﻿154 
  Prof. 
  Becquerel 
  on 
  Dispersion 
  of 
  Magnetic 
  Rotatory 
  

  

  each 
  side 
  o£ 
  a 
  band, 
  and 
  in 
  the 
  opposite 
  sense 
  in 
  the 
  interior 
  

   of 
  the 
  band. 
  

  

  Experiments 
  performed 
  with 
  vapours 
  exhibiting 
  the 
  

   Zeeman 
  effect 
  gave 
  results 
  in 
  entire 
  agreement 
  with 
  the 
  

   latter 
  theory*. 
  For 
  solid 
  bodies 
  and 
  solutions 
  not 
  merely 
  

   the 
  theories 
  but 
  the 
  experiments 
  themselves 
  have 
  been 
  subject 
  

   to 
  much 
  discussion. 
  The 
  results 
  obtained 
  by 
  Schmausz 
  f 
  

   with 
  solutions 
  of 
  colouring 
  matters 
  and 
  with 
  salts 
  of'didymium 
  

   were 
  in 
  agreement 
  with 
  the 
  molecular 
  current 
  theory 
  ; 
  but 
  

   according 
  to 
  Bates 
  J 
  these 
  experiments 
  are 
  not 
  free 
  from 
  

   criticism. 
  Moreover, 
  R. 
  W. 
  Wood§, 
  in 
  the 
  chloride 
  of 
  

   praseodymium, 
  found 
  opposite 
  rotations 
  on 
  the 
  two 
  sides 
  of 
  

   certain 
  bands. 
  

  

  Until 
  1906 
  no 
  observation 
  with 
  a 
  solid 
  body 
  or 
  a 
  solution 
  

   had 
  confirmed 
  the 
  truth 
  of 
  the 
  theory 
  based 
  on 
  the 
  Hall 
  

   effect 
  — 
  the 
  theory 
  which 
  at 
  the 
  same 
  time 
  represented 
  the 
  

   effects 
  in 
  vapours. 
  

  

  It 
  seemed 
  to 
  me 
  that 
  magnetic 
  rotatory 
  polarization 
  in 
  

   solid 
  bodies 
  should 
  result, 
  at 
  least 
  in 
  certain 
  cases, 
  as 
  with 
  

   vapours, 
  in 
  a 
  modification 
  of 
  the 
  bands 
  of 
  absorption 
  under 
  

   the 
  influence 
  of 
  magnetism. 
  I 
  then 
  endeavoured 
  to 
  observe 
  

   directly 
  whether 
  or 
  not 
  the 
  bands 
  were 
  displaced 
  in 
  a 
  magnetic 
  

   field. 
  

  

  My 
  first 
  experiments, 
  carried 
  out 
  with 
  solid 
  bodies 
  pos- 
  

   sessing 
  the 
  finest 
  possible 
  bands, 
  viz. 
  with 
  xenotime 
  (phosphate 
  

   of 
  yttrium 
  and 
  erbium) 
  gave 
  me 
  extremely 
  definite 
  results, 
  

   which 
  I 
  have 
  since 
  been 
  able 
  to 
  extend 
  to 
  all 
  crystals 
  and 
  to 
  

   those 
  solutions 
  which 
  possess 
  bands 
  sufficiently 
  narrow. 
  The 
  

   phenomenon 
  observed 
  with 
  crystals 
  is 
  of 
  the 
  same 
  nature 
  as 
  

   the 
  effect 
  discovered 
  by 
  Zeeman 
  in 
  the 
  spectra 
  of 
  vapours, 
  

   but 
  presenting 
  fundamental 
  differences 
  from 
  the 
  Zeeman 
  

   phenomenon 
  of 
  which 
  the 
  two 
  following 
  are 
  the 
  most 
  

   important 
  : 
  — 
  

  

  (1) 
  The 
  order 
  of 
  magnitude 
  of 
  the 
  displacements 
  for 
  many 
  

   of 
  the 
  bands 
  of 
  crystals 
  is 
  notably 
  greater 
  than 
  for 
  the 
  effects 
  

   observed 
  with 
  vapours. 
  The 
  separation 
  of 
  the 
  components 
  

   of 
  some 
  doublets 
  produced 
  by 
  the 
  magnetic 
  field 
  surpasses 
  in 
  

   fact 
  1 
  fjb/jL 
  in 
  a 
  field 
  of 
  30,000 
  gauss. 
  This 
  is 
  the 
  case 
  for 
  the 
  

   bands 
  of 
  wave-lengths 
  ()42"27 
  and 
  643*45 
  fJLfju 
  of 
  xenotime. 
  

  

  (2) 
  When 
  the 
  optic 
  axis 
  of 
  a 
  uniaxal 
  crystal 
  is 
  oriented 
  

  

  * 
  Macaluso 
  and 
  Corbino, 
  C<»»j>f(>s 
  Rendus, 
  127. 
  p. 
  548 
  (1898). 
  Henri 
  

   Becquerel, 
  Comptea 
  Rendus, 
  125. 
  p. 
  879 
  (1897). 
  and 
  127. 
  p. 
  8W 
  (1898) 
  ; 
  

   P. 
  Zeeman, 
  Arch, 
  nSerlandaieea, 
  vii. 
  p. 
  466 
  (1902), 
  

  

  t 
  A. 
  Schmausz, 
  Ann. 
  der 
  Fhyt, 
  ii. 
  p. 
  280 
  (1D00) 
  ; 
  viii. 
  p. 
  842 
  (11)02). 
  

   \. 
  p. 
  863 
  (1 
  908). 
  

  

  I 
  Bates, 
  Ann. 
  der 
  Phyt. 
  xii. 
  p. 
  L080 
  (1908). 
  

  

  § 
  U. 
  \Y. 
  Wood, 
  Fhyt. 
  ZeiUohr. 
  vi. 
  p. 
  41(> 
  (1906), 
  

  

  