1. 8. The Hering Illusion Are the green shapes straight or curved?
The Wundt (Wilhelm Lundt, German physician) illusion is the inverse of the Hering illusion. The red lines appear to bow inwards.
Look at the figure below. Are the 3 red squares perfectly concentric?
The Hering illusion is an optical illusion discovered by German physiologist Ewald Hering in The two horizontal lines are both straight, but they look as if they were bowed outwards. The distortion is produced by the lined pattern on the background that simulates a perspective design, and creates a false impression of depth.
Yes. They are perfectly concentric. The Hering illusion at work.

2. 9. The Café Wall Illusion Are the grey lines horizontal or slanted?
The illusion got its name based on a café in Bristol, England.
The café wall illusion, is produced by a black and white rectangular tessellation when the tiles are shifted in a zigzag pattern. While the pattern seems to slant to the upper and lower right corners, the gray (mortar) lines are parallel.
When the mortar lines are black, the effect is reduced somewhat.
It really is hard believing that the lines bordering the light blue stripes are actually parallel.
The illusion disappears if the dark green squares are directly beneath each other or in a checkerboard pattern.

3. 10. Stepping Feet Illusion
Observe the movement of blue and yellow “feet”. The feet seem to step alternately, rather than simultaneously. This is more pronounced if you do not look directly on the feet, but between them. (Place mouse over image above and Click)
Now we remove the high contrast and we see that the feet are moving simultaneously. (Place mouse over image above and Click)
The image above was created with an even number of black bars per unit distance. The image to the right was created with an odd number of black bars per unit distance. (Place mouse over image above and Click)
Even numbers give the stepping feet illusions. Odd numbers do not. Stuart Antis first demonstrated this illusion in 2003.

4. 11. Rotating Snakes Illusion
Stare at the image to the right. You will see strong rotation of the wheels, some clockwise, some counter-clockwise. Yet, there is no motion at all. If you focus on one area of it, the effect disappears. The illusion was developed by Professor Akiyoshi Kitaoka in 2003.
A variant of this illusion is below. Read this text while keeping the image in your peripheral vision. It should appear to expand. If you stare at it directly, it doesn’t.

5. 12. Motion Aftereffect Illusion
Gaze at the center of the rotating spiral for about 20 seconds, then look at something close. You will notice that whatever you look at now appears swirling. This sensation will disappear after a few seconds. The spiral aftereffect was first described by Joseph Plateau in 1849.

6. 13. Turning the Tables Illusion
You have to push one of these tables through a narrow doorway. The task would be easier with which table?
If there is an opportunity to interpret a drawing as a 3-dimensional object, we do. The two table tops have exactly the same 2-dimensional shape on the page, except for the rotation. Nobody believes this when they first look at the illusion. Roger Shepard’s illusion shows that we don’t see the 2-D shape drawn on the page, but instead we see the 3-D shape of the object in space.
We rotate the tabletop to move it from the left table to the right table.

7. Sources
8. Hering -
9. Café Wall -
10. Stepping Feet -
11. Rotating Snakes -
12. Motion Aftereffect -
13. Turning the Tables -