INVISIBLE  RADIATION  OF  THE  ELECTRIC  LIGHT.  217 
its  various  colors  in  succession,  and  to  search  the  spaces  beyond 
the  region  of  color  in  both  directions. 
As  in  the  case  of  the  solar  spectrum,  the  heat  was  found  to 
augment  from  the  violet  to  the  red,  while  the  maximum  heating 
effect  was  observed  beyond  the  red,  and  at  a  distance  from  the 
red,  in  one  direction,  equal  to  that  of  the  green  of  the  spec- 
trum in  the  other. 
The  augmentation  of  temperature  beyond  the  red  in  the  case 
of  the  electric  light  is  sudden  and  enormous.  Plotting  from  a 
datum  line  the  thermal  intensity  of  the  various  portions  of  the 
spectrum,  the  ordinates  suddenly  increase  in  length  beyond  the 
red,  reach  a  maximum,  and  then  fall  somewhat  more  suddenly 
on  the  other  side.  When  the  ends  of  the  ordinates  are  united, 
the  curve  beyond  the  rod  rises  in  a  steep  and  massive  peak, 
which  quite  dwarfs  the  luminous  portion  of  the  spectrum. 
The  comparative  height  and  steepness  of  this  peak  are  much 
greater  than  those  obtained  by  Professor  Miiller  for  the  solar 
spectrum.  Aqueous  vapor  acts  powerfully  upon  the  invisible 
rays,  and,  doubtless,  the  action  of  this  substance  in  our  atmos- 
phere has  toned  down  the  eminence  beyond  the  red  in  Professor 
Muller's  diagram.  A  solar  spectrum,  produced  beyond  the 
limits  of  the  atmosphere,  would  probably  exhibit  as  steep  a  peak 
as  that  of  the  electric  light. 
In  the  experiments  now  to  be  referred  to,  the  rays  from  the 
electric  light  were  converged  by  a  small  concave  mirror.  The 
glass  mirror  silvered  at  the  back,  which  usually  accompanies  the 
camera  of  Duboscq's  electric  lamp,  was  one  of  the  first  employ- 
ed. It  was  brought  so  near  the  electric  light  as  to  cast  an  image 
of  the  coal-points  five  or  six  inches  in  advance  of  the  light.  A 
solution  of  iodine  in  bisulphide  of  carbon,  contained  in  a  rock- 
salt  cell,  was  then  placed  in  front  of  the  lamp  :  the  light  was 
thereby  cut  off,  but  the  focus  of  dark  rays  remained,  and 
various  effects  of  combustion  and  incandescence  were  obtained 
at  the  focus.  A  mirror  four  inches  in  diameter,  and  silvered  in 
front,  will  enable  an  experimenter  to  obtain  most,  if  not  all  the 
results  now  to  be  mentioned.  I  also  employ  a  mirror  eight 
inches  in  diameter,  and  having  a  focal  length  of  eight  inches, 
with  excellent  effect. 
It  is  not  necessary  to  enclose  the  opaque  solution  in  a  rock 
