How To Build A Maxwell's Fishpond

How To Build A Maxwell's Fishpond: One mathematical solution stuck out as being particularly special: a lens in which all rays of light follow circular arcs. In this lens, light from any point in or on the sphere is always focused to another point on the other side of the sphere...
These guys have built a two dimensional version of Maxwell's fisheye lens which works with water waves rather than electromagnetic ones. Naturally, they call their device a Maxwell's fishpond.
The device is remarkably simply--a shallow dish with a curved bottom that rises towards the middle...When filled with water, waves on the surface behave in a remarkable way. The depth of the water determines the refractive index of waves on the surface. So the trick here is to design the shape of the bottom of the dish in a way that follows Maxwell's plan.
That turns out to be pretty straightforward with the result that a circular wave pattern does not simply spread out and disperse. Instead, it always reconverges to a point on the other side of the dish, regardless of where it formed in the first place.