﻿230 
  Messrs. 
  H. 
  G. 
  J. 
  Moseley 
  and 
  C. 
  G. 
  Darwin 
  on 
  

  

  Its 
  effect 
  will 
  only 
  become 
  important 
  when 
  the 
  constituent 
  

   of 
  which 
  the 
  wave-length 
  corresponds 
  with 
  the 
  first 
  order 
  is 
  

   very 
  weak. 
  The 
  steady 
  increase 
  in 
  the 
  absorption 
  coefficient 
  

   as 
  the 
  angle 
  is 
  increased 
  from 
  3° 
  to 
  8°, 
  and 
  the 
  way 
  in 
  which 
  

   the 
  maximum 
  shifts 
  towards 
  larger 
  angles 
  as 
  the 
  tube 
  softens, 
  

   have 
  already 
  been 
  described. 
  The 
  effect 
  of 
  the 
  different 
  

   orders 
  overlapping 
  is 
  very 
  marked 
  in 
  the 
  neighbourhood 
  of 
  

   the 
  special 
  angles 
  a 
  l1 
  &c. 
  Here 
  the 
  characteristic 
  radiation, 
  

   reflected 
  in 
  the 
  first 
  order, 
  is 
  stopped 
  by 
  a 
  few 
  tenths 
  of 
  1 
  mm. 
  

   of 
  aluminium. 
  A 
  sheet 
  of 
  this 
  thickness 
  has, 
  however, 
  very 
  

   little 
  effect 
  on 
  the 
  general 
  reflexion 
  at 
  a 
  neighbouring 
  angle. 
  

   In 
  the 
  reflexion 
  from 
  selenite 
  the 
  maximum 
  intensity 
  in 
  the 
  

   first 
  order 
  should 
  occur 
  at 
  about 
  1°*5. 
  In 
  this 
  case, 
  however, 
  

   the 
  second 
  order 
  is 
  more 
  important 
  than 
  the 
  first, 
  so 
  that 
  a 
  

   strong 
  second-order 
  effect, 
  with 
  maximum 
  at 
  about 
  3°0, 
  will 
  

   be 
  superimposed 
  on 
  it. 
  The 
  combined 
  effect 
  shows 
  in 
  

   curve 
  I. 
  a 
  strong 
  maximum 
  at 
  a 
  rather 
  smaller 
  angle 
  than 
  

   would 
  be 
  anticipated, 
  probably 
  partly 
  because 
  a 
  somewhat 
  

   harder 
  tube 
  was 
  used 
  in 
  this 
  case. 
  There 
  is 
  also, 
  however, 
  

   an 
  important 
  difference 
  between 
  the 
  behaviour 
  of 
  mono- 
  

   chromatic 
  radiation 
  and 
  a 
  continuous 
  spectrum. 
  In 
  the 
  

   latter 
  case, 
  owing 
  to 
  the 
  increased 
  dispersion, 
  the 
  second- 
  

   order 
  spectrum 
  will 
  be 
  spread 
  out 
  over 
  twice 
  the 
  range 
  of 
  

   angles 
  occupied 
  by 
  the 
  first 
  order, 
  and 
  the 
  intensity 
  at 
  any 
  

   angle 
  will 
  consequently 
  be 
  halved. 
  This 
  is 
  well 
  shown 
  in 
  

   curves 
  III. 
  and 
  IV. 
  taken 
  with 
  the 
  ferrocyanide 
  crystal. 
  

   Here 
  the 
  very 
  narrow 
  first 
  order 
  and 
  the 
  broader 
  third 
  order 
  

   are 
  conspicuous, 
  while 
  curve 
  IV. 
  shows 
  also 
  the 
  effect 
  of 
  the 
  

   weaker 
  second 
  order. 
  At 
  very 
  fine 
  angles 
  curve 
  IV. 
  falls 
  

   away 
  much 
  more 
  than 
  curve 
  III., 
  and 
  at 
  broader 
  angles 
  the 
  

   reverse 
  occurs, 
  all 
  in 
  accordance 
  with 
  theory. 
  

  

  In 
  the 
  reflexion 
  at 
  small 
  angles 
  from 
  rocksalt 
  the 
  first 
  

   order 
  predominates, 
  and 
  this 
  gives 
  an 
  opportunity 
  of 
  finding 
  

   the 
  distribution 
  of 
  energy 
  in 
  the 
  spectrum 
  of 
  the 
  X-rays. 
  

   The 
  complete 
  mathematical 
  treatment 
  of 
  this 
  question 
  is 
  much 
  

   too 
  long 
  for 
  this 
  paper, 
  and 
  a 
  simple 
  argument 
  which 
  leads 
  

   to 
  the 
  same 
  conclusion 
  must 
  here 
  suffice. 
  

  

  It 
  is 
  probable 
  in 
  the 
  first 
  place 
  that, 
  if 
  no 
  cooperation 
  

   or 
  interference 
  took 
  place, 
  the 
  amount 
  of 
  energy 
  scattered 
  

   from 
  the 
  surface 
  of 
  a 
  thick 
  sheet 
  of 
  any 
  material, 
  when 
  struck 
  

   by 
  the 
  X-rays, 
  would 
  be 
  approximately 
  independent 
  of 
  the 
  

   quality 
  of 
  the 
  radiation, 
  and 
  would 
  be 
  proportional 
  to 
  

   1 
  + 
  cos 
  2 
  cf> 
  where 
  (f> 
  is 
  the 
  angle 
  between 
  the 
  primary 
  and 
  

   scattered 
  rays. 
  The 
  experiments 
  of 
  Crowther 
  * 
  have 
  shown 
  

   that 
  when 
  <f> 
  is 
  small 
  the 
  scattering 
  is 
  much 
  greater 
  than 
  

   * 
  Crowther, 
  Proc. 
  Koy. 
  Soc. 
  A. 
  Ixxxvi. 
  p. 
  478 
  (1912). 
  

  

  