286  M.  S.  Lamansky  on  the  Heat- Spectrum  of 
Unfortunately  I  was  prevented  by  unfavourable  weather  from 
bringing  to  a  conclusion  the  investigations,  thus  commenced,  on 
the  limit  of  refrangibility ;  so  that  I  am  not  yet  in  a  position  to 
allege  direct  proofs  in  favour  of  the  above-mentioned  conjecture. 
As  regards  the  position  of  the  maximum  of  thermal  effect 
in  the  flint-glass  spectrum,  it  also  is  found  outside  the  red. 
In  all  the  experiments  made  in  June  and  July,  I  found  it  before 
the  first  break  ;  in  those  made  in  October,  on  the  contrary,  it  was 
after  the  same.  In  the  different  positions  of  the  maximum  of 
heat-effect  in  the  solar  spectrum  may  also  lie  the  reason  that  the 
statements  of  the  various  investigators  on  the  subject  have  been 
so  different. 
When  in  the  solar  spectrum  we  trace  the  heat-effect  from  the 
line  D  into  the  luminous  portion,  the  deflections  decrease  gra- 
dually ;  with  my  arrangement  I  could  perceive  distinct  thermal 
action  beyond  the  line  G.  That  this  did  not  arise  from  diffused 
obscure  heat  was  ascertained  by  means  of  the  two-prism  method 
above  mentioned. 
From  my  experiments  I  have  projected  the  heat-curve  for  the 
solar  spectrum,  by  taking  the  deflection  for  the  maximum  as  100, 
and  reducing  the  others  to  this.  All  that  has  been  said  above 
can  be  readily  seen  in  such  a  curve  as  the  following,  which  re- 
presents the  distribution  of  heat  in  the  rock-salt  spectrum. 
If,  in  such  a  curve  from  the  experiments  with  rock-salt  appa- 
ratus, we  compare  the  part  corresponding  to  the  obscure  heat 
with  that  which  corresponds  to  the  luminous,  we  find  that  the 
former  is  twice  as  great;  yet  it  is  not  strictly  correct  to  reckon 
in  this  manner  the  ratio  of  the  amount  of  the  obscure  to  that  of 
the  luminous  heat,  because,  as  I  shall  further  show,  a  certain 
amount  of  diffused  obscure  heat  is  spread  over  the  luminous  part 
of  that  spectrum. 
II.  Distribution  of  the  Heat  in  the  Spectrum  of  the  Lime-Light, 
These  experiments  were  carried  out  in  the  same  manner  as 
those  on  the  heat-spectrum  of  the  sun.  Between  the  incan- 
descent lime  cylinder  and  the  slit  a  lens  of  short  focal  distance 
