﻿212 
  Messrs. 
  H. 
  Gr. 
  J. 
  Moseley 
  and 
  C. 
  Gr. 
  Darwin 
  on 
  

  

  same 
  time 
  a 
  careful 
  study 
  of 
  the 
  general 
  reflexion 
  from 
  

   these 
  crystals 
  has 
  thrown 
  some 
  light 
  on 
  the 
  nature 
  of 
  the 
  

   ordinary 
  X-rays. 
  

  

  The 
  Apparatus. 
  

  

  In 
  the 
  course 
  of 
  the 
  present 
  work 
  the 
  experimental 
  

   arrangements 
  have 
  frequently 
  been 
  refined, 
  and 
  the 
  methods 
  

   which 
  finally 
  proved 
  satisfactory 
  will 
  alone 
  be 
  described. 
  

   The 
  general 
  plan 
  was 
  to 
  allow 
  a 
  beam 
  of 
  X-rays 
  to 
  fall 
  on 
  

   to 
  a 
  crystal 
  and 
  examine 
  the 
  radiation 
  from 
  its 
  surface 
  by 
  

   means 
  of 
  a 
  movable 
  ionization 
  chamber. 
  This 
  part 
  of 
  the 
  

   apparatus 
  will 
  in 
  future 
  be 
  called 
  the 
  detector. 
  

  

  The 
  X-rays. 
  — 
  The 
  source 
  of 
  the 
  X-rays 
  was 
  a 
  Miiller 
  tube 
  

   with 
  platinum 
  target. 
  The 
  beam 
  of 
  radiation 
  was 
  limited 
  

   by 
  two 
  parallel 
  lead 
  slits 
  3 
  cm. 
  long, 
  0'5 
  mm. 
  broad, 
  and 
  

   20 
  cm. 
  apart. 
  They 
  are 
  marked. 
  P 
  and 
  Q 
  in 
  fig. 
  1, 
  which 
  

  

  Fig. 
  1 
  B. 
  

  

  Fig. 
  1. 
  

  

  fROM 
  \ 
  \t/GHT 
  

   4ND 
  to\ 
  \TELESCOPE 
  S 
  

  

  Potentiometer 
  

  

  shows 
  the 
  general 
  arrangement 
  of 
  the 
  apparatus. 
  The 
  slits 
  

   stopped 
  the 
  radiation 
  from 
  all 
  but 
  a 
  small 
  strip 
  of 
  the 
  target, 
  

   and 
  it 
  was 
  therefore 
  important 
  that 
  the 
  cathode 
  stream 
  

   falling 
  on 
  it 
  should 
  be 
  focussed 
  to 
  a 
  point. 
  The 
  tube 
  was 
  

   excited 
  by 
  a 
  coil 
  with 
  a 
  Sanax 
  mercury 
  break. 
  A 
  current 
  

   of 
  some 
  6 
  amps, 
  was 
  sent 
  through 
  the 
  primary 
  circuit, 
  and 
  

  

  