It's a description of an optical illusion.
Imagine (or draw out) two sets of circles, one set on the right, and one set on the left. On the left, put a circle of size n and surround it with smaller circles. Do the same on the right, but surround it with larger circles.
Now show your imaginary circles to an imaginary friend ask the question: which of the two center circles is bigger, the one on the right or the one on the left?
The answer?
Like Naso said:
"The center circles on the left and the right are equal.
Yes -- they are identical: the surrounding circles fool the mind....
<Next Illusion>:
The Impossible Cube"
So where'd you get this?