﻿156 
  Prof. 
  Becquerel 
  on 
  Dispersion 
  of 
  Magnetic 
  Rotatory 
  

  

  based 
  on 
  the 
  Hall 
  effect 
  ever 
  observed 
  in 
  the 
  case 
  o£ 
  a 
  solid 
  

   body. 
  A 
  crystal 
  of 
  xenotime 
  behaves, 
  therefore, 
  from 
  the 
  

   magneto-optic 
  point 
  of 
  view, 
  like 
  a 
  vapour 
  exhibiting 
  the 
  

   Zeeman 
  effect. 
  Moreover, 
  one 
  obtains 
  experimentally 
  this 
  

   new 
  and 
  supremely 
  important 
  result 
  : 
  the 
  effect 
  of 
  an 
  ab- 
  

   sorption 
  band 
  depends, 
  assuming 
  the 
  existence 
  of 
  electrons 
  

   of 
  two 
  signs, 
  upon 
  the 
  sign 
  of 
  the 
  charge 
  of 
  the 
  electrons 
  

   producing 
  the 
  particular 
  band 
  under 
  consideration. 
  

  

  Fig. 
  3 
  represents 
  the 
  variations 
  of 
  the 
  magnetic 
  rotatory 
  

   power 
  of 
  xenotime, 
  at 
  the 
  temperature 
  of 
  liquid 
  air, 
  in 
  the 
  

   neighbourhood 
  of 
  the 
  bands 
  shown 
  in 
  the 
  group 
  in 
  fig. 
  1. 
  

   The 
  spectrum 
  (1) 
  of 
  fig. 
  3 
  shows 
  the 
  dislocation 
  of 
  the 
  black 
  

   fringes 
  obtained 
  in 
  the 
  spectrum 
  by 
  placing 
  against 
  the 
  slit 
  

   of 
  the 
  spectrograph 
  a 
  Babinet 
  compensator 
  placed 
  between 
  

   two 
  crossed 
  Nicol's 
  prisms 
  in 
  such 
  a 
  manner 
  that 
  the 
  central 
  

   fringe 
  is 
  normal 
  to 
  the 
  slit, 
  and 
  preceded 
  by 
  a 
  quarter-wave 
  

   plate 
  which 
  transforms 
  the 
  two 
  circular 
  components 
  of 
  

   opposite 
  senses 
  into 
  two 
  rectilinear 
  vibrations 
  parallel 
  to 
  the 
  

   two 
  principal 
  directions 
  of 
  the 
  compensator. 
  The 
  displace- 
  

   ments 
  of 
  the 
  fringe 
  measure 
  the 
  rotatory 
  power. 
  The 
  fringe 
  

   and 
  the 
  components 
  of 
  the 
  band 
  522*15 
  jjl^l 
  form 
  a 
  very 
  

   remarkable 
  Y-shaped 
  curve. 
  

  

  The 
  spectrum 
  2 
  of 
  fig. 
  3 
  was 
  obtained 
  by 
  polarizing, 
  at 
  

   45° 
  to 
  the 
  horizontal, 
  the 
  light 
  which 
  falls 
  on 
  the 
  crystal, 
  

   and 
  placing 
  in 
  front 
  of 
  the 
  slit 
  of 
  the 
  spectrograph 
  a 
  rhombo- 
  

   hedron 
  of 
  Iceland 
  spar 
  which 
  gives 
  two 
  images 
  polarized 
  

   horizontally 
  and 
  vertically 
  respectively. 
  The 
  two 
  contiguous 
  

   images 
  are 
  identical 
  if 
  the 
  field 
  is 
  not 
  excited 
  ; 
  but 
  when 
  the 
  

   crystal 
  is 
  submitted 
  to 
  a 
  magnetic 
  field, 
  a 
  rotation 
  of 
  the 
  

   plane 
  of 
  polarization 
  of 
  the 
  incident 
  vibration 
  corresponds 
  to 
  an 
  

   increase 
  in 
  the 
  luminous 
  intensity 
  in 
  one 
  of 
  the 
  images 
  and 
  

   a 
  decrease 
  of 
  that 
  of 
  the 
  other. 
  For 
  the 
  components 
  of 
  the 
  

   bands 
  522*15 
  and 
  525*17 
  /x/z. 
  the 
  phenomenon 
  is 
  exceedingly 
  

   definite. 
  

  

  Identical 
  results 
  were 
  obtained 
  with 
  all 
  crystals 
  or 
  solutions 
  

   of 
  salts 
  which 
  produce 
  bands 
  responsive 
  to 
  a 
  magnetic 
  field. 
  

  

  Some 
  months 
  after 
  my 
  first 
  experiments 
  on 
  the 
  magnetic 
  

   rotatory 
  polarization 
  in 
  xenotime 
  *, 
  Eliasf 
  obtained 
  quite 
  

   similar 
  effects 
  with 
  a 
  solution 
  of 
  chloride 
  of 
  erbium 
  which 
  

   he 
  recalls 
  in 
  a 
  recent 
  communication 
  %. 
  

  

  On 
  each 
  side 
  of 
  a 
  broad 
  band 
  of 
  a 
  solution 
  of 
  chloride 
  of 
  

   erbium 
  (about 
  520 
  ft/x-) 
  the 
  rotation 
  is 
  negative, 
  whilst 
  it 
  is 
  

   positive 
  iii 
  fche 
  midst 
  of 
  the 
  band. 
  The 
  experiments 
  of 
  Elias, 
  

  

  * 
  .lean 
  Becquerel, 
  Comptea 
  Rendus, 
  -1 
  May, 
  L906. 
  

  

  t 
  G. 
  .1. 
  Elias, 
  Physik. 
  Zeitschr. 
  vii. 
  p. 
  981, 
  15 
  Dec. 
  1006. 
  

  

  X 
  Tliil. 
  Mag. 
  [6] 
  vol. 
  w. 
  p. 
  588, 
  April 
  L908. 
  

  

  