﻿the 
  Reflexion 
  of 
  the 
  A-rays. 
  221 
  

  

  special 
  angles. 
  They 
  found 
  that 
  at 
  each 
  of 
  these 
  angles 
  a 
  

   particular 
  type 
  of 
  radiation 
  was 
  selectively 
  reflected. 
  The 
  

   special 
  angles 
  differed 
  from 
  crystal 
  to 
  crystal 
  but 
  the 
  three 
  

  

  tvpcs 
  of 
  radiation 
  were 
  always 
  the 
  same. 
  They 
  explained 
  

   these 
  results 
  by 
  assuming 
  that 
  the 
  platinum 
  target 
  emitted 
  

   three 
  characteristic 
  types 
  of 
  monochromatic 
  waves. 
  The 
  

   beams 
  reflected 
  from 
  the 
  successive 
  layers 
  of 
  atoms 
  in 
  the 
  

   crystal 
  generally 
  destroyed 
  each 
  other 
  by 
  interference. 
  

   When, 
  however, 
  the 
  wave-length 
  A, 
  the 
  distance 
  d 
  between 
  

   the 
  layers, 
  and 
  the 
  glancing 
  angle 
  6 
  were 
  connected 
  by 
  the 
  

   relation 
  

  

  rik 
  = 
  2d 
  sin 
  (1) 
  

  

  the 
  separate 
  beams 
  reinforced 
  one 
  another, 
  and 
  the 
  mono- 
  

   chromatic 
  radiation 
  was 
  selectively 
  reflected. 
  The 
  three 
  

   types 
  of 
  radiation 
  represented 
  three 
  different 
  values 
  of 
  X, 
  

   and 
  in 
  the 
  reflexion 
  from 
  rocksalt 
  special 
  angles 
  were 
  found 
  

   corresponding 
  to 
  ?i 
  = 
  l 
  and 
  to 
  n 
  — 
  2. 
  These 
  angles 
  may 
  by 
  

   analogy 
  be 
  said 
  to 
  refer 
  to 
  reflexion 
  in 
  the 
  first 
  and 
  second 
  

   order. 
  

  

  We 
  have 
  studied 
  in 
  detail 
  the 
  selective 
  reflexion 
  from 
  the 
  

   principal 
  cleavage-planes 
  of 
  rocksalt, 
  selenite, 
  and 
  potassium 
  

   ferrocyanide. 
  The 
  most 
  complete 
  search 
  was 
  made 
  in 
  the 
  

   case 
  of 
  ferrocyanide. 
  The 
  general 
  radiation 
  is 
  reflected 
  at 
  all 
  

   angles, 
  while 
  the 
  selective 
  reflexion 
  may 
  for 
  practical 
  

   purposes 
  be 
  said 
  to 
  occur 
  only 
  at 
  precisely 
  the 
  correct 
  angle 
  

   of 
  incidence. 
  For 
  this 
  reason 
  the 
  ratio 
  of 
  the 
  selective 
  to 
  

   the 
  general 
  reflexion 
  is 
  greatly 
  increased 
  by 
  limiting 
  the 
  

   breadth 
  of 
  the 
  slits 
  and 
  so 
  increasing 
  the 
  parallelism 
  of 
  the 
  

   primary 
  beam. 
  Unfortunately, 
  a 
  very 
  small 
  rotation 
  of 
  the 
  

   crystal 
  will 
  then 
  remove 
  all 
  traces 
  of 
  the 
  selective 
  effect. 
  It 
  

   therefore 
  proved 
  necessary 
  to 
  take 
  readings 
  with 
  the 
  crystal 
  

   set 
  at 
  every 
  5' 
  of 
  arc 
  between 
  1° 
  and 
  14°. 
  In 
  the 
  case 
  of 
  

   selenite 
  most 
  of 
  the 
  special 
  angles 
  were 
  already 
  approximately 
  

   known 
  from 
  the 
  singularities 
  in 
  the 
  original 
  intensity 
  curves, 
  

   the 
  meaning 
  of 
  which 
  had 
  not 
  been 
  appreciated 
  at 
  the 
  time. 
  

   Prof. 
  Bragg 
  most 
  kindly 
  told 
  us 
  the 
  positions 
  of 
  his 
  three 
  

   special 
  angles 
  in 
  rocksalt, 
  and 
  a 
  search 
  for 
  others 
  was 
  made 
  

   only 
  in 
  those 
  places 
  which 
  theory 
  suggested 
  as 
  possible. 
  

   The 
  positions, 
  of 
  the 
  special 
  angles 
  were 
  first 
  found 
  roughly, 
  

   and 
  then 
  the 
  crystal 
  was 
  turned 
  minute 
  by 
  minute 
  until 
  the 
  

   angle 
  which 
  gave 
  the 
  maximum 
  reflexion 
  was 
  definitely 
  

   determined. 
  The 
  opening 
  in 
  the 
  detector 
  was 
  wide 
  enough 
  

   to 
  include 
  the 
  whole 
  reflected 
  beam, 
  and 
  it 
  was 
  made 
  to 
  follow 
  

   this 
  beam 
  when 
  the 
  crystal 
  was 
  rotated. 
  Several 
  of 
  the 
  special 
  

   angles 
  were 
  found 
  with 
  the 
  crystal 
  set 
  on 
  both 
  sides 
  of 
  the 
  

  

  