﻿22± 
  Messrs. 
  H. 
  G. 
  J. 
  Moseley 
  and 
  C. 
  G. 
  Darwin 
  on 
  

  

  K=l; 
  and 
  if 
  it 
  is 
  the 
  atoms 
  which 
  are 
  so 
  arranged, 
  K=±. 
  

   If, 
  however, 
  the 
  atoms 
  are 
  arranged 
  at 
  the 
  corners 
  and 
  the 
  

   centres 
  of 
  faces 
  of 
  cubes, 
  K 
  = 
  §-. 
  As 
  the 
  arrangement 
  of 
  the 
  

   atoms 
  in 
  a 
  salt 
  crystal 
  is 
  still 
  in 
  doubt, 
  there 
  remains 
  this 
  factor 
  

   of 
  uncertainty 
  in 
  the 
  determination 
  of 
  the 
  wave-lengths. 
  

  

  It 
  will 
  be 
  seen 
  by 
  comparing 
  the 
  results 
  obtained 
  with 
  the 
  

   three 
  different 
  crystals 
  that 
  equation 
  (1) 
  is 
  accurately 
  

   satisfied. 
  First, 
  in 
  Tables 
  II. 
  and 
  III. 
  the 
  five 
  radiations 
  

   will 
  be 
  seen 
  repeated 
  in 
  different 
  orders, 
  that 
  is 
  with 
  different 
  

   values 
  of 
  n. 
  Higher 
  orders 
  were 
  also 
  observed 
  but 
  not 
  

   measured 
  accurately. 
  Secondly, 
  it 
  is 
  clear 
  that 
  d 
  is 
  a 
  constant 
  

   for 
  the 
  same 
  crystal, 
  since 
  the 
  sines 
  of 
  the 
  angles 
  have 
  the 
  

   same 
  ratio 
  in 
  each 
  case. 
  The 
  characteristic 
  radiations 
  /^ 
  2 
  

   and 
  Yx 
  in 
  ferrocyanide 
  and 
  in 
  selenite 
  were 
  too 
  close 
  together 
  

   to 
  be 
  properly 
  separated. 
  It 
  was, 
  however, 
  quite 
  clear 
  in 
  both 
  

   cases 
  that 
  two 
  components 
  were 
  present, 
  and 
  that 
  they 
  were 
  

   about 
  3' 
  apart. 
  For 
  the 
  purpose 
  of 
  finding 
  wave-lengths 
  the 
  

   values 
  found 
  in 
  the 
  higher 
  orders 
  are, 
  of 
  course, 
  the 
  more 
  

   accurate. 
  Measurements 
  of 
  the 
  absorption 
  in 
  aluminium 
  of 
  

   /3 
  3 
  from 
  ferrocyanide 
  and 
  ^ 
  l 
  and 
  /3 
  2 
  from 
  selenite 
  showed 
  

   that 
  these 
  were 
  all 
  the 
  same 
  radiation. 
  The 
  absorption 
  

   coefficients 
  were 
  found 
  by 
  taking 
  the 
  difference 
  between 
  the 
  

   intensities 
  of 
  reflexion 
  at 
  the 
  special 
  angle, 
  and 
  at 
  a 
  neigh- 
  

   bouring 
  angle, 
  first 
  with 
  the 
  detector 
  bare 
  and 
  then 
  with 
  

   aluminium 
  in 
  the 
  way. 
  The 
  approximate 
  values 
  found 
  for 
  

   the 
  mass-absorption 
  in 
  aluminium 
  of 
  the 
  four 
  radiations 
  a 
  3 
  , 
  /3 
  3 
  ,. 
  

   7 
  3 
  , 
  8 
  3 
  from 
  ferrocyanide 
  gave 
  34, 
  22, 
  19, 
  16 
  (g. 
  per 
  sq. 
  cm.) 
  -1 
  

   respectively. 
  The 
  absorption 
  of 
  e 
  3 
  was 
  not 
  measured, 
  as 
  this 
  

   type 
  of 
  radiation 
  was 
  much 
  weaker 
  than 
  the 
  others. 
  All 
  

   these 
  radiations 
  were 
  greatly 
  reduced 
  by 
  absorption 
  in 
  passing- 
  

   through 
  60 
  cm. 
  of 
  air 
  on 
  their 
  way 
  to 
  the 
  detector, 
  and 
  no 
  

   attempt 
  was 
  made 
  to 
  obtain 
  at 
  all 
  accurate 
  values 
  for 
  the 
  

   absorption 
  coefficients 
  with 
  this 
  apparatus. 
  As 
  one 
  would 
  anti- 
  

   cipate, 
  the 
  absorption 
  increases 
  rapidly 
  with 
  the 
  wave-length. 
  

   Thus 
  the 
  wave-lengths 
  of 
  a 
  and 
  8 
  are 
  in 
  the 
  ratio 
  of 
  1*37 
  : 
  1, 
  

   while 
  the 
  absorption 
  coefficients 
  are 
  roughly 
  as 
  2'12:1. 
  

   The 
  so-called 
  L 
  characteristic 
  radiation 
  from 
  platinum, 
  which 
  

   Chapman 
  found 
  to 
  have 
  a 
  mass-absorption 
  coefficient 
  of 
  

   22'2 
  * 
  in 
  aluminium, 
  seems 
  to 
  correspond 
  roughly 
  with 
  the 
  

   monochromatic 
  radiation 
  /?. 
  The 
  absorption 
  in 
  aluminium 
  of 
  

   ySi 
  from 
  ferrocyanide 
  appeared 
  to 
  be 
  abnormally 
  low. 
  Here, 
  

   however, 
  the 
  characteristic 
  radiation 
  contributes 
  only 
  25 
  per 
  

   cent, 
  to 
  the 
  whole 
  reflexion, 
  and 
  the 
  discrepancy 
  may 
  be 
  

   explained 
  by 
  supposing 
  that 
  the 
  general 
  reflexion, 
  which 
  is 
  

  

  * 
  Proc. 
  Roy. 
  Soc. 
  A. 
  lxxxvi. 
  p. 
  439 
  (1912). 
  

  

  