Here is the code:

// Demo scene for povray 3.5
// Features Gaussian distribution and use of trace function
// Composed by Greg M. Johnson 2001
// Uses a macro by Rico Reusser.
//

#declare UseRadiosity = no;
#declare S = seed(20173);
#declare CompRadius=0.05;//radius of blob components;
#declare CompNum=7500;//number of blob components

#include "colors.inc"
#if(UseRadiosity)
 global_settings {
  radiosity {
   pretrace_start 0.04
   pretrace_end 2/300
   count 400
   recursion_limit 2
   nearest_count 1
   error_bound 0.2
  }
 }

 #default {finish { ambient 0}}
#else
 light_source {<0, 200, -100> colour rgb 1.5}
#end


//This macro created by Rico Reusser <reu### [at] chorusnet>
#declare e = 2.718281828459;
#macro Gauss(RSR)
 sqrt(-2*log(rand(RSR))/log(e))*cos(2*pi*rand(RSR))
#end


plane {y,-0.9105  pigment {White}}


#declare Norm=<0,0,0>;//This variable is used to hold the normal vector
obtained
//using trace(). This vector is then used to determine whether an
intersection was found.
#declare Posy=array[CompNum]
#declare Posy[0]=<Gauss(S),0,Gauss(S)>;
#declare Ally=sphere {Posy[0], CompRadius}//This variable will hold a
collection of spheres,
//one for each blob component. The algorithm checks against this object to
decide
//if a component is "on top of" another.

#declare N=1;
#while(N<CompNum)
 #declare Test=<Gauss(S),0,Gauss(S)>;
 #declare Tracey=trace(Ally,Test+100*y,-y,Norm);

 #if (Norm.x = 0 & Norm.y = 0 & Norm.z = 0)
  #declare Posy[N]=Test;
  //put the new component at y=0
 #else
  #declare Posy[N]=Tracey+Norm*CompRadius;
  //put the new component at a point "above" the one it hit
  //The algorithm actually uses the normal to compute the position
 #end

 //Add new sphere to union
 #declare Ally=
 union {object {Ally}
  sphere {Posy[N], CompRadius}
 }

// #debug concat(str(N,4,0), "n")
 #declare N=N+1;
#end

blob {
 threshold 0.5

 #declare N=0;
 #while(N<CompNum)
  sphere {Posy[N], CompRadius*2.85, 1}
  #declare N=N+1;
 #end
 pigment {rgb <0.2,1,0.1>}
}


background {White}

sphere {<0.5,0,0>, 0.5
 inverse
 pigment {
  gradient x
  pigment_map {
   [0.0   rgb 0]
   [0.995   rgb 0.04]
   [1.0   rgb 2]
  }
 }
 finish {ambient 20}
 translate <-0.5,0,0>
 scale 500
}

camera {
 location <0,3,-15>
 look_at <0,0,0>
 angle 20
}

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