text: The next step is to apply a coordinate transformation, introducing the travel time (or wave phase)  given by
  :  so  
  and  are related through the celerity  Introducing the slow variable  and denoting derivatives of  and  with respect to  with a prime, e.g.  the -derivatives in the wave equation, Eq. (), become:
  Now the wave equation () transforms into:
  The next step is transform the equation in such a way that only deviations from homogeneity in the second order of approximation remain, i.e. proportional to