#declare firefly = sphere // transparent sphere containing media
 { 0,1 pigment { rgbt 1 } hollow
   interior
   { media
     { emission 1
       density
       { spherical density_map
         { [0 rgb 0]
           [0.4 rgb <0,1,.49>]
           [0.8 rgb <0,1,.49>]
           [1 rgb 1]
         }
       }
     }
  }
 }