﻿420 
  Scientific 
  Intelligence. 
  

  

  mined 
  by 
  McLennan 
  but, 
  at 
  present, 
  the 
  evidence 
  favors 
  the 
  

   strong 
  mercury 
  line 
  at 
  A, 
  2536*7. 
  In 
  none 
  of 
  the 
  experiments 
  

   with 
  the 
  coaxial 
  lamp 
  were 
  the 
  fluorescence 
  lines 
  discovered 
  by 
  

   Wood 
  detected. 
  The 
  complete 
  absence 
  of 
  these 
  series 
  lines 
  may 
  

   be 
  explained 
  as 
  due 
  either 
  to 
  the 
  absorption 
  in 
  the 
  long 
  column 
  

   of 
  iodine 
  vapor 
  with 
  the 
  temperature 
  gradient 
  down 
  towards 
  the 
  

   quartz 
  window, 
  or 
  to 
  a 
  different 
  constitution 
  of 
  iodine 
  vapor 
  at 
  

   the 
  relatively 
  high 
  temperature 
  of 
  the 
  coaxial 
  lamp 
  as 
  compared 
  

   with 
  the 
  ordinary 
  room 
  temperature 
  in 
  Wood's 
  investigations. 
  

   At 
  present, 
  there 
  is 
  experimental 
  evidence 
  in 
  favor 
  of 
  both 
  

   hypotheses. 
  — 
  Proc. 
  Roy. 
  Soc, 
  vol. 
  lxxxviii 
  (A), 
  p. 
  289. 
  

  

  II. 
  s. 
  u. 
  

  

  7. 
  Interference 
  of 
  Gamma 
  Rays. 
  — 
  Since 
  X-ra}^s 
  and 
  y-rays 
  

   seem 
  to 
  belong 
  to 
  the 
  same 
  type 
  of 
  radiation 
  and 
  since 
  it 
  has 
  

   been 
  shown 
  experimentally, 
  by 
  Friedrich, 
  Knipping, 
  Bragg, 
  and 
  

   others, 
  that 
  the 
  atomic 
  or 
  molecular 
  structure 
  of 
  certain 
  crystals 
  

   acts 
  like 
  a 
  space 
  grating 
  with 
  respect 
  to 
  X-rays, 
  it 
  is 
  natural 
  to 
  

   expect 
  a 
  similar 
  phenomenon 
  in 
  the 
  case 
  of 
  y-rays. 
  This 
  question 
  

   has 
  been 
  successfully 
  investigated 
  by 
  A. 
  Norman 
  Shaw. 
  A 
  

   general 
  idea 
  of 
  the 
  simple 
  apparatus 
  used 
  may 
  be 
  obtained 
  from 
  

   the 
  following 
  statements. 
  The 
  bulb 
  which 
  contained 
  about 
  15 
  

   mg. 
  of 
  radium 
  bromide 
  was 
  placed 
  at 
  the 
  bottom 
  of 
  a 
  deep 
  hole 
  

   bored 
  in 
  a 
  thick 
  block 
  of 
  lead. 
  The 
  primary 
  y-rays 
  passed 
  

   through 
  a 
  sheet 
  of 
  lead 
  2 
  mm 
  thick 
  which 
  covered 
  the 
  mouth 
  of 
  

   the 
  hole 
  and 
  which 
  prevented 
  the 
  emergence 
  of 
  the 
  direct 
  /3-rays. 
  

   The 
  primary 
  beam 
  continued 
  through 
  a 
  long 
  collimating 
  hole 
  in 
  a 
  

   second 
  block 
  of 
  lead. 
  A 
  magnetic 
  field 
  was 
  maintained 
  across 
  

   the 
  outer 
  end 
  of 
  this 
  hole 
  in 
  order 
  to 
  deviate 
  the 
  secondary, 
  

   " 
  emergence 
  " 
  /?-rays 
  from 
  the 
  axis 
  of 
  the 
  beam 
  of 
  primary 
  y-rays. 
  

   Beyond 
  the 
  electromagnet 
  the 
  latter 
  rays 
  passed 
  through 
  a 
  hole 
  

   in 
  a 
  lead 
  screen 
  and 
  then 
  struck 
  a 
  sheet 
  of 
  mica 
  at 
  approximately 
  

   grazing 
  incidence. 
  The 
  hole 
  in 
  this 
  screen 
  was 
  slightly 
  larger 
  

   than 
  the 
  one 
  in 
  the 
  collimating 
  block. 
  After 
  leaving 
  the 
  mica 
  

   the 
  y-rays 
  eventually 
  struck 
  a 
  photographic 
  plate 
  whose 
  plane 
  

   was 
  normal 
  to 
  the 
  impinging 
  beam. 
  " 
  It 
  was 
  found 
  that 
  the 
  

   sensitiveness 
  to 
  y-rays 
  was 
  increased 
  by 
  placing 
  thin 
  layers 
  of 
  

   matter" 
  [paper, 
  etc.] 
  ".in 
  front 
  of, 
  and 
  almost 
  in 
  contact 
  with, 
  

   the 
  photographic 
  plate." 
  " 
  More 
  /3-particles 
  were 
  thus 
  liberated 
  

   in 
  the 
  path 
  of 
  any 
  beam 
  of 
  y-rays 
  near 
  the 
  surface 
  of 
  the 
  plate, 
  

   and 
  the 
  impression 
  was 
  intensified 
  without 
  an 
  appreciable 
  amount 
  

   of 
  additional 
  scattering." 
  

  

  In 
  a 
  typical 
  experiment 
  the 
  sheet 
  of 
  mica 
  was 
  l 
  mm 
  thick 
  and 
  

   15 
  cm 
  distant 
  from 
  the 
  photographic 
  plate. 
  The 
  collimating 
  hole 
  

   had 
  a 
  diameter 
  of 
  2 
  mm 
  . 
  The 
  time 
  of 
  exposure 
  was 
  one 
  month 
  

   and 
  hence 
  rays 
  of 
  all 
  intensities 
  clown 
  to 
  1/1500 
  of 
  the 
  intensity 
  

   of 
  the 
  primary 
  beam 
  could 
  have 
  been 
  detected. 
  In 
  general, 
  the 
  

   central 
  image 
  could 
  be 
  clearly 
  recorded 
  in 
  40min. 
  In 
  addition 
  to 
  

   this 
  circular 
  spot 
  all 
  the 
  negatives 
  show 
  a 
  series 
  of 
  roughly 
  paral- 
  

   lel, 
  rectilinear 
  images 
  whose 
  common 
  normal 
  lies 
  in 
  a 
  direction 
  

   oblique 
  to 
  the 
  intersection 
  with 
  the 
  photographic 
  plate 
  of 
  a 
  plane 
  

  

  