III.    DEVELOPMENT  OF  THE  QUANTUM  THEORY

OF  SPECTRA

We  see  that  it  is  possible  by  making  use  of  a  few  simple  ideas

to  obtain  a  certain  insight  into  the  origin  of  the  series  spectra.
But  when  we  attempt  to  penetrate  more  deeply,  difficulties  arise.
