﻿194 
  Interference 
  Phenomena 
  ivith 
  Gamma 
  Rays. 
  

  

  Since 
  it 
  is 
  thus 
  apparent 
  that 
  we 
  get 
  faint 
  effects 
  with 
  

   7 
  rays 
  in 
  those 
  directions 
  in 
  which 
  very 
  hard 
  X-rays 
  give 
  

   their 
  most 
  intense 
  reflexions, 
  and 
  no 
  perceptible 
  effect 
  in 
  the 
  

   direction 
  in 
  which 
  soft 
  X-rays 
  give 
  their 
  strongest 
  reflexion, 
  

   we 
  may 
  conclude 
  that 
  the 
  wave-length 
  of 
  the 
  soft 
  7 
  rays 
  

   from 
  radium 
  is 
  less 
  than 
  that 
  of 
  hard 
  X-rays 
  by 
  an 
  amount 
  

   not 
  differing 
  greatly 
  from 
  the 
  difference 
  in 
  wave-length 
  

   between 
  the 
  softest 
  and 
  the 
  hardest 
  X-rays 
  that 
  can 
  be- 
  

   produced 
  with 
  an 
  ordinary 
  bulb 
  and 
  coil.! 
  

  

  This 
  agrees 
  with 
  an 
  approximate 
  estimation 
  which 
  may 
  be 
  

   based 
  on 
  a 
  theory 
  of 
  Planck. 
  If 
  we 
  assume 
  that 
  the 
  con- 
  

   clusions 
  with 
  regard 
  to 
  the 
  relation 
  between 
  the 
  velocity 
  of 
  

   emission 
  of 
  electrons 
  from 
  surfaces 
  and 
  the 
  frequency 
  of 
  the 
  

   ultra-violet 
  light 
  causing 
  them 
  * 
  can 
  be 
  extended 
  to 
  the 
  case 
  

   of 
  X-rays 
  f 
  and 
  also 
  to 
  that 
  of 
  7 
  rays, 
  then 
  it 
  follows 
  from 
  

   Planck's 
  expression, 
  ^mv 
  2 
  =hn 
  (where 
  m 
  is 
  the 
  mass 
  of 
  the 
  

   electron, 
  v 
  its 
  maximum 
  velocity, 
  h 
  is 
  Planck's 
  unit 
  and 
  n 
  is 
  

   the 
  frequency 
  of 
  the 
  incident 
  radiation) 
  that 
  

  

  Jl-h 
  

   v 
  2 
  2_ 
  V 
  

  

  where 
  Vi 
  is 
  the 
  maximum 
  velocity 
  of 
  secondary 
  corpuscular 
  

   radiation 
  when 
  X-rays 
  fall 
  on 
  a 
  given 
  substance, 
  V 
  2 
  their 
  

   maximum 
  velocity 
  when 
  7 
  rays 
  fall 
  on 
  the 
  substance, 
  X 
  : 
  is 
  

   the 
  wave-length 
  of 
  the 
  X-rays 
  and 
  X 
  2 
  that 
  of 
  the 
  7 
  rays. 
  

   Experimental 
  data 
  for 
  the 
  velocities 
  of 
  secondary 
  corpuscular 
  

   radiations 
  under 
  similar 
  conditions 
  for 
  the 
  two 
  types 
  of 
  

   incident 
  radiation 
  are 
  not 
  available, 
  but 
  a 
  substitution 
  of 
  

   approximate 
  values 
  at 
  once 
  shows 
  that 
  ordinary 
  7 
  rays 
  from 
  

   radium 
  have 
  on 
  these 
  assumptions 
  wave-lengths 
  probably 
  

   about 
  ten, 
  or 
  at 
  the 
  most 
  a 
  hundred 
  times 
  smaller 
  than 
  the 
  

   wave-length 
  of 
  the 
  hardest 
  Rontgen 
  radiations. 
  

  

  The 
  experiments 
  are 
  being 
  continued, 
  and 
  it 
  is 
  hoped 
  that 
  

   further 
  data 
  will 
  be 
  obtained 
  with 
  other 
  crystals. 
  In 
  con- 
  

   clusion 
  the 
  writer 
  desires 
  to 
  thank 
  Professor 
  Sir 
  J. 
  J. 
  

   Thomson 
  for 
  his 
  kind 
  interest 
  in 
  the 
  work. 
  

  

  The 
  Cavendish 
  Laboratory, 
  

   May 
  17, 
  1913. 
  

  

  * 
  A. 
  L. 
  Hughes, 
  Phil. 
  Trans. 
  A. 
  vol. 
  ccxii. 
  p. 
  205 
  (1912). 
  

   t 
  Barkla, 
  Phil. 
  Mag, 
  vol. 
  xxv. 
  p. 
  298 
  (1913). 
  

  

  