The  sunlight  is  originally  a  bundle  of  rays  with
their  edges  in  all  directions;  but  in  the  encounter
with  the  solid  particle  those  rays  which  are  edgewise
refuse  to  bend,  leaving  only  the  others,  so  that
the  ray  is  now  "  polarized  "  as  it  is  called.  If  the
corona  is  merely  a  mass  of  gas,  the  light  will  not  be
polarized.  If  the  corona  is  partly  gaseous  and
partly  solid  particles,  we  shall  get  some  light  from
the  solid  particles  which  is  polarized  and  some  from
the  gas  which  is  not.  How  much  is  there  of  each
kind?  Here  again  we  come  to  the  question  of
measurement,  of  finding  out  how  much  of  a  thing
there  is,  and  by  arranging  the  particular  photo-
graphs to  be  taken  we  can  find  it  out  by  measure-


THE  SUN


247


ments  of  the  same  kind  as  before,  viz.  of  the  density
of  the  negative  at  different  points.     In  Fig.  79  the


CORONA

SEEN     BY


ORDINARY
LIGHT


PARTIAL     POLARIZATION

Fig.  79.
light  of  the  corona  is  supposed  to  be  equal  in  all
directions  as  in  the  top  picture.  If  on  taking  a
pair  of  pictures,  one  polarized  for  bending  right
and  left,  and  the  other  up  and  down,  we  got  the


248  A  VOYAGE   IN   SPACE

results  shown  in  the  middle,  we  should  say  that
the  polarization  is  complete.  If  we  get  a  pair  as
shown  at  the  bottom,  we  should  say  that  it  is  only
partial.  The  actual  pair  I  got  in  Egypt  in  1905
is  shown  in  Fig.  80.  My  conclusion  is  that  there
are  a  large  number  of  solid  particles  in  the  corona,
because  there  is  a  great  deal  of  polarized  light;


Fig.  80. — Two  pictures  of  the  1905  Corona,  polarized  in
perpendicular  planes.
