I changed the settings to
global_settings {
  radiosity {
    pretrace_start 0.08           // start pretrace at this size
    pretrace_end   0.04           // end pretrace at this size
    count 60                      // higher -> higher quality (1..1600) [35]
    nearest_count 5               // higher -> higher quality (1..10) [5]
    error_bound 1.8               // higher -> smoother, less accurate [1.8]
    recursion_limit 5             // how much interreflections are 
calculated (1..5+) [3]
    low_error_factor .5           // reduce error_bound during last pretrace 
step
    gray_threshold 0.0            // increase for weakening colors (0..1) 
[0]
    minimum_reuse 0.015           // reuse of old radiosity samples [0.015]
    brightness 1                  // brightness of radiosity effects (0..1) 
[1]

//    adc_bailout 0.01/2
    adc_bailout 1/64
    normal on                     // take surface normals into account [off]
  }
  max_trace_level 26

This seems satisfactory to me, most disturbing black spots seem to be gone 
now.
I plan to do an animation with this scene (hopefully ready by christmas 
2007), so I guess I'll have to reduce the trace level to something like 17 
make it faster
Thanks for the advice.


> Each time you reflect on a sphere, the trace level goes up by 3: 1 getting 
> into
> the transparent outer shell, 1 relecting on the iner reflective part, and 
> 1 more
> geting OUT of the transparent shell.
> If the reflection hit 2 balls, it register 6 levels, 9 if it hits 3 balls. 
> Add
> one to reatch the background or any surrounding object.
> So, for one ball, you need a Max_trace_level of at least 4. For 2, you 
> need at
> least 7, and 10 if you hit 3.
> In your case, I'd go up to arount 15 or 17. For that scene, I'd go up to 
> the max
> of 255 and set adc_bailout to 1/128 or even as large as 1/64.
> Note that there are many more max_trace_level related black spots than 
> those you
> spotted. I can find at least 8 more, and I probably missed some.

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