﻿1026 
  Mr. 
  H. 
  G. 
  J. 
  Moseley 
  on 
  the 
  

  

  Now 
  it 
  is 
  known:* 
  that 
  X 
  rajs 
  consist 
  in 
  general 
  of 
  two 
  

   types, 
  the 
  heterogeneous 
  radiation 
  and 
  characteristic 
  radiations 
  

   of 
  definite 
  frequency. 
  The 
  former 
  of 
  these 
  is 
  reflected 
  from 
  

   such 
  a 
  surface 
  at 
  all 
  angles 
  of 
  incidence, 
  but 
  at 
  the 
  large 
  

   angles 
  used 
  in 
  the 
  present 
  work 
  the 
  reflexion 
  is 
  of 
  very 
  little 
  

   intensity. 
  The 
  radiations 
  of 
  definite 
  frequency, 
  on 
  the 
  other 
  

   hand, 
  are 
  reflected 
  only 
  when 
  they 
  strike 
  the 
  surface 
  at 
  

   definite 
  angles, 
  the 
  glancing 
  angle 
  of 
  incidence 
  6, 
  the 
  wave- 
  

   length 
  \, 
  and 
  the 
  " 
  grating 
  constant 
  " 
  d 
  of 
  the 
  crystal 
  being 
  

   connected 
  by 
  the 
  relation 
  

  

  n\=2d 
  sin 
  (9, 
  (1) 
  

  

  where 
  n, 
  an 
  integer, 
  may 
  be 
  called 
  the 
  " 
  order" 
  in 
  which 
  the 
  

   reflexion 
  occurs. 
  The 
  particular 
  crystal 
  used, 
  which 
  was 
  a 
  

   fine 
  specimen 
  with 
  face 
  6 
  cm. 
  square, 
  was 
  known 
  to 
  give 
  

   strong 
  reflexions 
  in 
  the 
  first 
  three 
  orders, 
  the 
  third 
  order 
  

   being 
  the 
  most 
  prominent. 
  

  

  If 
  then 
  a 
  radiation 
  of 
  definite 
  wave-length 
  happens 
  to 
  

   strike 
  any 
  part 
  P 
  of 
  the 
  crystal 
  at 
  a 
  suitable 
  angle, 
  a 
  small 
  

   part 
  of 
  it 
  is 
  reflected. 
  Assuming 
  for 
  the 
  moment 
  that 
  the 
  

   source 
  of 
  the 
  radiation 
  is 
  a 
  point, 
  the 
  locus 
  of 
  P 
  is 
  obviously 
  

   the 
  arc 
  of 
  a 
  circle, 
  and 
  the 
  reflected 
  rays 
  will 
  travel 
  along 
  

   the 
  generating 
  lines 
  of 
  a 
  cone 
  with 
  apex 
  at 
  the 
  image 
  of 
  the 
  

   source. 
  The 
  effect 
  on 
  a 
  photographic 
  plate 
  L 
  will 
  take 
  the 
  

   form 
  of 
  the 
  arc 
  of 
  an 
  hyperbola, 
  curving 
  away 
  from 
  the 
  direc- 
  

   tion 
  of 
  the 
  direct 
  beam. 
  With 
  a 
  fine 
  slit 
  at 
  S, 
  the 
  arc 
  

   becomes 
  a 
  fine 
  line 
  which 
  is 
  slightly 
  curved 
  in 
  the 
  direction 
  

   indicated. 
  

  

  The 
  photographic 
  plate 
  was 
  mounted 
  on 
  the 
  spectrometer 
  

   arm, 
  and 
  both 
  the 
  plate 
  and 
  the 
  slit 
  were 
  17 
  cm. 
  from 
  the 
  

   axis. 
  The 
  importance 
  of 
  this 
  arrangement 
  lies 
  in 
  a 
  geome- 
  

   trical 
  property, 
  for 
  when 
  these 
  two 
  distances 
  are 
  equal 
  the 
  

   point 
  L 
  at 
  which 
  a 
  beam 
  reflected 
  at 
  a 
  definite 
  angle 
  strikes 
  

   the 
  plate 
  is 
  independent 
  of 
  the 
  position 
  of 
  P 
  on 
  the 
  crystal 
  

   surface. 
  The 
  angle 
  at 
  which 
  the 
  crystal 
  is 
  set 
  is 
  then 
  im- 
  

   material 
  so 
  long 
  as 
  a 
  ray 
  can 
  strike 
  some 
  part 
  of 
  the 
  surface 
  

   at 
  the 
  required 
  angle. 
  The 
  angle 
  6 
  can 
  be 
  obtained 
  from 
  

   the 
  relation 
  26 
  = 
  180° 
  - 
  SFL 
  = 
  180° 
  -SAL. 
  

  

  The 
  following 
  method 
  was 
  used 
  for 
  measuring 
  the 
  angle 
  

   SAL. 
  Before 
  taking 
  a 
  photograph 
  a 
  reference 
  line 
  R 
  was 
  

   made 
  at 
  both 
  ends 
  of 
  the 
  plate 
  by 
  replacing 
  the 
  crystal 
  by 
  a 
  

   lead 
  screen 
  furnished 
  with 
  a 
  fine 
  slit 
  which 
  coincided 
  with 
  

   the 
  axis 
  of 
  the 
  spectrometer. 
  A 
  few 
  seconds' 
  exposure 
  to 
  the 
  

   X 
  rays 
  then 
  gave 
  a 
  line 
  R 
  on 
  the 
  plate, 
  and 
  so 
  defined 
  on 
  it 
  

  

  * 
  Mosfeley 
  and 
  Darwin, 
  loo, 
  cit. 
  

  

  