﻿326 
  ' 
  Mr. 
  T. 
  Preston 
  on 
  Radiation 
  Phenomena 
  

  

  what 
  appears 
  to 
  be 
  merely 
  a 
  broadening 
  of 
  the 
  bright 
  line 
  by 
  

   the 
  magnetic 
  field. 
  Once 
  we 
  know, 
  however, 
  that 
  the 
  

   central 
  line 
  is 
  polarized 
  in 
  a 
  plane 
  at 
  right 
  angles 
  to 
  that 
  of 
  

   the 
  two 
  side 
  lines 
  we 
  can, 
  by 
  suitably 
  interposing 
  a 
  NicoPs 
  

   prism, 
  obliterate 
  the 
  central 
  constituent 
  and 
  leave 
  the 
  two 
  

   side 
  lines 
  of 
  the 
  triplet 
  unextinguished. 
  The 
  overlapping 
  is 
  

   thus 
  removed 
  and 
  tbe 
  triplet 
  constitution, 
  as 
  distinguished 
  

   from 
  a 
  mere 
  broadening, 
  of 
  the 
  affected 
  line 
  becomes 
  to 
  

   some 
  extent 
  revealed 
  even 
  though 
  the 
  magnetic 
  field 
  has 
  not 
  

   been 
  strong 
  enough 
  to 
  completely 
  separate 
  the 
  constituents 
  

   of 
  the 
  triplet 
  from 
  each 
  other. 
  

  

  It 
  is 
  much 
  more 
  satisfactory, 
  however, 
  to 
  push 
  the 
  resolu- 
  

   tion 
  to 
  such 
  a 
  point 
  that 
  each 
  member 
  of 
  the 
  triplet 
  stands 
  

   revealed 
  and 
  isolated 
  from 
  the 
  others, 
  so 
  that 
  their 
  general 
  

   character 
  may 
  be 
  studied 
  separately. 
  In 
  fact, 
  until 
  this 
  has 
  

   been 
  effected, 
  we 
  cannot 
  assert 
  that 
  a 
  line 
  can 
  be 
  resolved 
  

   into 
  three 
  others 
  by 
  the 
  magnetic 
  field. 
  The 
  observations 
  

   mentioned 
  above 
  with 
  the 
  nicol 
  are 
  consistent 
  with 
  tripling, 
  

   but 
  they 
  do 
  not 
  absolutely 
  prove 
  it. 
  With 
  the 
  object 
  of 
  

   doing 
  this, 
  if 
  possible, 
  and 
  also 
  with 
  the 
  ulterior 
  object 
  of 
  

   making 
  a 
  general 
  survey 
  of 
  the 
  spectra 
  of 
  various 
  substances, 
  

   in 
  order 
  to 
  determine 
  if 
  the 
  spectra 
  of 
  different 
  substances 
  

   were 
  equally 
  or 
  similarly 
  affected, 
  or 
  if 
  the 
  various 
  spectral 
  

   lines 
  of 
  a 
  single 
  substance 
  were 
  equally 
  or 
  differently 
  affected, 
  

   I 
  gladly 
  availed 
  myself 
  of 
  the 
  opportunity 
  afforded 
  me 
  of 
  

   using 
  the 
  excellent 
  Rowland's 
  concave 
  grating 
  mounted 
  in 
  

   the 
  physical 
  laboratory 
  of 
  the 
  Royal 
  University 
  of 
  Ireland*. 
  

   This 
  instrument 
  is 
  of 
  the 
  usual 
  type, 
  having 
  a 
  radius 
  of 
  

   21 
  '5 
  ft., 
  and 
  ruled 
  with 
  about 
  14,438 
  lines 
  to 
  the 
  inch. 
  It 
  is 
  

   fitted 
  with 
  a 
  camera-box, 
  which 
  tikes 
  a 
  photographic 
  plate 
  

   50 
  cm. 
  long 
  and 
  6 
  cm. 
  broad, 
  so 
  that 
  a 
  length 
  of 
  the 
  spectrum 
  

   equal 
  to 
  half 
  a 
  metre 
  approximately 
  can 
  be 
  photographed 
  at 
  

   a 
  single 
  exposure. 
  

  

  As 
  a 
  consequence, 
  I 
  naturally 
  decided 
  to 
  study 
  the 
  

   phenomena 
  by 
  means 
  of 
  photography 
  as 
  well 
  as 
  by 
  eye 
  

   observation, 
  for 
  the 
  latter, 
  besides 
  being 
  applicable 
  in 
  the 
  

   visible 
  portion 
  of 
  the 
  spectrum 
  alone, 
  lends 
  itself 
  somewhat 
  

   to 
  the 
  personal 
  bias 
  of 
  the 
  observer 
  in 
  the 
  case 
  of 
  small 
  and 
  

   doubtful 
  effects. 
  The 
  photographic 
  plate, 
  on 
  the 
  other 
  hand, 
  

   gives 
  a 
  faithful 
  record 
  of 
  the 
  phenomena 
  as 
  they 
  actually 
  

   exist 
  in 
  the 
  image 
  focussed 
  upon 
  it, 
  and 
  this 
  record 
  can 
  be 
  

   examined 
  at 
  leisure. 
  Moreover, 
  as 
  a 
  considerable 
  length 
  of 
  

   the 
  spectrum 
  can 
  be 
  photographed 
  at 
  a 
  single 
  exposure, 
  the 
  

  

  * 
  A 
  description 
  of 
  the 
  mounting 
  of 
  this 
  instrument 
  by 
  Dr. 
  W. 
  E. 
  

   Adenev 
  and 
  Mr. 
  James 
  Carson 
  is 
  published 
  in 
  the 
  Proc. 
  Roy. 
  Dub. 
  Soc. 
  

   February 
  1898. 
  

  

  