the  Sun  and  the  Lime-Light.  287 
was  placed,  and  at  twice  that  distance  from  each  of  them.  The 
distribution  of  the  heat  in  the  spectrum  of  the  lime-light  was 
likewise  investigated  both  with  flint-glass  prisms  and  with  rock- 
salt  apparatus. 
In  the  experiments  with  flint-glass  prisms  I  was  obliged  to 
work  with  a  rather  broad  slit  (2  rnillims.),  as  the  thermal  effects 
were  very  feebly  expressed.  In  the  luminous  part  of  such  a 
spectrum,  I  could  only  verify  a  slight  heat-effect  in  the  red  and 
orange.  From  the  red  outwards  to  the  ultra-red  end  the  deflec- 
tions gradually  augmented  until  they  attained  a  certain  maxi- 
mum ;  then  a  gradual  diminution  commenced,  yet  without  that 
interruption  of  continuity  which  we  have  always  seen  in  the  solar 
spectrum.  I  will  here  mention  that  Tyndall,  in  the  account  of 
Ins  well-known  experiments  on  "  Calorescence "  (Phil.  Trans. 
1866),  called  attention  to  the  circumstance  that  the  discontinuous 
distribution  of  heat,  first  observed  by  Sir  John  Herschel  in  the 
-solar  spectrum,  does  not  exist  in  the  spectrum  of  artificial  sources 
of  light. 
On  comparing  the  distribution  of  heat  in  the  spectrum  of  the 
lime-light  with  that  in  the  solar  spectrum,  we  find  that  the  posi- 
tion of  the  maximum  of  heat-effect  is  much  further  from  the  end 
of  the  visible  red  in  the  former  than  in  the  latter.  In  other 
words,  with  the  feebler  sources  of  heat  the  intensity  of  heat- 
effect  attained  its  maximum  for  rays  of  greater  wave-length 
than  with  more  powerful  sources  of  heat.  Moreover  this  result 
was  to  be  expected ;  for  wre  have  here  the  same  case  as,  for  ex- 
ample, in  the  incandescence  of  a  platinum  wire — namely,  that 
the  higher  the  temperature  of  the  incandescence  ascends,  the 
more  rays  of  less  wave-length  are  emitted  by  the  wire. 
Besides,  also  in  the  flint-glass  spectrum  of  the  lime-light  we 
do  not  observe  the  sudden  diminution  of  heat-effect  which  we 
have  seen  in  the  solar  spectrum ;  this,  I  believe,  was  merely  in 
consequence  of  the  slit  being  broad  in  our  experiments  with 
artificial  sources  of  light.  But  in  the  experiments  with  rock- 
salt  apparatus,  where  the  slit  was  not  so  broad  as  in  those  with 
flint-glass  prisms,  though  broader  than  in  the  corresponding 
experiments  on  the  heat-spectrum  of  the  sun,  after  the  maximum 
a  place  can  always  be  pointed  out  where  a  very  sudden  diminu- 
tion is  perceptible.  However,  as  was  remarked  above,  the  ques- 
tion as  to  the  existence  of  an  abrupt  diminution,  or  the  probable 
limit  of  refrangibility,  can  only  be  decided  by  the  two-prism 
method  and  the  employment  of  very  fine  slits. 
We  have  already  mentioned  that,  in  the  flint-glass  spectrum 
of  the  lime-light,  only  feeble  heat-effects  could  be  ascertained  in 
the  red  and  orange;  but  when  the  same  lime-light  was  analyzed 
with  a  rock-salt  prism,  and  the  heat-effect  of  its  spectrum  inves- 
