Sounds like another filter/transmit, fog/atmosphere problem. All I can do is 
show you a test-file which may show similar thing even though it is using 
halo.
//BEGIN TEST FILE
camera
{
  location  <0.0 , 3.0 ,-8.0>
  angle 33
  look_at   <0.0 , 3.0 , 0.0>
}

#declare TestHalo = sphere { 0, 2
 // pigment { color rgbft <1, 1, 1, 1.125, 0> }  // type A equalization: no 
b.g., fog t=.5
  pigment { color rgbft <1, 1, 1, 1.2, .125> }  // type B equalization: b.g. 
or not, fog t=0
 // pigment { color rgbft <1, 1, 1, 1, (1/3)> }  // type C equalization: b.g. 
or not, fog t=0
    halo {
      emitting
      spherical_mapping
      linear
      color_map {
        [ 0 color rgbt <1, 0, 0, 1> ]
        [ 1 color rgbt <1, 1, 0, 0> ]
      }
      samples 10
      scale 2
    }
    hollow
    no_shadow
  }

//  transmit .5 (only, no b.g.) for type A,
//  0 (.5 only with b.g.) for type B,
//  0 (.5 only with b.g.) for type C
fog {
    distance 50
    colour rgbft<.5, .5, .5, .5, .5>
    fog_type 2
    fog_offset 3
    fog_alt .25
    }

light_source
{
  0*x
  color red 1.0  green 1.0  blue 1.0
  looks_like{ object {TestHalo}}
  translate <0, 2.5, 4>
}

// comment out below to see without b.g. objects
///*
plane {z,10 pigment {rgb <.33,.66,.99>} hollow}  // a background object messes 
it up again
plane {y,0 pigment {rgb <.99,.33,.99>} hollow}  // a background object messes 
it up again
cone {-3*y,1,3*y,0 pigment {rgb<0,1,0>} translate <0,3,6> no_shadow}
//*/
//END OF TEST FILE

Message <01bdea3b$c8ad1bc0$6b43a8c2@wphnvffu>, Mick Hazelgrove  typed...
>
>Help
>
>I've being trying to put an image map of an insects wing in a scene with
>fog
>
>Where the box on which the image map is projected meets the fog it shows up
>and the background shows through the fog
>
>Does anyone know how to get round this - I've tried upping Max_Trace level
>but it has know effect.
>
>Hope someone knows the answer to this one
>
>Mick Hazelgrove

-- 
 omniVERSE: beyond the universe
  http://members.aol.com/inversez/POVring.html
=Bob

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