An exact solution for the Casimir force in a spherically symmetric   medium
  We calculated the force of the quantum vacuum, the Casimir force, in a spherically symmetric medium, Maxwell's fish eye, surrounded by a perfect mirror and derived an exact analytic solution. Our solution questions the idea that the Casimir force of a spherical mirror is repulsive - we found an attractive force that diverges at the mirror. 
Ulf Leonhardt and William M. R. Simpson
quant-ph