The Hermann Grid features a black area covered in a thin white grid. As you view the grid, you seem to detect a dark spot at each intersection of the white lines. However, it doesn't occur in the center of your field of vision. The spot at which you're looking directly never has any spots. As you move your focus to each individual intersection, the spots disappear.
These are drawings or sculptures of three-dimensional objects that toy with the sense of perspective. Artists use shading to imply a light source in an image. By looking at the degree of shading on each surface, we can get a sense of each surface's position in relation to the light source. With Impossible Objects, artists deliberately use incorrect levels of shading on the edges, creating objects that appear to fold inside themselves. A good example of this is M.C. Escher's "Waterfall," which features a stream of water that seems to run both uphill and down.
To see an afterimage, it is necessary to stare intently at a high-contrast image, usually rendered in black and white. After about a minute, you will be instructed to stare at a blank white surface and a second image will be revealed. By using thin white lines in the black, the artist is able to create fine detail in the initial image that won't be visible in the "imprint" on the eye, allowing the artist to disguise the hidden image below.
Motion illusions use complex repeating patterns to create a sensation of movement or animation within a static image. Sometimes they are dependent on the viewer moving her head left and right to create a sense of depth. Some motion-based optical illusions use a spiral pattern that is stretched toward the center using a mathematical algorithm. It counteracts the way our eye muscles contract to refocus, creating a sensation of being "drawn in."