﻿786 
  Method 
  of 
  Testing 
  Photographic 
  Shutters, 
  

  

  whole 
  length 
  of 
  the 
  plate, 
  and 
  this 
  curve 
  serves 
  now 
  simply 
  

   as 
  a 
  time 
  record. 
  A 
  line 
  source 
  of 
  light 
  B 
  is 
  focussed 
  by 
  a 
  

   lens 
  L3 
  on 
  to 
  a 
  narrow 
  horizontal 
  slit 
  b 
  h' 
  placed 
  in 
  a 
  diametral 
  

   plane 
  of 
  the 
  shutter 
  as 
  close 
  as 
  possible 
  to 
  the 
  shutter-leaves. 
  

   An 
  image 
  /3 
  /3' 
  of 
  this 
  slit 
  is 
  formed 
  by 
  the 
  concave 
  mirror 
  

   C 
  on 
  the 
  photographic 
  plate 
  by 
  the 
  side 
  of 
  the 
  vibrating 
  

   beam 
  of 
  light. 
  As 
  the 
  plate 
  falls 
  the 
  shutter 
  is 
  opened 
  as 
  

   before, 
  and 
  a 
  record 
  is 
  obtained 
  giving 
  at 
  every 
  instant 
  of 
  

   time 
  the 
  length 
  of 
  slit 
  through 
  which 
  light 
  was 
  passed 
  by 
  

   the 
  shutter 
  leaves 
  (fig. 
  5). 
  Measurements 
  are 
  then 
  taken 
  of 
  

  

  Fig. 
  5. 
  

  

  the 
  area 
  of 
  shutter 
  aperture 
  corresponding 
  to 
  a 
  number 
  of 
  

   lengths 
  of 
  the 
  slit 
  opening. 
  Fig. 
  (5 
  shows 
  some 
  stages 
  in 
  

   the 
  opening 
  of 
  a 
  particular 
  type 
  of 
  shutter, 
  the 
  white 
  line 
  

   representing 
  the 
  position 
  of 
  the 
  slit. 
  From 
  a 
  pair 
  of 
  records 
  

   such 
  as 
  shown 
  in 
  figs. 
  5 
  and 
  6 
  a 
  curve 
  is 
  drawn 
  showing 
  at 
  

   every 
  instant 
  of 
  the 
  exposure 
  the 
  area 
  of 
  the 
  shutter 
  aperture 
  

   through 
  which 
  light 
  can 
  pass 
  to 
  the 
  plate. 
  It 
  is 
  now 
  evidently 
  

   a 
  simple 
  matter 
  to 
  find 
  the 
  amount 
  of 
  light 
  cut 
  ofE 
  owing 
  to 
  

   the 
  finite 
  length 
  of 
  time 
  taken 
  by 
  the 
  shutter 
  leaves 
  to 
  open 
  

   and 
  close, 
  and 
  hence 
  to 
  calculate 
  the 
  efficiency. 
  

  

  