﻿On 
  Radiation 
  Phenomena 
  in 
  the 
  Magnetic 
  Field. 
  325 
  

  

  Kew 
  Observatory 
  I 
  am 
  indebted 
  to 
  the 
  Kew 
  Observatory 
  

   Committee. 
  To 
  one 
  of 
  its 
  members, 
  Mr. 
  W. 
  N. 
  Shaw, 
  F.R.S., 
  

   I 
  am 
  indebted 
  for 
  a 
  variety 
  of 
  suggestions 
  which 
  have 
  added 
  

   to 
  the 
  lucidity 
  and 
  completeness 
  of 
  the 
  paper. 
  For 
  any 
  ex- 
  

   pressions 
  of 
  opinion, 
  and 
  for 
  the 
  accuracy 
  of 
  the 
  facts, 
  I 
  am, 
  

   however, 
  personally 
  exclusively 
  responsible. 
  

  

  XXXVI. 
  Radiation 
  Phenomena 
  in 
  the 
  Magnetic 
  Field. 
  

   By 
  Thomas 
  Preston, 
  M.A.* 
  

  

  

  [Plate 
  XXII 
  L] 
  

  

  TN 
  March 
  1807, 
  Dr. 
  P. 
  Zeeman 
  announced 
  in 
  the 
  pages 
  of 
  

   -*■ 
  this 
  Magazine 
  the 
  important 
  discovery 
  that 
  the 
  bright 
  

   lines 
  of 
  the 
  spectra 
  become 
  sensibly 
  modified 
  in 
  appearance 
  

   and 
  constitution 
  when 
  the 
  source 
  of 
  light 
  is 
  placed 
  in 
  a 
  

   strong 
  magnetic 
  field. 
  This 
  effect 
  has 
  often 
  been 
  sought 
  for 
  

   both 
  by 
  men 
  who 
  have 
  published 
  the 
  results 
  of 
  their 
  investi- 
  

   gations 
  as 
  well 
  as 
  by 
  many 
  who 
  have 
  not, 
  but 
  it 
  is 
  only 
  now, 
  

   thanks 
  to 
  the 
  work 
  of 
  Dr. 
  Zeeman, 
  that 
  the 
  scientific 
  world 
  

   has 
  become 
  convinced 
  of 
  its 
  existence 
  and 
  nature. 
  

  

  Considerable 
  difficulty 
  attends 
  the 
  experimental 
  investiga- 
  

   tion 
  of 
  this 
  effect 
  owing 
  to 
  its 
  smallness, 
  and 
  it 
  is 
  only 
  when 
  

   a 
  very 
  strong 
  magnetic 
  field 
  is 
  used 
  in 
  combination 
  with 
  

   high 
  dispersion 
  that 
  it 
  can 
  be 
  placed 
  distinctly 
  in 
  evidence. 
  

   Its 
  investigation, 
  however, 
  has 
  been 
  greatly 
  assisted 
  by 
  the 
  

   present 
  advanced 
  state 
  of 
  electromagnetic 
  theory, 
  which 
  

   pointed 
  out 
  what 
  the 
  essential 
  features 
  of 
  the 
  phenomenon 
  

   were 
  which 
  should 
  be 
  looked 
  for. 
  Thus, 
  the 
  theory 
  informed 
  

   us 
  that 
  each 
  bright 
  line 
  of 
  a 
  line-spectrum 
  should 
  be 
  con- 
  

   verted 
  into 
  a 
  doublet, 
  or 
  a 
  triplet, 
  according 
  as 
  the 
  source 
  of 
  

   light 
  is 
  viewed 
  along, 
  or 
  across, 
  the 
  lines 
  of 
  magnetic 
  force, 
  

   and 
  further, 
  that 
  each 
  member 
  of 
  a 
  doublet 
  should 
  be 
  

   circularly 
  polarized, 
  whereas 
  each 
  member 
  of 
  a 
  triplet 
  should 
  

   be 
  plane 
  polarized, 
  the 
  plane 
  of 
  polarization 
  of 
  the 
  central 
  

   line 
  being 
  at 
  right 
  angles 
  to 
  that 
  of 
  the 
  two 
  side 
  lines. 
  

  

  This 
  information 
  proved 
  of 
  special 
  importance 
  to 
  the 
  

   experimental 
  inquiry 
  owing 
  to 
  the 
  fact 
  that 
  a 
  bright 
  spectral 
  

   line 
  is 
  not 
  a 
  mathematical 
  line, 
  but, 
  on 
  the 
  contrary, 
  possesses 
  

   a 
  finite 
  width, 
  so 
  that 
  in 
  order 
  to 
  observe 
  the 
  triplet 
  effect 
  

   the 
  resolution 
  must 
  be 
  great 
  enough 
  to 
  separate 
  the 
  lateral 
  

   components 
  from 
  each 
  other 
  by 
  a 
  distance 
  greater 
  than 
  the 
  

   width 
  of 
  the 
  spectral 
  line 
  in 
  question. 
  For 
  resolutions 
  less 
  

   than 
  this 
  the 
  central 
  member 
  of 
  the 
  triplet 
  overlaps 
  the 
  

   literal 
  components, 
  and 
  the 
  effect 
  presented 
  to 
  the 
  eye 
  is 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  