﻿234 
  Prof. 
  H. 
  A. 
  Bumstead 
  on 
  the 
  

  

  as 
  electronic 
  velocities 
  go, 
  not 
  very 
  different, 
  in 
  fact, 
  from 
  

  

  those 
  which 
  are 
  observed 
  in 
  the 
  photo-electric 
  effect. 
  The 
  

  

  estimates 
  by 
  different 
  observers 
  of 
  the 
  maximum 
  velocity 
  of 
  

  

  the 
  rays 
  have 
  been 
  discordant 
  ; 
  they 
  have 
  varied 
  from 
  zero 
  

  

  cm 
  

   to 
  3" 
  9 
  x 
  10 
  8 
  — 
  "j 
  the 
  latter 
  corresponding 
  to 
  a 
  fall 
  of 
  potential 
  

  

  of 
  about 
  42 
  volts. 
  

  

  Last 
  year 
  it 
  was 
  shown 
  by 
  Dr. 
  McGougan 
  and 
  the 
  present 
  

   writer 
  * 
  that 
  electrons 
  were 
  present 
  in 
  a 
  beam 
  of 
  S-rays 
  which 
  

   had 
  much 
  greater 
  velocities 
  than 
  any 
  of 
  the 
  above 
  estimates; 
  

   an 
  opposing 
  potential 
  difference 
  of 
  1700 
  volts 
  was 
  not 
  

   sufficient 
  to 
  stop 
  all 
  of 
  them, 
  and 
  very 
  marked 
  effects 
  were 
  

   produced 
  by 
  electrons 
  having 
  velocities 
  corresponding 
  to 
  

   several 
  hundred 
  volts. 
  The 
  present 
  paper 
  contains 
  the 
  

   results 
  of 
  some 
  further 
  experiments 
  upon 
  these 
  swifter 
  

   rays. 
  It 
  will 
  be 
  seen 
  that 
  there 
  is 
  no 
  gap 
  between 
  the 
  swifter 
  

   and 
  the 
  slower 
  electrons, 
  but 
  that 
  all 
  intermediate 
  speeds 
  

   are 
  found 
  between 
  the 
  highest 
  and 
  the 
  lowest. 
  It 
  seems 
  

   reasonable, 
  therefore, 
  to 
  include 
  under 
  the 
  name 
  " 
  delta 
  rays 
  ,; 
  

   all 
  the 
  electrons 
  which 
  are 
  projected 
  from 
  the 
  atoms 
  of 
  

   bodies 
  by 
  the 
  direct 
  action 
  of 
  the 
  a- 
  rays, 
  the 
  recently 
  

   discovered 
  swift 
  ones 
  as 
  well 
  as 
  the 
  slower 
  ones 
  previously 
  

   known 
  ; 
  this 
  nomenclature 
  will 
  be 
  adopted 
  in 
  the 
  present 
  

   paper. 
  It 
  is 
  however 
  to 
  be 
  observed 
  that 
  some, 
  at 
  least, 
  of 
  

   the 
  slower 
  electrons 
  (under 
  10 
  volts) 
  must 
  be 
  caused, 
  not 
  

   directly 
  by 
  the 
  a-rays, 
  but 
  by 
  the 
  swifter 
  S-rays. 
  When 
  

   there 
  is 
  occasion 
  to 
  refer 
  to 
  these 
  they 
  will 
  be 
  called 
  tertiary 
  

   rays. 
  It 
  is 
  impossible 
  at 
  present 
  to 
  make 
  a 
  sharp 
  distinction 
  

   between 
  the 
  slower 
  S-rays 
  and 
  the 
  tertiary 
  rays 
  which 
  come 
  

   from 
  the 
  source 
  of 
  S-rays, 
  or 
  to 
  determine 
  the 
  proportion 
  of 
  

   each 
  ; 
  so 
  that 
  in 
  all 
  numerical 
  estimates 
  the 
  tertiary 
  rays 
  

   must 
  be 
  included 
  among 
  the 
  S-rays. 
  

  

  §2. 
  

   Tertiary 
  electrons 
  are 
  emitted 
  also 
  by 
  any 
  body 
  on 
  which 
  

   S-rays 
  fall, 
  and 
  in 
  numbers 
  considerably 
  in 
  excess 
  of 
  the 
  

   swifter 
  ^-electrons 
  which 
  cau^e 
  them. 
  When 
  an 
  electric 
  

   field 
  is 
  used 
  to 
  hold 
  back 
  the 
  slower 
  ^-electrons, 
  this 
  is 
  the 
  

   most 
  conspicuous 
  effect 
  of 
  the 
  remaining 
  swift 
  rays; 
  for 
  

   the 
  field 
  which 
  opposes 
  the 
  &-rays 
  assists 
  the 
  tertiary 
  electrons 
  

   to 
  escape, 
  and 
  their 
  larger 
  number 
  magnifies 
  the 
  experimental 
  

   effect. 
  The 
  unsuspected 
  presence 
  of 
  this 
  phenomenon 
  has 
  

   undoubtedly 
  had 
  an 
  influence 
  upon 
  the 
  results 
  obtained 
  in 
  

  

  * 
  Bumstead 
  and 
  McGougan, 
  Phil. 
  Mag. 
  xxiv. 
  p. 
  474 
  (1912) 
  ; 
  Am. 
  

   Journ. 
  Sci. 
  xxxiv. 
  p. 
  309 
  (1911). 
  

  

  