POV global media does not work if the rays go into infinity. You either have 
to use a media container or put a object into the scene where all rays hit, 
like e.g. 
 sphere{0,100 hollow}

> 
> 
> #declare SphereCutout = difference {
>         sphere { <0,0,0>,1 }
>         box { <-.1,-2,-2>,
<snip>

You can construct this CSG object more compactly: 
#declare SphereShell = intersection{
        sphere { <0,0,0>,1 }    
        box { <-.1,-2,-2>,<.1, 2, 2> inverse}
        box { <-.1,-2,-2>,<.1, 2, 2> rotate z*90 inverse}
        sphere { <0,0,0>,0.95 inverse}
}

Also 10 samples won't be enough to get a nice picture in this case.

- Micha

-- 
POV-Ray Objects Collection: http://objects.povworld.org

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