﻿192 
  Mr. 
  A. 
  Norman 
  Shaw 
  on 
  Interference 
  

  

  a 
  definite 
  beam 
  is 
  obtained 
  through 
  two 
  holes 
  in 
  thick 
  lead- 
  

   blocks 
  C 
  and 
  D, 
  the 
  opening 
  in 
  D 
  being 
  always 
  larger 
  than 
  

   that 
  in 
  C. 
  F 
  is 
  a 
  plate 
  of 
  lead 
  about 
  2 
  mm. 
  thick 
  which 
  

   was 
  used 
  to 
  cut 
  off 
  the 
  direct 
  ft 
  rays, 
  and 
  MM 
  represent 
  

   the 
  pole-pieces 
  by 
  means 
  of 
  which 
  the 
  secondary 
  " 
  emer- 
  

   gence" 
  ft 
  radiation 
  from 
  F 
  is 
  deflected 
  as 
  much 
  as 
  possible 
  

   from 
  the 
  hole 
  in 
  D. 
  H 
  represents 
  a 
  plate 
  of 
  mica 
  at 
  right 
  

   angles 
  to 
  the 
  plane 
  of 
  the 
  figure 
  and 
  arranged 
  at 
  almost 
  

   grazing 
  incidence 
  (or 
  actually 
  in 
  the 
  direction 
  of 
  the 
  beam).. 
  

   The 
  photographic 
  plate 
  is 
  represented 
  by 
  P, 
  in 
  contact 
  with 
  

   which 
  layers 
  of 
  paper 
  E, 
  or 
  paper 
  and 
  thin 
  metal 
  foil, 
  could; 
  

   be 
  placed. 
  The 
  photographic 
  part 
  was 
  enclosed 
  in 
  a 
  light- 
  

   proof 
  box, 
  A. 
  

  

  It 
  was 
  found 
  that 
  the 
  sensitiveness 
  to 
  7 
  rays 
  was 
  increased! 
  

   by 
  placing 
  the 
  thin 
  layers 
  of 
  matter 
  in 
  front 
  of, 
  and 
  almost 
  

   in 
  contact 
  with, 
  the 
  photographic 
  plate. 
  More 
  ft 
  particles 
  

   were 
  thus 
  liberated 
  in 
  the 
  path 
  of 
  any 
  beam 
  of 
  7 
  rays 
  near 
  

   the 
  surface 
  of 
  the 
  plate, 
  and 
  the 
  impression 
  was 
  intensified 
  

   without 
  an 
  appreciable 
  amount 
  of 
  additional 
  scattering. 
  

  

  In 
  the 
  experiments 
  recorded 
  below 
  the 
  exposures 
  varied 
  

   in 
  length 
  from 
  eight 
  hundred 
  to 
  fifteen 
  hundred 
  times 
  the 
  

   time 
  necessary 
  for 
  the 
  production 
  of 
  a 
  visible 
  spot 
  by 
  the 
  direct 
  

   rays. 
  Figures 
  2, 
  3, 
  and 
  4 
  (see 
  PLY.) 
  are 
  drawings 
  of 
  three 
  

   photographs 
  which 
  indicated 
  the 
  interference 
  phenomena 
  for 
  

   7 
  rays. 
  The 
  intensity 
  of 
  the 
  markings 
  has 
  been 
  consider- 
  

   ably 
  exaggerated 
  ; 
  in 
  the 
  actual 
  photograph 
  the 
  effects 
  were 
  

   very 
  faint, 
  although 
  quite 
  definite, 
  and 
  there 
  was 
  consider- 
  

   able 
  fogging 
  due 
  to 
  penetration 
  of 
  some 
  of 
  the 
  7 
  radiation 
  

   through 
  the 
  lead. 
  

  

  In 
  the 
  case 
  of 
  the 
  result 
  shown 
  in 
  fig. 
  2, 
  a 
  sheet 
  of 
  mica 
  

   about 
  3 
  cm. 
  square 
  and 
  0*3 
  mm. 
  thick 
  was 
  placed 
  at 
  almost 
  

   grazing 
  incidence. 
  The 
  hole 
  in 
  C 
  was 
  1 
  mm. 
  in 
  diameter,, 
  

   and 
  the 
  plate 
  P 
  was 
  about 
  5 
  cm. 
  from 
  the 
  mica. 
  The 
  

   sensitiveness 
  was 
  such 
  that 
  an 
  exposure 
  of 
  10 
  minutes 
  to 
  the 
  

   direct 
  beam 
  Avas 
  necessary 
  to 
  produce 
  an 
  impression 
  on 
  the 
  

   plate. 
  An 
  exposure 
  of 
  one 
  month 
  was 
  taken 
  for 
  this 
  photo- 
  

   graph, 
  and 
  any 
  rays 
  down 
  to 
  r™ 
  of 
  the 
  intensity 
  of 
  the 
  main 
  

  

  beam 
  were 
  thus 
  detectable. 
  

  

  In 
  the 
  case 
  of 
  fig. 
  3 
  a 
  sheet 
  of 
  mica 
  1 
  mm. 
  thick 
  was 
  used 
  

   at 
  complete 
  grazing 
  incidence. 
  The 
  distance 
  of 
  the 
  plate 
  

   from 
  the 
  mica 
  was 
  15 
  cm. 
  and 
  the 
  hole 
  in 
  C 
  was 
  2 
  mm. 
  in 
  

   diameter. 
  An 
  exposure 
  of 
  one 
  month 
  was 
  taken, 
  which 
  in 
  

   this 
  case 
  was 
  found 
  sufficient 
  to 
  detect 
  rays 
  down 
  to 
  about 
  

  

  -*- 
  the 
  intensity 
  of 
  the 
  direct 
  beam. 
  

   1500 
  J 
  

  

  The 
  result 
  in 
  fig. 
  4 
  was 
  produced 
  with 
  thicker 
  mica 
  (about 
  

  

  