←The standard Hering illusion (1861).
Placing the mouse over the image¹ causes the oblique lines to disappear, revealing the fact that the red bars were completely straight all the time.
[¹On a touch device, tap the image. To untap, tap on the icon top right.]
The picture on the right is a mess to look at. But if you place the mouse over it, you can see how beautifully it changes the apparent orientation of the red squares.
This is a dynamic variation of “Three in One” by Hans Kuiper which, in turn, is based on “Square of Three” by Reginald H. Neal (colour lithograph, 1964).
←Another variant, a combination of Hering’s and Zöllner’s illusion. Again, judge the straightness of the lines, then place the mouse over the image. The combination of circles and squares is also known as Ehrenstein illusion (but more common is his other one, to be added).