﻿112 
  Mr. 
  C. 
  A. 
  Sadler 
  on 
  the 
  

  

  small 
  compared 
  with 
  the 
  readings 
  obtained 
  when 
  metallic 
  

   radiators 
  were 
  employed. 
  

  

  The 
  portion 
  o£ 
  the 
  radiator 
  R, 
  exposed 
  to 
  the 
  primary 
  

   beam 
  was 
  limited 
  to 
  that 
  opposite 
  to 
  the 
  tube 
  D 
  by 
  means 
  of 
  

   a 
  suitable 
  stop 
  placed 
  in 
  the 
  tube 
  B 
  as 
  indicated 
  in 
  the 
  

   diagram. 
  

  

  Nature 
  of 
  the 
  Tertiary 
  Rays. 
  

  

  It 
  was 
  perhaps 
  reasonable 
  to 
  expect 
  that 
  the 
  tertiary 
  

   radiation 
  emitted 
  by 
  any 
  member 
  of 
  the 
  group 
  Cr-Ag 
  when 
  

   subjected 
  to 
  a 
  more 
  penetrating 
  homogeneous 
  beam 
  from 
  the 
  

   same 
  group 
  would 
  be 
  identical 
  in 
  character 
  with 
  that 
  emitted 
  

   as 
  secondary 
  radiation 
  by 
  the 
  same 
  substance 
  when 
  excited 
  

   by 
  a 
  suitable 
  primary. 
  

  

  Direct 
  experiments 
  were 
  carried 
  out 
  to 
  test 
  how 
  far 
  this 
  

   was 
  true, 
  and 
  for 
  this 
  purpose 
  pure 
  iron 
  was 
  chosen 
  as 
  the 
  

   tertiary 
  radiator. 
  

  

  The 
  radiator 
  consisted 
  of 
  a 
  small 
  rectangle 
  3 
  cms. 
  high 
  by 
  

   2 
  cms. 
  broad, 
  formed 
  of 
  pure 
  iron 
  wire 
  interlaced 
  so 
  as 
  to 
  

   expose 
  as 
  large 
  a 
  surface 
  as 
  possible 
  to 
  the 
  exciting 
  beam. 
  

   This 
  was 
  placed 
  in 
  the 
  position 
  indicated 
  by 
  R2? 
  the 
  centre 
  of 
  

   the 
  radiator 
  was 
  at 
  the 
  intersection 
  of 
  the 
  axis 
  of 
  the 
  tube 
  D 
  

   with 
  the 
  normal 
  from 
  the 
  centre 
  of 
  the 
  aperture 
  to 
  the 
  ioni- 
  

   zation 
  -chamber 
  I, 
  the 
  plane 
  of 
  the 
  radiator 
  making 
  equal 
  

   angles 
  with 
  these 
  directions. 
  

  

  As 
  a 
  secondary 
  radiator 
  in 
  the 
  position 
  R^ 
  a 
  plate 
  of 
  pure 
  

   copper 
  was 
  used. 
  

  

  The 
  aperture 
  YY 
  of 
  the 
  ionization-chamber 
  I 
  in 
  these 
  

   experiments 
  was 
  3 
  cms. 
  high 
  by 
  2 
  cms. 
  broad, 
  and 
  the 
  

   distance 
  from 
  the 
  centre 
  of 
  the 
  aperture 
  to 
  the 
  centre 
  of 
  

   R2, 
  4 
  cms. 
  

  

  Owing 
  to 
  the 
  obliquity 
  of 
  some 
  of 
  the 
  tertiary 
  rays, 
  it 
  was 
  

   evident 
  that 
  the 
  absorption 
  coeflBcients, 
  obtained 
  by 
  studying 
  

   the 
  absorption 
  by 
  thin 
  plates 
  of 
  diffeient 
  substances 
  placed 
  

   parallel 
  to 
  the 
  aperture 
  YY 
  in 
  the 
  path 
  of 
  the 
  beam, 
  would 
  

   be 
  greater 
  than 
  would 
  have 
  been 
  the 
  case 
  had 
  it 
  been 
  possible 
  

   to 
  utilize 
  a 
  pencil 
  of 
  tertiary 
  rays. 
  

  

  A 
  control 
  experiment 
  conducted 
  with 
  a 
  fairly 
  soft 
  primary 
  

   beam 
  falling 
  upon 
  the 
  same 
  iron 
  radiator 
  similarly 
  situated 
  

   before 
  an 
  aperture 
  of 
  the 
  same 
  size 
  as 
  that 
  in 
  the 
  screen 
  YY 
  

   in 
  an 
  electroscope 
  of 
  the 
  Wilson 
  type 
  placed 
  in 
  the 
  secondary 
  

   beam 
  from 
  the 
  iron, 
  gave 
  an 
  increase 
  in 
  the 
  value 
  of 
  the 
  

   absorption 
  coefficient 
  by 
  an 
  aluminium 
  plate 
  "00297 
  cm. 
  

   thick 
  of 
  6 
  per 
  cent, 
  over 
  the 
  value 
  found 
  when 
  .a 
  narrow 
  

   pencil 
  of 
  secondary 
  radiation 
  was 
  used. 
  

  

  