﻿Asymmetry 
  of 
  Illumination 
  Curves 
  in 
  Oblique 
  Refraction. 
  113 
  

  

  obliquity, 
  which, 
  of 
  course, 
  is 
  not 
  the 
  same 
  at 
  all 
  points 
  in 
  

   the 
  diffraction 
  pattern. 
  In 
  other 
  words, 
  the 
  ordinates 
  o£ 
  the 
  

   illumination 
  curve 
  were 
  found 
  to 
  be 
  proportional 
  to 
  the 
  ex- 
  

   pression 
  cos 
  2 
  # 
  sin 
  2 
  S/S 
  2 
  . 
  

  

  The 
  question 
  arises 
  whether 
  these 
  results, 
  practically 
  those 
  

   indicated 
  in 
  (b) 
  and 
  (c) 
  above, 
  are 
  peculiar 
  to 
  the 
  case 
  of 
  a 
  

   surface 
  of 
  rectangular 
  form, 
  or 
  whether 
  similar 
  phenomena 
  

   might 
  be 
  expected 
  with 
  other 
  forms 
  of 
  surface 
  as 
  well. 
  

   The 
  cases 
  which 
  it 
  seemed 
  of 
  particular 
  interest 
  to 
  examine 
  

   are 
  tiose 
  in 
  which 
  the 
  reflecting 
  surface 
  is 
  not 
  a 
  single 
  

   individual 
  area 
  but 
  consists 
  of 
  two, 
  three, 
  or 
  more 
  parallel 
  

   elements 
  lying 
  in 
  the 
  same 
  plane. 
  A 
  satisfactory 
  surface 
  of 
  

   this 
  kind 
  which 
  can 
  be 
  used 
  at 
  very 
  oblique 
  incidences 
  may 
  

   be 
  prepared 
  by 
  etching 
  out 
  deep 
  grooves 
  on 
  the 
  optically 
  

   plane 
  surface 
  of 
  a 
  thick 
  plate 
  of 
  glass 
  with 
  hydrofluoric 
  acid, 
  

   the 
  edges 
  of 
  the 
  reflecting 
  strips 
  left 
  on 
  the 
  surface 
  being 
  

   subsequently 
  ground 
  so 
  as 
  to 
  be 
  sharp, 
  straight, 
  and 
  parallel. 
  

   I 
  have 
  prepared 
  several 
  such 
  surfaces 
  containing 
  two 
  and 
  

   three 
  equidistant 
  reflecting 
  strips 
  respectively. 
  By 
  placing 
  

   one 
  of 
  these 
  on 
  the 
  table 
  of 
  a 
  spectrometer, 
  the 
  diffraction 
  

   pattern 
  produced 
  by 
  reflexion 
  at 
  very 
  oblique 
  incidences 
  

   may 
  be 
  readily 
  observed 
  through 
  the 
  telescope 
  of 
  the 
  instru- 
  

   ment. 
  The 
  present 
  paper 
  describes 
  the 
  results 
  of 
  the 
  

   quantitative 
  study 
  of 
  the 
  phenomena 
  thus 
  obtained. 
  Inci- 
  

   dentally 
  the 
  opportunity 
  has 
  also 
  been 
  taken 
  of 
  testing 
  the 
  

   results 
  obtained 
  by 
  Raman 
  for 
  the 
  case 
  of 
  a 
  single 
  aperture 
  

   using 
  improved 
  optical 
  and 
  photographic 
  appliances. 
  The 
  

   experiments 
  and 
  determinations 
  have 
  throughout 
  been 
  made 
  

   using 
  monochromatic 
  light. 
  This 
  was 
  secured 
  by 
  illuminat- 
  

   ing 
  the 
  slit 
  of 
  the 
  spectrometer 
  with 
  light 
  of 
  a 
  definite 
  

   wave-length 
  isolated 
  by 
  a 
  monochromator 
  from 
  sunlight 
  or 
  

   arc 
  light. 
  

  

  Unsymmetrical 
  Interference-fringes 
  due 
  to 
  two 
  

   parallel 
  apertures. 
  

  

  Fig. 
  I 
  (PI. 
  Y.) 
  reproduces 
  a 
  photograph 
  of 
  the 
  diffraction 
  

   pattern 
  due 
  to 
  a 
  surface 
  containing 
  two 
  reflecting 
  elements 
  

   each 
  of 
  width 
  0*48 
  cm., 
  and 
  3*60 
  cm. 
  apart. 
  The 
  direct 
  

   image 
  of 
  the 
  slit 
  of 
  the 
  spectrometer 
  also 
  appears 
  in 
  the 
  

   figures 
  to 
  the 
  right 
  of 
  the 
  diffraction-pattern. 
  The 
  photo- 
  

   graph 
  is 
  reproduced 
  from 
  a 
  dense 
  negative 
  taken 
  to 
  show 
  

   the 
  perfect 
  blackness 
  of 
  the 
  minima 
  of 
  illumination, 
  and 
  the 
  

   progressive 
  increase 
  (from 
  left 
  to 
  right) 
  in 
  the 
  width 
  of 
  the 
  

   interference-fringes 
  of 
  the 
  light 
  diffracted 
  by 
  the 
  two 
  re- 
  

   flecting 
  elements. 
  It 
  was 
  obtained 
  by 
  replacing 
  the 
  telescope 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  35. 
  No. 
  205. 
  Jan. 
  1918. 
  I 
  

  

  