﻿the 
  Reflexion 
  of 
  the 
  X-rays. 
  2 
  1 
  3 
  

  

  this 
  gave 
  a 
  current 
  of 
  about 
  0*3 
  niilliamp. 
  through 
  the 
  tube. 
  

   The 
  tube 
  was 
  enclosed 
  in 
  a 
  thick 
  lead 
  box, 
  and 
  the 
  coil 
  

   surrounded 
  by 
  an 
  earthed 
  tin 
  case. 
  

  

  The 
  Reflector. 
  — 
  Three 
  different 
  crystals 
  were 
  used 
  for 
  

   reflecting 
  the 
  radiation 
  : 
  rocksalt 
  (Nad), 
  selenite 
  (CaS0 
  4 
  , 
  

   2H 
  2 
  0), 
  and 
  potassium 
  ferrocyanide 
  (K 
  4 
  FeCy 
  6 
  , 
  3H 
  2 
  0). 
  'Hie 
  

   last 
  of 
  these 
  was 
  a 
  magnificent 
  specimen, 
  with 
  a 
  perfect 
  

   face 
  6 
  cm. 
  square. 
  A 
  perfect 
  face 
  of 
  selenite 
  was 
  easily 
  

   obtained 
  by 
  cleavage. 
  All 
  the 
  available 
  specimens 
  of 
  rock- 
  

   salt 
  were 
  contorted, 
  but 
  one 
  was 
  eventually 
  found 
  with 
  a 
  

   sufficient 
  area 
  of 
  flat 
  surface. 
  The 
  faces 
  used 
  were 
  a 
  cube 
  

   face 
  in 
  rocksalt, 
  and 
  the 
  principal 
  cleavage-planes 
  in 
  selenite 
  

   and 
  potassium 
  ferrocyanide. 
  

  

  The 
  Mounting. 
  — 
  A 
  dismantled 
  spectrometer 
  was 
  used 
  to 
  

   carry 
  the 
  principal 
  parts 
  of 
  the 
  apparatus. 
  The 
  crystal 
  was 
  

   set 
  up 
  with 
  its 
  surface 
  on 
  the 
  axis 
  of 
  the 
  spectrometer. 
  

   The 
  slits 
  P 
  and 
  Q 
  were 
  fixed 
  in 
  the 
  place 
  of 
  the 
  collimator, 
  

   and 
  the 
  detector 
  on 
  the 
  arm 
  which 
  usually 
  carries 
  the 
  

   telescope, 
  while 
  each 
  crystal 
  was 
  permanently 
  mounted 
  on 
  

   a 
  table, 
  the 
  legs 
  of 
  which 
  could 
  rest 
  in 
  the 
  V 
  grooves 
  of 
  

   the 
  prism 
  table. 
  The 
  slits 
  were 
  parallel 
  to 
  this 
  axis 
  and 
  

   pointed 
  at 
  it. 
  The 
  beam 
  therefore 
  struck 
  the 
  crystal 
  surface 
  

   in 
  a 
  fine 
  line, 
  and 
  however 
  the 
  crystal 
  was 
  turned, 
  the 
  line 
  

   remained 
  stationary 
  on 
  the 
  axis 
  of 
  the 
  spectrometer. 
  This 
  

   line 
  was 
  perpendicular 
  to 
  the 
  direction 
  of 
  the 
  incident 
  beam 
  

   and 
  so 
  the 
  angles 
  of 
  incidence 
  of 
  the 
  rays 
  at 
  the 
  ends 
  and 
  

   at 
  the 
  centre 
  of 
  the 
  slit 
  were 
  almost 
  exactly 
  equal. 
  

  

  The 
  Adjustment. 
  — 
  A 
  telescope 
  T 
  was 
  kept 
  permanently 
  

   pointing 
  at 
  the 
  axis 
  of 
  the 
  spectrometer. 
  The 
  focus 
  spot 
  on 
  

   the 
  target 
  A, 
  the 
  slits 
  P 
  and 
  Q, 
  and 
  the 
  surface 
  of 
  the 
  crystal 
  

   were 
  set 
  accurately 
  in 
  the 
  phme 
  defined 
  by 
  the 
  axes 
  of 
  the 
  

   telescope 
  and 
  spectrometer. 
  The 
  first 
  of 
  these 
  adjustments 
  

   Was 
  made 
  by 
  covering 
  with 
  lead 
  all 
  but 
  the 
  centre 
  of 
  P, 
  and 
  

   moving 
  the 
  X-ray 
  tube 
  until 
  the 
  bright 
  spot 
  on 
  a 
  fluorescent 
  

   screen 
  placed 
  at 
  Q 
  was 
  brought 
  on 
  to 
  the 
  axis 
  of 
  the 
  

   telescope. 
  

  

  Measuring 
  the 
  angles. 
  — 
  The 
  position 
  of 
  the 
  arm 
  which 
  

   carried 
  the 
  detector 
  could 
  be 
  read 
  on 
  the 
  divided 
  circle 
  of 
  

   the 
  spectrometer 
  to 
  within 
  30". 
  The 
  prism-table, 
  however, 
  

   carried 
  no 
  vernier 
  and 
  a 
  second 
  telescope 
  S 
  was 
  used 
  to 
  

   define 
  the 
  position 
  of 
  the 
  crystal. 
  A 
  piece 
  of 
  plate-glass 
  

   was 
  fixed 
  on 
  to 
  the 
  crystal 
  at 
  such 
  an 
  angle 
  that 
  when 
  the 
  

   crystal 
  >urface 
  was 
  parallel 
  to 
  the 
  X-ray 
  beam 
  a 
  fixed 
  light 
  

   was 
  reflected 
  by 
  the 
  glass 
  on 
  to 
  the 
  cross-wires 
  of 
  S, 
  which 
  was 
  

   3 
  metres 
  distant. 
  The 
  crystal 
  was 
  first 
  Bet 
  to 
  this 
  standard 
  

   position 
  and 
  then 
  turned 
  together 
  with 
  the 
  detector 
  through 
  

  

  