Perhaps  one  of  the  most  brilliant  applications  of  the  Zeeman
effect  is  that  by  Hale  ^  to  the  investigation  of  sunspots.  It  had
been  previously  shown  by  him  that  sunspots  consist  of  vortices.
He  succeeded  in  doing  this  by  photographing  the  solar  surface  by
means  of  the  Ha  hydrogen  rays  with  the  large  spectro-helio-
graph  at  Mount  Wilson  observatory..  Since  the  solar  pro-
minences are  known  to  consist  of  glowing  hydrogen  it  is
evident  that  such  photographs  will  show  clearly  the  distri-
bution of  hydrogen  upon  the  surface  of  the  sun.  These  photo-
graphs showed  clear  evidences  of  powerful  cyclonic  movement
and  also  the  absorption  of  a  dark  hydrogen  flocculus  into  the
disturbed  area  of  a  spot  on  July  3rd,  1908.  The  mean  of  six
observations  gave"  the  velocity  of  the  flocculus  as  106  kilo-
metres per  second.  Discussing  the  niagnetic  field  associated
with  large  sunspots,  Hale  suggests  that  there  should  exist
magnetic  lines  of  force  perpendicular  to  the  plane  of  the
vortex.  If  this  were  true,  the  light  from  the  spot  radiated  along
the^e  lines,  should,  of  course,  produce  a  doubling  of  the
spectrum  lines,  with  the  two  components  circularly  and
oppositely  polarised.  Hale  found  the  iron  line  A.  =  630271
to  be  doubled,  thus  >  clearly  proving  the  existence  of  the
magnetic  field  within  the  spot.
