Ah, yes, 19/20 the second time (only the last one wrong).
The first time I kept my eyes fixed in the same place roughly in the middle which clearly wasn't a good idea. On the second attempt I glanced between each circle in turn, trying to discern the difference over two points in time rather than two points in space.
The first time I kept my eyes fixed in the same place roughly in the middle which clearly wasn't a good idea. On the second attempt I glanced between each circle in turn, trying to discern the difference over two points in time rather than two points in space.