Do you see the white square to the right? That’s odd, because there’s not actually a square there, just four black shapes. Our brains tend to fill in the gaps and allow us to see things that aren’t really there. This illusion has other subtle effects. Since our brains are telling us that a white square is superimposed on top of four black circles, placing the object on top of text can have an illusory “magnifying glass” effect. Since the white square seems to be on top of the circles, which are on top of the underlying text, the text within the square may seem larger, even though the square is actually transparent and the text is the same size. In the frame below, you can click and drag the object around to see the effect on different parts of the text.
Kanizsa illusions, invented by Italian psychologist Gaetano Kanizsa, can also affect our perception of brightness and depth. In the Kanizsa triangle below on the left, the white triangle (which is not really there) may appear brighter than the background, though they are both the same white. More recently, researchers have discovered the shaded illusion below, in which a pyramid may appear to emerge from a corner.
If you like this kind of illusion, you may want to check out Optical Illusions: The Science of Visual Perception, a massive collection of visual trickery compiled by Al Seckel. This book contains 300 pages of amazing illusions, along with explanations of what’s happening when we perceive weird things in the images.