﻿394 
  Dr. 
  H. 
  Stansfield 
  on 
  

  

  Width 
  of 
  the 
  Components. 
  

  

  The 
  mean 
  values 
  of 
  the 
  widths 
  of 
  the 
  photographic 
  images 
  

   of 
  the 
  components 
  are 
  given 
  in 
  Table 
  III. 
  If 
  the 
  echelon 
  

   acted 
  perfectly, 
  the 
  width 
  of 
  a 
  principal 
  light 
  maximum 
  

   constituting 
  one 
  order 
  of 
  the 
  primary 
  echelon 
  spectrum 
  of 
  a 
  

   monochromatic 
  radiation, 
  with 
  a 
  narrow 
  slit, 
  would 
  be 
  2/33 
  

   of 
  the 
  interval 
  between 
  the 
  orders, 
  which 
  corresponds 
  in 
  this 
  

   case 
  to 
  a 
  width 
  of 
  30 
  m.A. 
  The 
  narrowest 
  lines 
  in 
  the 
  pho- 
  

   tographs 
  which 
  still 
  have 
  a 
  measurable 
  width, 
  are 
  about 
  

   10 
  m.A. 
  wide. 
  The 
  widths 
  of 
  the 
  brighter 
  lines 
  vary 
  consi- 
  

   derably 
  with 
  their 
  exposure, 
  while 
  the 
  width 
  of 
  the 
  com- 
  

   panion 
  line 
  8, 
  which 
  is 
  too 
  faint 
  to 
  be 
  overexposed, 
  is 
  the 
  

   same 
  on 
  each 
  of 
  the 
  three 
  plates 
  on 
  \Yhich 
  its 
  width 
  could 
  be 
  

   measured. 
  

  

  Measurement 
  of 
  Secondary 
  Spectrum. 
  

  

  The 
  results 
  obtained 
  by 
  measuring 
  the 
  secondary 
  point 
  

   spectrum 
  are 
  also 
  entered 
  in 
  Table 
  III. 
  The 
  photograph 
  

   measured 
  was 
  exposed 
  for 
  an 
  hour, 
  and 
  is 
  the 
  one 
  reproduced 
  

   in 
  Plate 
  XI., 
  No. 
  3. 
  It 
  will 
  be 
  seen 
  that 
  the 
  agreement 
  of 
  

   these 
  results, 
  obtained 
  by 
  an 
  independent 
  method, 
  with 
  the 
  

   ordinary 
  echelon 
  values, 
  is 
  fairly 
  close, 
  except 
  for 
  the 
  com- 
  

   ponent 
  2. 
  The 
  methods 
  agree 
  very 
  closely 
  as 
  to 
  the 
  position 
  

   of 
  the 
  central 
  line. 
  They 
  both 
  give 
  the 
  centre 
  of 
  the 
  double 
  

   line 
  at 
  232 
  and 
  the 
  dividing 
  dark 
  line 
  at 
  228. 
  The 
  two 
  

   components 
  of 
  the 
  central 
  line 
  were 
  not 
  measured 
  separately 
  

   in 
  the 
  primary 
  spectrum 
  photographs, 
  but 
  the 
  position 
  of 
  the 
  

   dark 
  dividing 
  line 
  was 
  sometimes 
  recorded. 
  The 
  positions 
  of 
  

   the 
  centres 
  of 
  the 
  components 
  given 
  in 
  brackets 
  in 
  the 
  second 
  

   column 
  of 
  Table 
  III., 
  are 
  deduced 
  from 
  the 
  position 
  and 
  width 
  

   of 
  the 
  whole 
  line 
  and 
  the 
  position 
  of 
  the 
  dark 
  dividing 
  line 
  

   (neglecting 
  its 
  width). 
  

  

  Spectrum 
  given 
  hy 
  a 
  Bastian 
  Lamp. 
  

  

  A 
  striking 
  variation 
  in 
  the 
  spectrum 
  of 
  the 
  green 
  line 
  

   was 
  observed 
  with 
  a 
  Bastian 
  mercury 
  arc-lamp. 
  The 
  glass 
  

   tube 
  through 
  which 
  the 
  discharge 
  passes 
  is 
  bent 
  nearly 
  into 
  

   the 
  form 
  of 
  an 
  S 
  in 
  a 
  horizontal 
  plane, 
  so 
  that 
  when 
  one 
  

   part 
  of 
  the 
  discharge 
  is 
  parallel 
  to 
  the 
  slit 
  plate, 
  another 
  part 
  

   may 
  be 
  normal 
  to 
  it. 
  When 
  the 
  image 
  of 
  the 
  ^' 
  end-on 
  " 
  

   portion 
  of 
  the 
  discharge 
  is 
  put 
  on 
  the 
  slit, 
  the 
  change 
  in 
  the 
  

   spectrum 
  is 
  so 
  great 
  that 
  it 
  is 
  difficult 
  at 
  first 
  to 
  recog- 
  

   nize 
  the 
  components. 
  On 
  measuring 
  a 
  photograph 
  of 
  the 
  

   '' 
  end-on" 
  spectrum, 
  however, 
  it 
  was 
  found 
  that 
  the 
  companion 
  

   lines 
  keep 
  their 
  relative 
  positions, 
  although 
  some 
  become 
  

  

  