﻿Power 
  in 
  the 
  neighbourhood 
  of 
  Bands 
  of 
  Absorption. 
  157 
  

  

  made 
  by 
  a 
  method 
  having 
  great 
  precision, 
  permit 
  one 
  to 
  

   follow 
  the 
  details 
  of 
  the 
  variation 
  of 
  rotatory 
  power 
  in 
  the 
  

   neighbourhood 
  and 
  in 
  the 
  interior 
  of 
  the 
  band, 
  and 
  it 
  is 
  

   manifest, 
  from 
  the 
  numerous 
  irregularities 
  of 
  the 
  curve, 
  that 
  

   the 
  band 
  is 
  complex, 
  i. 
  e. 
  it 
  is 
  formed 
  by 
  the 
  superposition 
  

   of 
  several 
  components. 
  It 
  is 
  quite 
  impossible 
  to 
  deduce 
  from 
  

   these 
  res 
  alts 
  any 
  certain 
  conclusion 
  relative 
  to 
  the 
  effect 
  of 
  a 
  

   simple 
  band. 
  

  

  On 
  the 
  contrary, 
  with 
  crystals 
  which 
  exhibit 
  fine 
  bands, 
  

   such 
  as 
  xenotime, 
  tysonite, 
  porisite, 
  apatite 
  ; 
  and 
  especially 
  

   if 
  we 
  study 
  the 
  absorption 
  at 
  a 
  very 
  low 
  temperature 
  (as 
  we 
  

   shall 
  show 
  later 
  on), 
  we 
  have 
  the 
  advantage 
  of 
  being 
  able 
  to 
  

   observe 
  simple 
  bands 
  ; 
  and 
  we 
  then 
  see 
  the 
  connexion 
  which 
  

   exists 
  between 
  magnetic 
  rotatory 
  dispersion 
  and 
  the 
  modifi- 
  

   cation 
  which 
  each 
  band 
  undergoes. 
  The 
  variation 
  of 
  the 
  

   magnetic 
  rotatory 
  powers 
  appears 
  indubitably 
  as 
  the 
  conse- 
  

   quence 
  of 
  the 
  simultaneous 
  effect 
  of 
  anomalous 
  dispersion 
  

   and 
  of 
  the 
  separation, 
  under 
  the 
  influence 
  of 
  the 
  magnetic 
  

   field, 
  of 
  bands 
  into 
  two 
  components 
  absorbing 
  circular 
  

   vibrations. 
  

  

  In 
  tysonite, 
  after 
  having 
  studied, 
  at 
  ordinary 
  temperatures 
  

   and 
  at 
  the 
  temperature 
  of 
  liquid 
  air, 
  the 
  variations 
  of 
  the 
  

   index 
  of 
  refraction 
  in 
  the 
  neighbourhood 
  of 
  one 
  of 
  the 
  bands 
  

   (523*5 
  fjifi) 
  and 
  measured 
  the 
  separation 
  of 
  the 
  components 
  

   in 
  a 
  magnetic 
  field, 
  I 
  was 
  able 
  to 
  predict 
  from 
  the 
  formula 
  

   given 
  by 
  Voigt, 
  what 
  the 
  magnitude 
  of 
  the 
  rotation 
  should 
  

   be 
  in 
  the 
  interior 
  of 
  a 
  band, 
  at 
  20° 
  C. 
  and 
  -188° 
  C. 
  The 
  

   calculated 
  and 
  observed 
  numbers 
  are 
  in 
  as 
  satisfactory 
  

   agreement 
  with 
  one 
  another 
  as 
  possible 
  *. 
  

  

  R. 
  W. 
  Wood, 
  in 
  a 
  note 
  j 
  published 
  in 
  the 
  month 
  of 
  

   February 
  last, 
  shows 
  that 
  very 
  complex 
  effects 
  occur 
  in 
  the 
  

   neighbourhood 
  of 
  a 
  group 
  of 
  bands 
  of 
  nitrate 
  of 
  neodymium. 
  

   In 
  crystals, 
  I 
  have 
  for 
  a 
  long 
  time 
  observed 
  and 
  pointed 
  out 
  

   the 
  existence 
  of 
  phenomena 
  near 
  certain 
  bands 
  which 
  do 
  not 
  

   possess 
  the 
  simplicity 
  of 
  those 
  which 
  I 
  have 
  just 
  described 
  ; 
  

   in 
  particular, 
  a 
  strong 
  band 
  of 
  tysonite 
  (577 
  /jljjl) 
  gives 
  a 
  

   negative 
  rotation 
  on 
  the 
  side 
  of 
  smaller 
  w 
  r 
  ave-length 
  and 
  a 
  

   positive 
  rotation 
  on 
  the 
  side 
  of 
  greater 
  wave-length. 
  This 
  

   result 
  appears 
  at 
  first 
  to 
  be 
  in 
  agreement 
  with 
  the 
  molecular 
  

   current 
  theory 
  ; 
  but, 
  before 
  coming 
  to 
  any 
  conclusion, 
  it 
  is 
  

   important 
  to 
  learn 
  whether 
  one 
  has 
  to 
  deal 
  with 
  a 
  simple 
  

   band, 
  or 
  if, 
  on 
  the 
  contrary, 
  the 
  band 
  is 
  a 
  complex 
  group. 
  

   It 
  is 
  evident 
  that 
  a 
  combination 
  of 
  bands 
  of 
  negative 
  and 
  

  

  * 
  Jean 
  Becquerel, 
  Comptes 
  Rendus, 
  11 
  & 
  12 
  Nov., 
  1967 
  ; 
  Le 
  Radium.) 
  

   v.No. 
  1. 
  p. 
  11 
  (1908). 
  

  

  t 
  R. 
  W. 
  Wood, 
  Phil. 
  Mag. 
  [6] 
  vol. 
  xv. 
  p. 
  270, 
  Feb. 
  1908. 
  

  

  