﻿93 
  

  

  Errors 
  in 
  Ruling 
  a 
  Diffraction 
  Grating. 
  

  

  two 
  emergent 
  cones 
  of 
  light 
  are 
  bounded 
  by 
  the 
  correspond- 
  

   ing 
  parts 
  of 
  the 
  image 
  of 
  the 
  grating. 
  The 
  light 
  from 
  the 
  

   wide 
  ruling 
  AB 
  is 
  bounded 
  by 
  the 
  image 
  A'B', 
  that 
  from 
  the 
  

   narrow 
  is 
  bounded 
  by 
  B'C 
  Apart 
  from 
  interference-effects, 
  

  

  a 
  screen 
  p 
  

  

  ^d 
  at 
  A'B'C 
  .would 
  be 
  uniformlv 
  illuminated 
  

  

  Ffr.l 
  

  

  If 
  the 
  screen 
  is 
  moved 
  nearer 
  to 
  the 
  lens 
  there 
  will 
  be 
  a 
  

   portion 
  at 
  B" 
  receiving 
  light 
  from 
  both 
  the 
  cones, 
  so 
  that 
  the 
  

   illumination 
  here 
  is 
  increased. 
  If 
  the 
  screen 
  is 
  moved 
  away 
  

   from 
  the 
  lens, 
  the 
  part 
  at 
  B'" 
  does 
  not 
  receive 
  light 
  from 
  

   either 
  ot 
  the 
  cones, 
  so 
  that 
  a 
  dark 
  band 
  is 
  formed 
  on 
  the 
  

   screen. 
  

  

  In 
  fig. 
  2 
  (p. 
  94) 
  the 
  second 
  case 
  is 
  shown 
  in 
  which 
  the 
  relative 
  

   position 
  of 
  the 
  rulings 
  is 
  reversed 
  — 
  the 
  wide 
  ruling 
  is 
  now 
  on 
  

   the 
  right, 
  the 
  narrow 
  one 
  on 
  the 
  left. 
  The 
  positions 
  of 
  F 
  x 
  

   and 
  F 
  2 
  with 
  regard 
  to 
  the 
  rulings 
  are 
  reversed, 
  and 
  it 
  is 
  clear 
  

   at 
  once 
  from 
  the 
  figure 
  that 
  there 
  will 
  now 
  be 
  darkness 
  at 
  B" 
  

   and 
  increased 
  illumination 
  at 
  W". 
  

  

  If 
  the 
  figures 
  are 
  drawn 
  for 
  orders 
  on 
  th_> 
  other 
  side 
  — 
  that 
  

  

  