Subtracting two isosurfaces to create a shell, portions of the shell where the
inner (subtracted) surface should be visible are completely transparent.  Some
shadows are cast by the invisible shell.  Other shapes I've tried do not do
this.

I've tried this on Linux (Kubuntu 9.10 fully patched, POV-ray 3.6.1 as compiled
by Debian) and Windows Vista (32-bit fully patched, POV-ray 3.6.2 and
3.7.beta.35a) on a P4 dual core 3GHz, 3GB RAM computer, and on WinXP Home
(POV-ray 3.6.2 and 3.7.beta.35a, Core 2 Duo 2.33GHz, 1GB RAM, Japanese).

Sample code:


background { color <0,0.1,0.2> }

camera {
  orthographic
  location <2, 6, -20>
  right <8, 0, 0>
  up <0, 6, 0>
  look_at <0, 0, 0>
}

light_source { <50, 50, -20> colour <1,1,1> }
light_source { <-20, -5, -10> colour <1,1,1> }
light_source { <-2, 5, -10> colour <1,1,1> }

difference{
isosurface{
    function{ pow(x,2) + pow(y,2) + pow(z,2) - pow(2.5,2) }
    contained_by { sphere { <0,0,0>, 2.51 } }
    max_gradient 5.1
    pigment{color <1,1,0>}
           }
isosurface{
    function{ pow(x,2) + pow(y,2) + pow(z,2) - pow(2.4,2) }
    contained_by { sphere { <0,0,0>, 2.51 } }
    max_gradient 5.1
    pigment{color <1,1,0>}
           }
  box { <0, -5, 0>, <5, 5, -5>
    rotate <0, 45, 0>
    pigment{ color <1,1,0> }
       }
}

// I found the following helpful in demonstrating the issue

difference{
 torus{ 1, 0.2
      pigment{ color <1,0,0> }
      translate <0,-2.35,0>
       }
  box { <0, -5, 0>, <5, 5, -5>
    rotate <0, 45, 0>
    pigment{ color <1,0,0> }
       }
}

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