isosurface { //-------------------------------------------------------------

  function{
    f_dupin_cyclid(
       x,y,z,
       -0.05, // field strength,
       1,     // major radius of torus,
       0.25,  // minor radius of torus,
       0.25,  // x displacement of torus,
       0.0,   // y displacement of torus,
       0.75   // radius of inversion
     ) 
  }
  contained_by {sphere {0,1.25}}
 
  texture{ pigment{ color rgb <0.6,0.5,1>} 
           finish { specular 0.1 roughness 0.1 phong 0.2 }
           }
  scale 1.2
  rotate<90,0,0>
  translate< 0,1.3,0>
} // end of isosurface -----------------------------------------------------