﻿High-Frequency 
  Spectra 
  of 
  the 
  Elements. 
  1027 
  

  

  the 
  line 
  joining 
  8 
  and 
  A. 
  A 
  second 
  line 
  B/ 
  was 
  made 
  in 
  the 
  

   same 
  way 
  after 
  turning 
  the 
  spectrometer 
  arm 
  through 
  a 
  

   definite 
  angle. 
  The 
  arm 
  was 
  then 
  turned 
  to 
  the 
  position 
  

   required 
  to 
  catch 
  the 
  reflected 
  beam 
  and 
  the 
  angles 
  LAP 
  i'cr 
  

   any 
  lines 
  which 
  were 
  subsequently 
  found 
  on 
  the 
  plate 
  de- 
  

   duced 
  from 
  the 
  known 
  value 
  of 
  HAP 
  and 
  the 
  position 
  of 
  the 
  

   lines 
  on 
  the 
  plate. 
  The 
  angle 
  LAP 
  was 
  measured 
  with 
  an 
  

   error 
  of 
  not 
  more 
  than 
  o, 
  l, 
  by 
  superposing 
  on 
  the 
  negative 
  

   a 
  plate 
  on 
  which 
  reference 
  lines 
  had 
  been 
  marked 
  in 
  the 
  

   same 
  way 
  at 
  intervals 
  of 
  1°. 
  In 
  finding 
  from 
  this 
  the 
  

   glancing 
  angle 
  of 
  reflexion 
  two 
  small 
  corrections 
  were 
  

   necessary 
  in 
  practice, 
  since 
  neither 
  the 
  face 
  of 
  the 
  crystal 
  

   nor 
  the 
  lead 
  slit 
  coincided 
  accurately 
  with 
  the 
  axis 
  of 
  the 
  

   spectrometer. 
  Wave-lengths 
  varying 
  over 
  a 
  range 
  of 
  about 
  

   30 
  per 
  cent, 
  could 
  be 
  reflected 
  for 
  a 
  given 
  position 
  of 
  the 
  

   crystal. 
  

  

  In 
  almost 
  all 
  cases 
  the 
  time 
  of 
  exposure 
  was 
  five 
  minutes. 
  

   Ilford 
  X-ray 
  plates 
  were 
  used 
  and 
  were 
  developed 
  with 
  

   rodinal. 
  The 
  plates 
  were 
  mounted 
  in 
  a 
  plate-holder, 
  the 
  

   front 
  of 
  which 
  was 
  covered 
  with 
  black 
  paper. 
  In 
  order 
  to 
  

   determine 
  the 
  wave-length 
  from 
  the 
  reflexion 
  angle 
  6 
  it 
  is 
  

   necessary 
  to 
  know 
  both 
  the 
  order 
  n 
  in 
  which 
  the 
  reflexion 
  

   occurs 
  and 
  the 
  grating 
  constant 
  d. 
  n 
  was 
  determined 
  by 
  

   photographing 
  every 
  spectrum 
  both 
  in 
  the 
  second 
  order 
  and 
  

   the 
  third. 
  This 
  also 
  gave 
  a 
  useful 
  check 
  on 
  the 
  accuracy 
  of 
  

   the 
  measurements 
  ; 
  d 
  cannot 
  be 
  calculated 
  directly 
  for 
  the 
  

   complicated 
  crystal 
  potassium 
  ferrocyanide. 
  The 
  grating 
  

   constant 
  of 
  this 
  particular 
  crystal 
  had, 
  however, 
  previously 
  * 
  

   been 
  accurately 
  compared 
  with 
  d' 
  , 
  the 
  constant 
  of 
  a 
  specimen 
  

   of 
  rocksalt. 
  It 
  was 
  found 
  that 
  

  

  •1988 
  

  

  Now 
  W. 
  L. 
  Bragg 
  f 
  has 
  shown 
  that 
  the 
  atoms 
  in 
  a 
  rock- 
  

   salt 
  crystal 
  are 
  in 
  simple 
  cubical 
  array. 
  Hence 
  the 
  number 
  

   of 
  atoms 
  per 
  c.c. 
  

  

  2t= 
  X 
  

  

  M 
  (d'Y 
  ' 
  

  

  N, 
  the 
  number 
  of 
  molecules 
  in 
  a 
  gram-mol., 
  =6*05xl0 
  23 
  , 
  

   assuming 
  the 
  charge 
  on 
  an 
  electron 
  to 
  bo 
  4*89 
  xlO" 
  10 
  ; 
  

   <r, 
  the 
  density 
  of 
  this 
  crystal 
  of 
  rocksalt, 
  was 
  2*167, 
  and 
  

   M 
  the 
  molecular 
  weight 
  =58'46. 
  

  

  * 
  Moseley 
  & 
  Darwin, 
  loc. 
  cit. 
  

  

  t 
  Proc. 
  Roy. 
  Soc, 
  A. 
  lxxxix. 
  p. 
  248 
  (1913), 
  

  

  • 
  

  

  

  