The surface normal is usually directed outward, but will be directed inward if the given radius is negative. (This typically matters only for one-sided materials.) A zero radius is illegal.
# Create a thick glass sphere with a hollow inside m glass = sides 1 ir 1.52 0 c rs .06 0 ts .88 0 v cent = p 0 0 1.1 # The outer shell sph cent .1 # The inner bubble sph cent -.08