If  now,  in  the  second  place,  the  light  be  viewed  across  the
lines  of  force,  then  effects  will  be  produced  by  all  three
components.  The  first  component,  that  is  the  electrons
vibrating  parallel  to  the  magnetic  lines  of  force,  will  give  rise
to  waves  which  are  plane  polarised,  the  vibrations  being  parallel
to  the  lines  of  force.  Since  these  motions  are  not  influenced  by
the  magnetic  field,  the  oscillation  frequency  of  the  resulting
waves  will  be  the  same  as  that  of  the  undisturbed  light.  The
two  circular  components  will  now.be  seen  sideways,  and  the
electrons  will  act  as  if  they  were  simply  vibrating  in  straight
lines  up  and  down,  or  perpendicular  to  the  lines  of  force.
THE  ZEE  MAN  EFFECT  531

They  will  thus  give  rise  to  plane  polarised  light  waves,  the
vibrations  being  perpendicular  to  the  lines  of  force.  As  in  the
previous  case,  since  the  two  circular  motions  have  not  the  same
period,  therefore  two  of  these  plane  polarised  rays  will  be
emitted,  one  having  an  oscillation  frequency  greater  and  the
other  less  than  that  of  the  original  light.  When,  therefore,  the
light  is  viewed  across  the  lines  of  force,  each  spectrum  line
should  be  seen  to  be  split  symmetrically  into  three  lines,  and
each  line  should  be  plane  polarised,  the  two  outside  com-
ponents having  their  plane  of  polarisation  parallel  to  the  lines
of  force,  and  the  middle  component  with  its  plane  of  polarisa-
tion perpendicular  to  the  lines  of  force.
With  the  apparatus  at  his  disposal,  Zeeman  ^  was  not  at  first
able  to  split  the  spectrum  lines  into  doublets  or  triplets,  but  he
found  that  the  lines  were  widened,  and  that  the  edges  were
polarised  exactly  in  the  way  he  expected  from  the  theory.
