﻿Power 
  in 
  the 
  neighbourhood 
  of 
  Bands 
  of 
  Absorption. 
  159 
  

  

  invisible 
  at 
  —259° 
  C. 
  Some 
  bands 
  which 
  made 
  their 
  first 
  

   appearance 
  at 
  low 
  temperatures 
  still 
  continue 
  to 
  increase 
  in 
  

   intensity 
  at 
  —259° 
  C. 
  ; 
  we 
  think 
  they 
  all 
  should 
  possess 
  a 
  

   maximum, 
  but 
  at 
  a 
  temperature 
  which 
  we 
  have 
  not 
  yet 
  

   attained. 
  

  

  The 
  study 
  of 
  magneto-optic 
  phenomena 
  at 
  very 
  low 
  tem- 
  

   peratures 
  has 
  considerably 
  extended 
  the 
  field 
  of 
  research 
  

   and 
  has 
  led 
  to 
  new 
  results 
  of 
  very 
  great 
  interest. 
  In 
  the 
  

   first 
  place, 
  the 
  separation 
  between 
  bands 
  o£ 
  absorption 
  

   of 
  the 
  two 
  circular 
  components, 
  that 
  is 
  to 
  say, 
  the 
  changes 
  of 
  

   period 
  of 
  oscillating 
  systems 
  under 
  the 
  influence 
  of 
  an 
  external 
  

   magnetic 
  field 
  are 
  independent 
  of 
  the 
  temperature, 
  liven 
  in 
  

   paramagnetic 
  bodies 
  these 
  changes 
  of 
  period 
  constitute 
  pheno- 
  

   mena 
  which 
  are 
  connected 
  with 
  diamagnetism*. 
  

  

  Moreover, 
  at 
  very 
  low 
  temperatures, 
  an 
  unforeseen 
  effect 
  

   is 
  observed. 
  The 
  bands 
  which 
  correspond 
  to 
  the 
  absorption 
  of 
  

   the 
  opposite 
  circular 
  components 
  are 
  very 
  often 
  of 
  unequal 
  

   intensity. 
  At 
  temperatures 
  of 
  —253° 
  and 
  —259° 
  the 
  com- 
  

   ponents 
  which 
  the 
  magnetic 
  field 
  displaces 
  towards 
  the 
  blue 
  

   increases 
  at 
  the 
  expense 
  of 
  the 
  other 
  component. 
  The 
  dis- 
  

   symmetry 
  is 
  greater 
  to 
  such 
  an 
  extent 
  as 
  the 
  field 
  is 
  more 
  

   intense 
  and 
  the 
  temperature 
  is 
  lower. 
  

  

  An 
  example 
  of 
  this 
  dissymmetry 
  is 
  visible 
  on 
  fig. 
  1 
  for 
  

   the 
  band 
  525*17 
  fjufi 
  of 
  xenotime 
  at 
  —188°. 
  We 
  clearly 
  see 
  

   that 
  in 
  one 
  of 
  the 
  two 
  spectra 
  of 
  the 
  circular 
  components 
  the 
  

   band 
  525 
  which 
  is 
  displaced 
  by 
  the 
  field 
  towards 
  the 
  violet 
  

   side 
  is 
  more 
  intense 
  than 
  the 
  band 
  corresponding 
  to 
  the 
  

   other 
  spectrum. 
  

  

  To 
  return 
  to 
  the 
  question 
  with 
  which 
  this 
  note 
  is 
  specially 
  

   concerned 
  : 
  the 
  variations 
  in 
  the 
  intensity 
  of 
  the 
  bands 
  and 
  

   their 
  magnetic 
  components 
  draw 
  in 
  their 
  train, 
  as 
  we 
  might 
  

   expect, 
  modifications 
  in 
  the 
  magnetic 
  rotatory 
  dispersion. 
  

   The 
  magnitude 
  of 
  the 
  effects 
  produced 
  for 
  each 
  band 
  depends 
  

   upon 
  the 
  intensity 
  of 
  absorption 
  and 
  on 
  the 
  breadth 
  of 
  the 
  

   band. 
  When 
  a 
  band 
  becomes 
  finer 
  and 
  more 
  intense 
  the 
  

   variations 
  of 
  rotatory 
  power 
  increase 
  at 
  the 
  same 
  time 
  ; 
  

   moreover, 
  the 
  dissymmetry 
  of 
  intensity 
  for 
  the 
  two 
  magnetic 
  

   components 
  is 
  accompanied 
  by 
  a 
  dissymmetry 
  in 
  the 
  rotation 
  

   produced 
  by 
  the 
  band. 
  I 
  have 
  indicated 
  on 
  fig. 
  1 
  the 
  inequality 
  

   of 
  the 
  circular 
  components 
  of 
  the 
  band 
  525*17 
  fjufju 
  of 
  xeno- 
  

   iime 
  ; 
  in 
  fig. 
  3 
  it 
  is 
  seen 
  that, 
  on 
  the 
  edges 
  of 
  the 
  band, 
  

   the 
  two 
  maxima 
  of 
  positive 
  rotation 
  are 
  unequal 
  ; 
  the 
  more 
  

   intense 
  are 
  on 
  the 
  violet 
  side 
  because 
  the 
  circular 
  component 
  

   displaced 
  towards 
  the 
  violet 
  is 
  more 
  intense 
  than 
  the 
  other 
  

  

  * 
  Jean 
  Becquerel, 
  Le 
  Radium, 
  v. 
  No. 
  1. 
  p. 
  5 
  (1908) 
  ; 
  Jean 
  Becquerel 
  

   .and 
  Kainerlingh 
  Onnes, 
  loc. 
  cit. 
  

  

  