Kinds  of  Spectra.  —  If,  with  any  form  of  spectroscope,  we
look  at  the  flame  of  a  candle  or  any  other  artificial  light,  not
very  far  away,  the  spectrum  will  show  the  whole  series  of
spectral  colors  without  any  dark  lines;  but  if  the  light  at
which  we  look  is  several"  miles  away,  a  distant  gaslight,  for
example,  we  shall  see  that  the  spectrum  is  crossed  by  a
number  of  dark  lines.  Why  do  these  lines  appear  when  the
light  is  at  a  distance,  and  not  when  the  light  is  near  ?  Experi-
ments show  that  it  is  because  some  of  the  light  is  absorbed  by


74


ASTRONOMY


[indigo


(Green


'ellow


I  Orange


Red


FIG.  42.
The  solar  spectrum  with
its  dark  lines.
the  air  through  which  it  passes.  The
light  absorbed  is  that  which  belongs
in  the  place  of  the  dark  lines.  The
process  by  which  this  is  brought  about
is  called  selective  absolution.  The  word
selective  implies  that  the  air  does  not
absorb  all  the  different  kinds  of  light
equally,  but  picks  out,  or  selects,  cer-
tain'kinds.
Spectrum  of  the  Sun.  —  If,  instead
of  looking  at  a  distant  light,  we  ex-
amine the  light  of  the  sun  by  the
spectroscope,  we  shall  find  that,  in  ad-
dition to  the  lines  produced  by  the  air,
there  are  a  great  number  of  other
dark  lines  in  the  spectrum.  These  are
formed  by  the  selective  absorption  of
the  sun's  atmosphere,  which  is  dif-
ferent from  the  atmosphere  of  the
earth.
