting category.
