When  the  lines  of  the  arc  spectrum  of  a  metal  are  indi-
vidually examined,  they  will  be  found  as  a  general  rule  to
differ  materially  in  appearance  amongst  themselves.  They
are  by  no  means  all  equally  well  defined ;  some  of  them
are  sharply  defined  on  both  sides,  while  others  are  sharply
defined  on  one  side,  the  other  side  being  nebulous  and  diffused  ;
there  are  further  to  be  seen  frequently  lines  which  are  nebulous
on  both  sides.  Distinction  may  therefore  be  drawn  between
four  types  of  lines,  those  which  are  well  defined  at  both  edges,
those  which  are  diffused  only  towards  the  red,  those  which  are
diffused  towards  the  blue,  and  lastly,  those  which  are  diffused
on  both  sides.  It  is  interesting  to  note  that  the  lines  which
belong  to  the  same  spectral  series  are  always  of  the  same
character,  and  that  we  have,  for  example,  in  this  way  a  sharp
series  and  a  diffuse  series  belonging  to  each  alkali  metal.
The  importance  of  these  characteristic  appearances  of  the  lines
is  thus  manifest,  and  in  all  spectral  work  care  should  be  taken
to  note  the  character  of  all  the  lines  measured,  because  in  any
'  Handbuch  der  Spectroscopic,  vol.  ii.  p.  222.
502  SPECTROSCOPY

attempt  to  arrange  the  lines  in  series,  such  observations  will  be
of  the  greatest  importance.
These  differences  in  character  of  the  lines  of  the  spectrum
of  a  substance  are  chiefly  noticeable  in  the  case  of  arc  and
spark  spectra.  In  the  spectra  of  gases  in  vacuum  tubes  the
lines  are  almost  always  very  narrow  and  sharply  defined ;  this
is  no  doubt  due  to  the  very  low  pressure  that  the  gas  is  exerting.
