Though the blue lines are parallel to each other, they don't appear to be.


Though the two brown lines are of same size, they don't appear to be so. This was invented by Orbison.


Though the quadrilateral is a rectangle, it doesn't appear so. This is another version of the one by Orbison, given by Ponzo.


Though the brown lines are parallel to each other, they don't appear so. This was by Hering.


Look at the bar in the center. It is of same color from its left end to right end, but appears as if lighter on the left and darker on the right.


Do you see that at the center of the above picture, there is a bulge... But all the lines are parallel and all the circles are equally spaced.


Look at the center and move your head forth and back. What do you see.


All the squares in black lines are parallel to each other.. but ... what do you feel...