public SurfaceIntersection(VolumeSphere s1, VolumeSphere s2)
 {
     this.s1 = s1;
     this.s2 = s2;
 }
 public SurfaceDifference(VolumeSphere s1, VolumeSphere s2)
 {
     this.s1 = s1;
     this.s2 = s2;
 }