In  1840  Sir  John  Herschel  continued  the  investigation
of  the  subject,  and  succeeded  in  proving  the  fact  that  the-
spectrum  actually  extended  beyond  the  visible  limit,  and  that
the  Fraunhofer  lines  were  continued  into  this  region!  His
method  of  proving  this  was  one  of  great  ingenuity.  He
painted  a  sheet  of  paper  with  gum  and  lampblack  so  as  to
make  it  readily  absorptive  of  heat ;  this  paper  was  then  dipped
into  alcohol  and  exposed  to  the  sun's  spectrum.  Had  the
invisible  portion  been  quite  continuous,  the  alcohol,  being  so
volatile  a  liquid,  would  have  entirely  evaporated,  leaving  a  dry
strip  where  the  invisible  spectrum  had  been  projected  on  to
the  paper.  This,  however,  Herschel  found  not  to  be  the  case,
for  only  partial  drying  took  place,  and  three  or  four  damp
patches  were  left  which  marked  the  presence  of  absorption
bands  in  the  invisible  region.  The  investigation  was  taken  up
more  fully  soon  afterwards  by  many  experimenters,  and  the
spectrum  was  proved  to  extend  far  beyond  the  limit  of  visibility
in  the  red,  and  to  contain  many  of  the  Fraunhofer  absorption
lines.  This  portion  of  the  invisible  spectrum,  which  is  called
the  infra-red,  consists  of  the  rays  of  longer  wave-lengths  which
evidence  themselves  as  radiant  heat,  and,  therefore  may  be
readily  examined  by  means  of  a  thermopile,  and  most  of  the
investigations  have  been  carried  out  with  the  help  of  such
apparatus.  These  apparatus  improved  in  delicacy  as  time
went  on,  and  the  emission  spectra  were  found  to  extend  a  Very
great  distance  in  this  direction.  Amongst  other  important
work  upon  this  part  of  the  spectrum  may  be  mentioned  that  of
Langley  in  America,  who  made  use  of  an  exceedingly  delicate
electrical  resistance  thermometer,  which  he  called  a  bolometer,
and  made  investigations  chiefly  into  the  infra-red   portion  of


32  ,        SPECTROSCOPY

the  solar  spectrum.  Very  important  researches  have  also  been
carried  out  by  Paschen  and  by  Rubens,  a  full  account  of  whose
methods  will  be  given  in  Chapter  VILI.  It  is  interesting  to
note  that  photographic  plates  have  been  made  which  are  sensi-
tive to  these  heat  rays,  and  Abney  and  others  have  succeeded
in  obtaining  photographic  records  which  extend  very  far  into
this  region.
