﻿536 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  High 
  Purity 
  Interference 
  

  

  I 
  have 
  marked 
  on 
  the 
  spectrum 
  immediately 
  above 
  this 
  one. 
  

   (The 
  marks 
  refer 
  to 
  the 
  lower 
  spectrum 
  only.) 
  

  

  By 
  increasing 
  the 
  angle 
  of 
  incidence, 
  the 
  band 
  can 
  be 
  

   inade 
  to 
  move 
  down 
  the 
  spectrum, 
  widening 
  as 
  it 
  moves. 
  

   When 
  in 
  the 
  green 
  it 
  appears 
  as 
  in 
  spectrum 
  " 
  h 
  " 
  and 
  is 
  

   accompanied 
  by 
  fainter 
  lateral 
  minima. 
  The 
  narrowness 
  of 
  

   the 
  rejected 
  region 
  has 
  been 
  shown 
  by 
  Lord 
  Rayleigh 
  to 
  be 
  

   due 
  to 
  multiple 
  twin 
  planes, 
  sensibly 
  equidistant. 
  From 
  the 
  

   width 
  of 
  the 
  reflected 
  region 
  we 
  can 
  form 
  an 
  estimate 
  of 
  the 
  

   number 
  of 
  laminae 
  present 
  in 
  the 
  crystal 
  plate. 
  Each 
  lamina 
  

   gives 
  us 
  by 
  reflexion 
  a 
  virtual 
  image 
  of 
  the 
  source, 
  these 
  

   images 
  being 
  in 
  line, 
  one 
  behind 
  the 
  other, 
  at 
  normal 
  incidence. 
  

   The 
  action 
  is 
  not 
  unlike 
  that 
  of 
  a 
  diffraction 
  grating 
  when 
  

   the 
  diffracted 
  ray 
  is 
  at 
  grazing 
  emergence. 
  If 
  we 
  are 
  dealing 
  

   with 
  a 
  first-order 
  spectrum, 
  1000 
  lines 
  are 
  necessary 
  to 
  

   resolve 
  the 
  D 
  lines. 
  As 
  I 
  have 
  shown 
  in 
  a 
  previous 
  paper 
  

   (" 
  Interference-phenomena 
  of 
  Chlorate 
  of 
  Potash 
  crystals 
  ^'*)* 
  

   we 
  have 
  in 
  general 
  a 
  number 
  of 
  orders 
  present, 
  and 
  if 
  we 
  

   photograph 
  those 
  in 
  the 
  ultra-violet, 
  we 
  can 
  determine 
  the 
  

   order 
  of 
  the 
  one 
  (or 
  more) 
  in 
  the 
  visible. 
  (Each 
  order 
  is 
  

   represented 
  by 
  a 
  narrow 
  region 
  of 
  wave-lengths 
  selectively 
  

   reflected.) 
  

  

  In 
  the 
  present 
  case 
  since 
  the 
  visible 
  spectrum 
  shows 
  but 
  a 
  

   single 
  reflected 
  band 
  (in 
  the 
  yellow) 
  we 
  are 
  clearly 
  dealing 
  

   with 
  the 
  first 
  order. 
  The 
  second 
  order 
  will 
  be 
  found 
  at 
  

   wave-length 
  29 
  or 
  thereabouts. 
  If 
  our 
  visible 
  band 
  at 
  wave- 
  

   length 
  X 
  was 
  a 
  second 
  order 
  one 
  we 
  should 
  have 
  a 
  third 
  order 
  

   band 
  at 
  wave-length 
  fX 
  or 
  4000 
  which 
  would 
  be 
  easily 
  

   visible. 
  

  

  If 
  now 
  we 
  compare 
  the 
  width 
  of 
  the 
  band 
  in 
  the 
  photo- 
  

   graph 
  with 
  the 
  distance 
  between 
  the 
  D 
  lines, 
  it 
  is 
  clear 
  that 
  

   the 
  crystal 
  plate 
  is 
  very 
  nearly 
  able 
  to 
  separate 
  or 
  resolve 
  

   the 
  D 
  lines. 
  In 
  other 
  words, 
  if 
  we 
  inclme 
  the 
  plate 
  a" 
  little 
  

   less, 
  causing 
  the 
  band 
  to 
  move 
  up 
  the 
  spectrum, 
  it 
  will 
  reflect 
  

   D2 
  before 
  it 
  reflects 
  Dj. 
  It 
  was, 
  however, 
  not 
  quite 
  able 
  to 
  

   do 
  this, 
  but 
  would 
  easily 
  separate 
  lines 
  of 
  twice 
  the 
  separation 
  

   of 
  the 
  D's. 
  From 
  this 
  we 
  may 
  infer 
  that 
  the 
  number 
  of 
  twin 
  

   planes 
  in 
  the 
  crystal 
  is 
  somewhere 
  between 
  500 
  and 
  1000 
  

   say 
  roughly 
  700. 
  If 
  we 
  are 
  dealing 
  with 
  a 
  first-order 
  spec- 
  

   trum 
  the 
  path 
  difference 
  between 
  rays 
  reflected 
  from 
  two 
  

   adjacent 
  twin-planes 
  must 
  be 
  equal 
  to 
  the 
  wave-length 
  of 
  the 
  

   light 
  in 
  the 
  crystal. 
  Assuming 
  no 
  phase-change 
  this 
  makes 
  

   the 
  thickness 
  of 
  each 
  lamina 
  about 
  0*0002 
  mm., 
  and 
  multi- 
  

   plying 
  this 
  by 
  700 
  gives 
  us 
  0"14: 
  mm. 
  as 
  the 
  thickness 
  of 
  the 
  

   crystal 
  plate, 
  which 
  was 
  not 
  very 
  far 
  from 
  the 
  truth. 
  

   * 
  Phil. 
  Mag. 
  [6] 
  xii. 
  p. 
  67. 
  

  

  