Illusion

set them equal by eye. a ruler checks.

nine optical illusions printed as Op Art: set each one right by eye, then a ruler measures how far you were fooled

Each plate is a classic illusion. You set a length, a size, a grey, a tilt or a line by eye, and the card measures how far your setting is from the truth: lengths and sizes in percent, greys in points of the black-to-white scale, angles in degrees. The first plate has no illusion, to show how precise your eye is on its own; the last repeats the second, so the ruler can say whether knowing the trick helped.

Concepts: perception, optical-illusions.

An interactive you play with one thumb, on Wundr.