Tilted Table Illusion
On the right you see a balancing table, which is clearly tilted (left down) by the heavy blue weight. Or is it?
What to do
Using the button “toggle weight” you can remove the weight, but it remains tilted ;-). You can flip the hatch orientation of the top and bottom rectangles separately (toggle top, toggle bottom). Whenever they are not parallel, the illusion of a tilt occurs. 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 horizontal slider allows to change the angle of the table top from –5° to +5°. Try the following: adjust the slider to let table top and bottom appear parallel. Then toggle both top and bottom; I was amazed at the effect size (which is thus doubled, of course).
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