﻿542 
  Dr. 
  J. 
  A. 
  Gray 
  : 
  Notes 
  

  

  passing 
  through 
  small 
  thicknesses 
  of 
  platinum. 
  The 
  scat- 
  

   tering 
  is 
  so 
  great 
  that 
  after 
  passing 
  through 
  the 
  platinum 
  

   many 
  ft 
  rays 
  have 
  traversed 
  much 
  longer 
  paths 
  and 
  so 
  have 
  

   lost 
  more 
  energy. 
  It 
  is 
  possible 
  also 
  that 
  when 
  ft 
  rays 
  

   are 
  scattered 
  through 
  large 
  angles, 
  they 
  lose 
  energy. 
  In 
  

   the 
  case 
  of 
  small 
  thicknesses 
  of 
  aluminium 
  the 
  chance 
  of 
  a 
  

   ft 
  ray 
  which 
  has 
  once 
  been 
  deflected 
  through 
  a 
  large 
  angle 
  

   being 
  again 
  so 
  deflected 
  is 
  small, 
  so 
  that 
  the 
  transmitted 
  rays 
  

   remain 
  more 
  homogeneous. 
  

  

  The 
  theory 
  of 
  Schmidt 
  only 
  applies 
  strictly 
  to 
  cases 
  where 
  

   the 
  rays 
  are 
  unaltered 
  in 
  quality 
  by 
  transmission 
  through 
  

   matter. 
  It 
  is 
  easily 
  seen, 
  however, 
  that 
  if 
  the 
  percentage 
  of 
  

   rays 
  reflected 
  is 
  large 
  there 
  will 
  always 
  be 
  a 
  similar 
  decrease 
  

   in 
  the 
  first 
  part 
  of 
  an 
  ionization 
  curve 
  due 
  to 
  scattering. 
  In 
  

   the 
  case 
  of 
  y 
  rays 
  the 
  percentage 
  reflected 
  is 
  usually 
  so 
  small 
  

   that 
  this 
  effect 
  is 
  not 
  noticeable, 
  and 
  a 
  greater 
  absorption 
  at 
  

   first 
  is 
  in 
  most 
  cases 
  correctly 
  interpreted 
  by 
  saying 
  that 
  

   " 
  softer 
  " 
  rays 
  are 
  being 
  cut 
  out, 
  the 
  rays 
  as 
  a 
  whole 
  becoming 
  

   "harder." 
  

  

  There 
  is, 
  however, 
  another 
  way 
  by 
  which 
  scattered 
  <y 
  rays 
  

   can 
  alter 
  ionization 
  curves 
  (see 
  fig. 
  1). 
  Let 
  A 
  represent 
  a 
  

  

  Fig. 
  1. 
  

  

  • 
  A 
  

  

  source 
  of 
  7 
  rays, 
  E 
  a 
  measuring 
  electroscope 
  10 
  or 
  20 
  cm. 
  

   away, 
  and 
  suppose 
  we 
  take 
  ionization 
  curves 
  firstly 
  by 
  placing 
  

   absorption 
  plates 
  directly 
  above 
  A, 
  secondly 
  by 
  placing 
  them 
  

   directly 
  under 
  E. 
  It 
  has 
  been 
  found 
  by 
  several 
  observers 
  

   that 
  lower 
  values 
  for 
  the 
  absorption 
  coefficient 
  are 
  found 
  in 
  

   the 
  first 
  case. 
  This 
  is 
  really 
  due 
  to 
  the 
  scattering 
  of 
  the 
  

   rays. 
  When 
  the 
  absorption 
  plates 
  are 
  placed 
  directly 
  above 
  

   the 
  active 
  material 
  they 
  subtend 
  a 
  comparatively 
  large 
  solid 
  

   angle, 
  and 
  so 
  a 
  greater 
  percentage 
  of 
  scattered 
  rays 
  enters 
  

  

  