A concave lens<>is<>thicker at the edges$$$it<>is<>in the middle
As in the grindings this polisher is fastened to the pedestal, the concave lens rubbed over it until there is enough OshineO to perform a knife-edge and Ronchi test.
A concave lens is thicker at the edges than it is in the middle.
What type of lens is thicker at the edges than it is in the middle?