Some optical illusions require creative talent. You have to draw spirals, and pick the right colors, or at least have the ability to make two faces in profile look like one vase. Not these two illusions. The Hermann Illusion and the Scintillating Illusion require nothing more than a checkerboard.

The more you look into optical illusions the more you realize how very much the connection between your brain and your eyes malfunctions. Sometimes it seems like just about anything can mess it up, even a simple grid. Two illusions prove exactly that. The Hermann Illusion is literally just a black background with a grid of white stripes on it. Most of you will have noticed the illusion when glancing at checkerboards or bathroom tiling. The parts of the white stripes that run along the borders of the black squares are fine. But the intersections of those white lines, phantom dark splotches appear, and then disappear when you look right at them.

If a no-doubt frustrated bathroom cleaner tries to rub out these illusory dark spots by applying selective bleach to the intersections, or even little white dabs of paint, they would fall face-first into the Scintillating Illusion. This starts with the same black-and-white grid, and adds little white disks at the intersections. That should stop the dark splotches, or at least even them out. Actually, it makes them more dramatic. Instead of vague gray splotches, the white disks develop well-defined black circles inside them. The dots shimmer quickly in and out of reality as the eye passes over the grid.


The best explanation that's given for these illusions involves the way the eye compares information. Put a bright patch next to a dark place and it will look bright. Put a bright patch next to another bright patch, and it will look a little darker. Intersections are surrounded on four sides by bright patches, and appear darker in comparison. Any given spot on a line, though, has two bright places and two dark places next to it. It looks bright compared to the darkness.

Some people think this explanation is wrong. For example, tilting the head forty-five degrees reduces the effect of the Scintillating Illusion, even though it doesn't change the composition of light and dark. Meanwhile, both illusions stop working if the lines are curved to any degree. Do you have an idea as to why these illusions fool us?


Top Image: Michael Phillips

Via Psylux and Wolfram Mathworld.