﻿Secondary 
  Radiation 
  of 
  Positive 
  Bays. 
  

  

  61 
  

  

  Several 
  similar 
  exposures 
  were 
  made 
  with 
  an 
  induction-coil 
  

   and 
  others 
  with 
  an 
  influence- 
  machine. 
  The 
  duration 
  of 
  ex- 
  

   posure 
  was 
  varied 
  between 
  2*5 
  and 
  22 
  minutes, 
  the 
  potential 
  

   between 
  25 
  and 
  40 
  mm. 
  spark-gap, 
  and 
  the 
  pressure 
  between 
  

   '0007 
  and 
  *0037 
  mm. 
  mercury. 
  

  

  The 
  annexed 
  photograph 
  was 
  obtained 
  with 
  an 
  influence- 
  

   machine 
  and 
  an 
  exposure 
  of 
  22 
  minutes. 
  The 
  spark-gap 
  was 
  

   25-30 
  mm. 
  and 
  the 
  pressure 
  "0037 
  mm. 
  mercury. 
  

  

  When 
  photographs 
  were 
  taken 
  with 
  lead 
  foil, 
  with 
  

   potentials 
  ranging 
  between 
  25 
  mm. 
  and 
  40 
  mm. 
  spark-gap, 
  

   the 
  darkening 
  of 
  the 
  plate 
  was 
  only 
  faint, 
  and 
  no 
  difference 
  

   was 
  visible 
  in 
  intensity 
  of 
  the 
  impression 
  on 
  either 
  half 
  of 
  the 
  

   image. 
  As 
  soon, 
  however, 
  as 
  the 
  spark-gap 
  was 
  increased 
  to 
  

   45 
  mm. 
  a 
  very 
  distinct 
  contrast 
  became 
  visible 
  in 
  the 
  darkening 
  

   of 
  the 
  two 
  halves. 
  

  

  The 
  intensity 
  of 
  the 
  photograph 
  was 
  greatest 
  on 
  that 
  half 
  

   where 
  the 
  positive 
  rays 
  fell 
  upon 
  the 
  lead 
  surface, 
  and 
  there 
  

   only 
  on 
  that 
  part 
  where 
  the 
  intensity 
  of 
  the 
  positive 
  rays 
  

   would 
  be 
  a 
  maximum. 
  

  

  From 
  this 
  it 
  would 
  seem 
  that 
  the 
  energy 
  required 
  for 
  the 
  

   excitation 
  of 
  characteristic 
  X-rays 
  has 
  a 
  lower 
  limit, 
  just 
  as 
  

   Duane, 
  Hunt, 
  Hull, 
  Webster 
  and 
  others 
  have 
  observed 
  in 
  the 
  

   case 
  of 
  cathode 
  rays 
  in 
  a 
  Coolidge 
  tube. 
  

  

  For 
  cathode 
  rays 
  the 
  relation 
  e>hv 
  has 
  been 
  established, 
  

   where 
  e 
  is 
  the 
  minimum 
  energy 
  of 
  the 
  electron 
  necessary 
  for 
  

   exciting 
  the 
  K-series, 
  and 
  v 
  the 
  maximum 
  frequency 
  of 
  the 
  

   line 
  K/3 
  2 
  that 
  corresponds 
  to 
  this 
  series. 
  Supposing 
  this 
  

   relation 
  to 
  hold 
  also 
  for 
  the 
  excitation 
  of 
  characteristic 
  rays 
  

   by 
  positive 
  particles, 
  then 
  to 
  excite 
  the 
  K/3j 
  line 
  of 
  tin 
  

   (X 
  = 
  *432.10 
  -8 
  cm.) 
  the 
  voltage 
  required 
  would 
  have 
  to 
  

   exceed 
  57 
  KV. 
  In 
  the 
  described 
  experiments, 
  however, 
  the 
  

  

  