To  facilitate  an  accurate  comparison,  the  solar  spectrum
is  photographed  side  by  side  on  the  same  plate  with  the
spectrum  of  the  substance  whose  presence  in  the  Sun  is  to
be  determined.  In  order  to  accomplish  this,  one-half  of  the
slit  is  covered,  and  the  sunlight  is  admitted  through  the
other  half.  Thus  the  solar  spectrum  is  photographed  on
one  side  of  the  plate.  After  this  exposure  is  completed,  the
sunlight  is  shut  off,  and  the  screen  in  front  of  the  slit  moved
so  as  to  cover  the  half  previously  open,  and  to  uncover  the
other  half.  The  image  of  the  Sun  on  the  slit  of  the  spectro-
scope is  then  replaced  by  an  image  of  an  electric  arc  light,
burning  between  two  poles  of  iron.  The  spectrum  of  the  iron
vapor  is  thus  produced  on  the  plate,  and  a  strip  of  this
spectrum  is  photographed  beside  the  strip  of  solar  spectrum.
This  is  illustrated  in  Fig.  2,  Plate  XXII,  where  the  upper


Grating  Spectroscopes  61

strip  is  a  small  part  of  the  spectrum  of  iron.  It  will  be  seen
by  a  glance  at  this  photograph  that  these  bright  lines  of  iron
are  represented  in  the  solar  spectrum  by  corresponding  dark
lines,  which  accurately  match  them  in  position.  In  Rowland's
work  on  the  solar  sj^ectrum  thousands  of  lines  were  found  to
correspond  with  iron  lines  given  by  the  electric  arc.
