﻿78 
  Prof. 
  L. 
  R. 
  Iiigersoll 
  on 
  Magnetic 
  

  

  spectrometer, 
  to 
  reach 
  finally, 
  after 
  dispersion, 
  the 
  strip 
  of 
  a 
  

   bolometer. 
  On 
  exciting 
  the 
  magnet 
  the 
  rotation 
  of 
  the 
  

   polarization 
  plane 
  would 
  cause 
  a 
  change 
  of 
  intensity 
  of 
  the 
  

   radiation 
  passing 
  the 
  analyser 
  and 
  falling 
  on 
  the 
  bolometer, 
  

   which 
  change, 
  when 
  divided 
  by 
  the 
  actual 
  intensity 
  of 
  trans- 
  

   mitted 
  radiation 
  of 
  any 
  given 
  wave-length, 
  gave 
  a 
  ratio 
  

   proportional 
  to 
  the 
  rotation 
  for 
  this 
  particular 
  w^ave-length. 
  

  

  1 
  he 
  Compensating 
  Pinnciple. 
  — 
  The 
  foregoing 
  arrangement 
  

   was 
  capable 
  of 
  yielding 
  very 
  fair 
  results 
  at 
  a 
  great 
  expendi- 
  

   ture 
  of 
  labour 
  in 
  observing, 
  but 
  difficulty 
  arose 
  from 
  small 
  

   and 
  unavoidable 
  changes 
  of 
  brilliancy 
  of 
  the 
  glower 
  which 
  

   would 
  appear 
  as 
  spurious 
  rotations. 
  After 
  trying 
  many 
  ways 
  

   of 
  compensating 
  for 
  this, 
  there 
  was 
  finally 
  adopted 
  the 
  scheme 
  

   of 
  using 
  as 
  analyser 
  a 
  large 
  double-image 
  prism, 
  placed 
  so 
  

   that 
  each 
  of 
  its 
  principal 
  planes 
  made 
  an 
  angle 
  of 
  45° 
  with 
  

   the 
  original 
  plane 
  of 
  polarization. 
  The 
  two 
  transmitted 
  

   beams 
  of 
  equal 
  intensity 
  were 
  carried 
  through 
  the 
  spectro- 
  

   meter, 
  and 
  after 
  dispersion 
  were 
  allowed 
  to 
  fall 
  on 
  the 
  two 
  

   strips 
  of 
  a 
  specially 
  constructed 
  bolometer. 
  Any 
  rotation 
  

   would 
  thus 
  diminish 
  one 
  beam 
  and 
  increase 
  the 
  other 
  by 
  an 
  

   equal 
  amount 
  — 
  thus 
  producing 
  a 
  doubled 
  effect 
  — 
  while 
  any 
  

   small 
  change 
  in 
  brilliancy 
  of 
  the 
  glower 
  would 
  affect 
  each 
  

   beam 
  equally, 
  and 
  hence 
  give 
  rise 
  to 
  no 
  error. 
  This 
  was 
  

   immeasurably 
  superior 
  to 
  the 
  former 
  arrangement, 
  and 
  more 
  

   than 
  made 
  amends 
  for 
  the 
  small 
  loss 
  of 
  range 
  of 
  spectrum 
  

   its 
  use 
  involved 
  ; 
  for 
  the 
  thick 
  calcite 
  prisms 
  (the 
  f)olarizing 
  

   plates 
  were 
  also 
  replaced 
  by 
  calcite) 
  would 
  transmit 
  little 
  

   beyond 
  X=2*5//'. 
  

  

  The 
  very 
  simple 
  mathematics 
  of 
  this 
  method 
  have 
  already 
  

   been 
  worked 
  out 
  in 
  a 
  former 
  paper*, 
  and 
  we 
  need 
  take 
  from 
  

   this 
  only 
  the 
  formula 
  

  

  g 
  = 
  45/7r.^I/I, 
  

  

  which 
  gives 
  for 
  any 
  wave-length 
  the 
  rotation 
  5 
  in 
  degrees, 
  

   w^hen 
  I 
  is 
  the 
  intensity 
  of 
  radiation 
  on 
  either 
  bolometer 
  strip 
  

   for 
  this 
  wave-length, 
  and 
  c/I 
  the 
  relative 
  change 
  of 
  intensity 
  

   ■of 
  the 
  two 
  beams. 
  

  

  Use 
  of 
  High 
  and 
  Low 
  Dispersion. 
  — 
  One 
  of 
  the 
  problems 
  

   to 
  be 
  met 
  in 
  this 
  work 
  was 
  that 
  of 
  securing, 
  in 
  spite 
  of 
  the 
  

   many 
  losses, 
  a 
  sufficiently 
  intense 
  spectrum 
  such 
  that 
  the 
  

   small 
  changes 
  of 
  intensity 
  amounting 
  to 
  perhaps 
  less 
  than 
  

   1/10 
  per 
  cent, 
  would 
  still 
  be 
  readily 
  measurable. 
  Every 
  

   ^effort 
  had 
  been 
  made 
  to 
  secure 
  this 
  in 
  the 
  earlier 
  form 
  of 
  

   apparatus 
  by 
  the 
  use 
  of 
  large 
  apertures 
  and 
  a 
  brilliant 
  source, 
  

   hence 
  as 
  a 
  still 
  further 
  increase 
  was 
  desired 
  for 
  the 
  present 
  

   * 
  Phys. 
  Eev. 
  loc. 
  cit. 
  

  

  