What to see
Above there is a balancing table, its top clearly tilted (left down) by the heavy blue weight. Or is it?
What to do
Deselecting the checkbox “Weight” you can remove the weight, but the table remains tilted ;-).
Try the following: using the angle slider at the bottom, balance the table so it’s nicely horizontal. Now use the button “Toggle <” to toggle the hatching angles of top and bottom. Now the table is out of balance again. Toggle back, table back in balance? Now deselect the “Hatching” checkbox. This reveals that the tilt of the table was actually illusory. The weight is really irrelevant here.
The numerical stepper (on the right, preset at 20) allows to change the angle of the oblique lines. There is a broad optimum for maximal illusion at angles below 45°.
The Tilted Table is a cute variant of the Zöllner illusion. In the image on the right, depicting the original Zöllner illusion, the long lines going up-right are all parallel but do not seem so. This is due to interaction with the intersecting short lines which are alternatively horizontal and vertical.
This demonstration was inspired by an image floating around the internet, appropriately called “tilting table illusion”. I would love to credit the originator, suggestions welcome.
Zöllner F (1860) Ueber eine neue Art von Pseudoskopie und ihre Beziehungen zu den von Plateau und Oppel beschrieben Bewegungsphaenomenen. Annalen der Physik: 500–523. DOI