sentences:  This property is the key for a parametric representation of the canal surface.The center of the circle (for parameter formula_20) has the distance 
  formula_23 (s. condition above)
  from the center of the corresponding sphere and its radius is formula_24.Hence
  where the vectors formula_26 and the tangenten vector formula_27 form an orthonormal basis, is a parametric representation of the canal surface.