Question

A technician wishes to produce the lens +2.00DS/+2.00DC x90 in toric form, in CR39 (n=1.498) with a -5.00 base curve. After finishing the toroidal surface with the CR39 tool it is discovered that, in error, they have used a plastics material of refractive index 1.56. Which CR39 tool must now be used to work the front surface?