﻿in 
  the 
  Magnetic 
  Field. 
  327 
  

  

  effect? 
  produced 
  on 
  many 
  lines, 
  under 
  exactly 
  the 
  same 
  cir- 
  

   cumstances, 
  can 
  be 
  compared 
  with 
  precision. 
  Still 
  it 
  is 
  to 
  

   be 
  remembered 
  that 
  the 
  photographic 
  plate 
  is 
  not 
  capable 
  of 
  

   reproducing 
  detail 
  beyond 
  a 
  certain 
  order 
  of 
  fineness, 
  and, 
  

   further, 
  that 
  the 
  negative 
  is 
  of 
  the 
  nature 
  of 
  an 
  integral 
  and 
  

   reproduces 
  the 
  sum 
  total 
  of 
  all 
  the 
  effects 
  produced 
  on 
  it 
  

   during 
  the 
  time 
  of 
  exposure. 
  For 
  this 
  reason, 
  if 
  the 
  image 
  

   varies, 
  owing 
  to 
  variations 
  going 
  on 
  in 
  the 
  source 
  of 
  light, 
  

   or 
  elsewhere, 
  during 
  the 
  time 
  of 
  exposure, 
  the 
  photographic 
  

   negative 
  will 
  be 
  the 
  muddled 
  integral 
  of 
  these 
  various 
  images, 
  

   and 
  may 
  not 
  in 
  the 
  end 
  be 
  similar 
  to 
  any 
  one 
  of 
  the 
  various 
  

   images 
  which 
  produced 
  it. 
  For 
  this 
  reason, 
  the 
  examination 
  

   of 
  the 
  visible 
  portion 
  of 
  the 
  spectrum 
  by 
  means 
  of 
  the 
  eye 
  

   cannot 
  be 
  dispensed 
  with, 
  as 
  will 
  appear 
  from 
  what 
  follows. 
  

  

  In 
  the 
  case 
  of 
  a 
  phenomenon 
  which 
  exhibits 
  itself 
  as 
  a 
  

   very 
  small 
  effect, 
  and 
  one 
  which 
  it 
  is 
  difficult 
  to 
  obtain 
  and 
  

   observe, 
  it 
  is 
  natural 
  that 
  some 
  doubt 
  should 
  exist 
  at 
  first 
  as 
  

   to 
  what 
  it 
  is 
  that 
  actually 
  is 
  presented 
  to 
  the 
  observer, 
  and 
  

   that 
  discrepancies 
  should 
  occur 
  in 
  the 
  statements 
  of 
  different 
  

   observers 
  regarding 
  the 
  phenomenon 
  now 
  under 
  consideration 
  

   is 
  not 
  surprising. 
  Thus, 
  while 
  Dr. 
  Zeeman 
  distinctly 
  states 
  

   that 
  he 
  obtained 
  a 
  tripling 
  of 
  the 
  spectral 
  lines 
  when 
  the 
  

   source 
  of 
  light 
  is 
  viewed 
  across 
  the 
  lines 
  of 
  magnetic 
  force, 
  

   and 
  a 
  doubling 
  when 
  it 
  is 
  viewed 
  along 
  the 
  lines 
  of 
  force 
  ; 
  

   yet 
  these 
  effects 
  were 
  obtained 
  only 
  after 
  the 
  theory 
  of 
  the 
  

   phenomena 
  pointed 
  in 
  this 
  direction, 
  and 
  other 
  reliable 
  

   observers 
  investigating 
  the 
  same 
  lines 
  have 
  given 
  expression 
  

   to 
  the 
  opinion 
  that 
  when 
  viewed 
  across 
  the 
  lines 
  of 
  force 
  a. 
  

   doubling 
  (like 
  a 
  reversal) 
  or 
  a 
  broadening 
  combined 
  with 
  a 
  

   doubling 
  occurred. 
  So 
  far 
  as 
  I 
  am 
  aware, 
  all 
  these 
  observers 
  

   have 
  failed 
  to 
  obtain 
  distinct 
  triplets 
  when 
  the 
  light 
  is 
  

   viewed 
  across 
  the 
  lines 
  of 
  force, 
  that 
  is, 
  they 
  have 
  not 
  

   succeeded 
  in 
  separating 
  the 
  constituents 
  sufficiently 
  to 
  enable 
  

   them 
  to 
  decide 
  what 
  the 
  exact 
  composition 
  of 
  the 
  modified 
  

   spectral 
  line 
  is. 
  By 
  placing 
  a 
  Nicol's 
  prism 
  in 
  the 
  path 
  of 
  

   the 
  light 
  it 
  has 
  been 
  determined 
  that 
  the 
  central 
  part 
  of 
  the 
  

   modified 
  line 
  is 
  plane 
  polarized, 
  and 
  that 
  the 
  edges 
  are 
  also 
  

   plane 
  polarized, 
  but 
  in 
  a 
  plane 
  at 
  right 
  angles 
  to 
  the 
  plane 
  

   of 
  polarization 
  of 
  the 
  middle 
  part. 
  Thus 
  with 
  the 
  nicol 
  in 
  

   one 
  position 
  the 
  middle 
  may 
  be 
  cut 
  out, 
  leaving 
  the 
  two 
  

   border 
  lines, 
  and 
  with 
  the 
  nicol 
  turned 
  through 
  a 
  right 
  angle 
  

   the 
  two 
  border 
  lines 
  may 
  be 
  removed, 
  leaving 
  the 
  middle 
  

   portion 
  alone. 
  It 
  is 
  to 
  be 
  remarked, 
  however, 
  that 
  observa- 
  

   tions 
  of 
  this 
  nature, 
  although 
  they 
  are 
  in 
  accordance 
  with 
  

   the 
  supposition 
  that 
  the 
  modified 
  line 
  is 
  a 
  triplet, 
  yet 
  they 
  

   do 
  not 
  absolutely 
  prove 
  tripling 
  pure 
  and 
  simple. 
  For 
  this 
  

  

  