﻿the 
  Reflexion 
  of 
  the 
  X-rays, 
  217 
  

  

  was 
  very 
  marked. 
  Potassium 
  ferrocyanide 
  was 
  here 
  used 
  as 
  

   reflector. 
  

  

  (4) 
  Absorption. 
  — 
  The 
  primary 
  and 
  reflected 
  beams 
  were 
  

   usually 
  not 
  equally 
  penetrating. 
  The 
  cause 
  of 
  this 
  was 
  

   thought 
  to 
  be 
  the 
  unequal 
  reflexion 
  of 
  different 
  constituents. 
  

   Ir 
  was 
  conceivable, 
  however, 
  that 
  a 
  special 
  type 
  of 
  radiation 
  

   was 
  manufactured 
  by 
  the 
  crystal. 
  This 
  point 
  was 
  tested 
  in 
  

   an 
  experiment 
  in 
  which 
  mica 
  was 
  used 
  as 
  reflector, 
  and 
  the 
  

   reflected 
  beam 
  happened 
  to 
  be 
  considerably 
  the 
  more 
  pene- 
  

   trating-. 
  An 
  aluminium 
  sheet 
  was 
  placed 
  alternately 
  in 
  the 
  

   path 
  of 
  the 
  primary 
  and 
  reflected 
  beams. 
  The 
  reflected 
  

   radiation 
  was 
  the 
  same 
  in 
  the 
  two 
  cases. 
  

  

  All 
  these 
  experiments 
  show 
  clearly 
  that 
  the 
  primary 
  and 
  

   reflected 
  radiations 
  consist 
  essentially 
  of 
  the 
  same 
  constituents, 
  

   but 
  that 
  these 
  constituents 
  are 
  present 
  in 
  different 
  proportions 
  

   in 
  the 
  two 
  beams. 
  

  

  2 
  he 
  General 
  Reflexion 
  at 
  different 
  Angles 
  of 
  Incidence. 
  

   The 
  next 
  step 
  w 
  r 
  as 
  to 
  determine 
  how 
  the 
  intensity 
  of 
  

   reflexion 
  depends 
  on 
  the 
  angle 
  at 
  which 
  the 
  radiation 
  strikes 
  

   the 
  surface 
  of 
  the 
  crystal. 
  It 
  was 
  soon 
  apparent 
  that 
  the 
  

   strongest 
  reflexion 
  occurs 
  at 
  almost 
  glancing 
  incidence, 
  

   and 
  that 
  at 
  an 
  angle 
  less 
  than 
  55° 
  the 
  reflexion 
  becomes 
  too 
  

   small 
  for 
  measurement. 
  For 
  the 
  future 
  therefore 
  the 
  

   glancing 
  angle, 
  that 
  is 
  the 
  angle 
  made 
  by 
  the 
  rays 
  with 
  the 
  

   surface 
  of 
  the 
  crystal, 
  will 
  be 
  used 
  instead 
  of 
  the 
  angle 
  of 
  

   incidence. 
  The 
  crystals 
  used 
  were 
  all 
  so 
  thick 
  that 
  at 
  no 
  

   angle 
  would 
  an 
  increase 
  in 
  thickness 
  have 
  improved 
  the 
  

   reflexion. 
  For 
  the 
  smaller 
  angles 
  the 
  primary 
  beam 
  was 
  

   limited 
  by 
  fine 
  slits 
  at 
  P 
  and 
  Q. 
  As 
  the 
  crystal 
  was 
  turned 
  

   round 
  the 
  detector 
  was 
  turned 
  through 
  twice 
  the 
  angle, 
  so 
  

   that 
  it 
  always 
  caught 
  the 
  whole 
  reflected 
  beam. 
  At 
  the 
  

   larger 
  angles 
  it 
  was 
  necessar}^ 
  to 
  use 
  wider 
  slits, 
  and 
  then 
  

   the 
  detector 
  only 
  intercepted 
  the 
  central 
  part 
  of 
  the 
  reflexion. 
  

   When 
  changing 
  the 
  slits 
  careful 
  comparative 
  measurements 
  

   were 
  made 
  at 
  one 
  or 
  two 
  angles, 
  so 
  that 
  all 
  the 
  results 
  could 
  

   be 
  reduced 
  to 
  a 
  common 
  standard. 
  There 
  were 
  two 
  reasons 
  

   for 
  using 
  fine 
  slits 
  at 
  small 
  angles. 
  In 
  the 
  first 
  place 
  it 
  

   was 
  essentia] 
  that 
  the 
  whole 
  beam 
  should 
  strike 
  the 
  crystal, 
  

   and 
  no 
  radiation 
  get 
  past 
  it. 
  Secondly, 
  the 
  variation 
  of 
  

   reflexion 
  with 
  angle 
  was 
  sometimes 
  so 
  rapid 
  that 
  it 
  was 
  

   necessary 
  to 
  avoid 
  any 
  sensible 
  divergence 
  in 
  the 
  beam. 
  

   This 
  was 
  particularly 
  important 
  near 
  those 
  angles 
  at 
  which 
  

   the 
  selective 
  reflexion 
  occurred. 
  When 
  these 
  special 
  angles 
  

   had 
  been 
  accurately 
  measured, 
  they 
  could 
  always 
  be 
  avoided 
  

   in 
  finding 
  the 
  shape 
  of 
  the 
  general 
  curve. 
  In 
  giving 
  the 
  

   results 
  obtained 
  for 
  the 
  general 
  reflexion 
  we 
  have 
  as 
  far 
  as 
  

  

  