﻿330 
  

  

  Mr. 
  T. 
  Preston 
  on 
  Radiation 
  Phenomena 
  

  

  recorded 
  on 
  the 
  photographic 
  plate, 
  but 
  which 
  presents 
  itself 
  

   with 
  great 
  distinctness 
  to 
  the 
  naked 
  eye 
  when 
  the 
  lines 
  are 
  

   viewed 
  through 
  a 
  well-focussed 
  eyepiece. 
  One 
  of 
  these 
  

   modifications 
  occurs 
  in 
  the 
  case 
  of 
  lines 
  which 
  photograph 
  

   as 
  doublets 
  when 
  the 
  source 
  of 
  light 
  is 
  viewed 
  across 
  the 
  lines 
  

   of 
  force, 
  and 
  may 
  be 
  described 
  by 
  saying 
  that 
  the 
  affected 
  

   line 
  is 
  a 
  modified 
  triplet 
  in 
  which 
  the 
  central 
  line 
  is 
  absent, 
  

   and 
  in 
  which 
  at 
  the 
  same 
  time 
  each 
  of 
  the 
  side 
  lines 
  is 
  

   reversed. 
  Thus 
  the 
  appearance 
  presented 
  to 
  the 
  eye 
  is 
  that 
  

   of 
  two 
  pairs 
  of 
  lines 
  rather 
  than 
  a 
  doublet 
  or 
  triplet. 
  The 
  

   other 
  modification 
  of 
  note 
  is 
  that 
  in 
  which 
  each 
  constituent 
  of 
  

   the 
  triplet 
  shows 
  as 
  a 
  pair 
  of 
  lines, 
  or, 
  in 
  other 
  words, 
  the 
  

   whole 
  appears 
  not 
  as 
  a 
  triplet, 
  but 
  as 
  a 
  sextet 
  of 
  fine 
  equally 
  

   spaced 
  well-marked 
  and 
  sharp 
  lines. 
  

  

  All 
  the 
  modifications 
  so 
  far 
  observed, 
  when 
  the 
  source 
  of 
  

   light 
  is 
  viewed 
  across 
  the 
  lines 
  of 
  force, 
  are 
  illustrated 
  by 
  the 
  

   accompanying 
  woodcut. 
  Thus 
  in 
  fig. 
  1 
  we 
  have 
  the 
  normal 
  

  

  triplet, 
  such 
  as 
  is 
  presented 
  by 
  the 
  violet 
  line 
  of 
  cadmium 
  4678 
  

   and 
  the 
  vast 
  majority 
  of 
  other 
  lines. 
  Then 
  we 
  have 
  in 
  fig. 
  2 
  

   the 
  weak 
  middled 
  " 
  quartet" 
  in 
  which 
  nearly 
  all 
  the 
  light 
  is 
  

   concentrated 
  in 
  the 
  two 
  side 
  lines. 
  Next 
  we 
  have 
  in 
  fig. 
  3 
  

   the 
  doublet 
  in 
  which 
  the 
  central 
  line 
  has 
  completely 
  disappeared. 
  

   Next 
  in 
  fig. 
  4 
  we 
  have 
  the 
  double 
  doublet, 
  or 
  two 
  pairs 
  of 
  fine 
  

   lines, 
  and 
  finally 
  in 
  fig. 
  5 
  the 
  sextet 
  or 
  three 
  pairs 
  of 
  equally 
  

   spaced 
  sharp 
  lines. 
  

  

  The 
  Sodium 
  Lines. 
  

  

  It 
  is 
  interesting 
  to 
  remark 
  that 
  the 
  two 
  D-lines 
  of 
  sodium 
  

   do 
  not 
  belong 
  to 
  the 
  class 
  which 
  show 
  as 
  triplets 
  in 
  the 
  mag- 
  

   netic 
  field. 
  Owing 
  to 
  the 
  ease 
  with 
  which 
  these 
  lines 
  reverse 
  

   and 
  to 
  the 
  constant 
  variations 
  going 
  on 
  in 
  the 
  vapour-density 
  

   in 
  the 
  spark 
  obtained 
  from 
  a 
  solution 
  of 
  the 
  salt, 
  it 
  is 
  not 
  easy 
  

   to 
  obtain 
  the 
  sodium 
  Lines 
  in 
  a 
  steady 
  state 
  for 
  any 
  length 
  of 
  

   time. 
  But 
  when 
  the 
  sodium 
  salt 
  is 
  contained 
  in 
  small 
  quantity 
  

   in 
  the 
  solution 
  the 
  lines 
  may 
  be 
  obtained 
  sharp 
  and 
  fairly 
  

   steady. 
  Under 
  these 
  circumstances 
  one 
  of 
  the 
  sodium 
  lines 
  

   D 
  2 
  shows 
  as 
  a 
  sextet 
  (fig. 
  5) 
  of 
  fine 
  sharp 
  equally-spaced 
  line3. 
  

  

  