﻿782 
  Messrs. 
  A. 
  Campbell 
  and 
  T. 
  Smith 
  on 
  a 
  

  

  In 
  conclusion 
  we 
  have 
  to 
  admit 
  that 
  the 
  various 
  phenomena 
  

   observed 
  have 
  not 
  been 
  reduced 
  to 
  any 
  simple 
  law, 
  but 
  

   subsequent 
  researches 
  will 
  perhaps 
  throw 
  further 
  light 
  on 
  

   the 
  matter 
  and 
  clear 
  up 
  the 
  present 
  obscurities. 
  

  

  Finally, 
  I 
  have 
  to 
  express 
  my 
  thanks 
  to 
  Prof. 
  E. 
  H. 
  Barton, 
  

   at 
  whose 
  suggestion 
  the 
  above 
  experiments 
  were 
  undertaken. 
  

   University 
  College, 
  Nottingham, 
  

   April 
  27, 
  1909. 
  

  

  LXXXI. 
  On 
  a 
  Method 
  of 
  Testing 
  Photographic 
  Shutters. 
  

   By 
  A. 
  Campbell, 
  B,A,, 
  and 
  T. 
  Smith, 
  B.A.^ 
  

  

  (From 
  the 
  National 
  Physical 
  Laboratory 
  ) 
  

   [Plate 
  XXVII.] 
  

  

  I. 
  Principle 
  of 
  the 
  Method, 
  

  

  THE 
  importance 
  o£ 
  having 
  accurate 
  methods 
  of 
  testing- 
  

   photographic 
  shutters 
  is 
  well 
  shown 
  by 
  the 
  number 
  

   and 
  variety 
  of 
  the 
  methods 
  that 
  have 
  from 
  time 
  to 
  time 
  been 
  

   proposed 
  and 
  used 
  for 
  this 
  purpose. 
  The 
  method 
  described 
  

   herein 
  is 
  intended 
  to 
  provide 
  a 
  rapid 
  test, 
  while 
  ensuring 
  the 
  

   maximum 
  of 
  accuracy. 
  The 
  total 
  duration 
  of 
  exposure 
  for 
  

   low 
  speeds 
  (say 
  longer 
  than 
  '1 
  sec.) 
  is 
  determined 
  with 
  an 
  

   error 
  not 
  exceeding 
  '005 
  sec, 
  and 
  for 
  high 
  speeds 
  not 
  

   exceeding 
  '0005 
  sec. 
  For 
  exceptionally 
  fast 
  exposures, 
  

   measurements 
  can 
  probably 
  be 
  made 
  to 
  within 
  -0001 
  sec. 
  

   Attention 
  has 
  been 
  specially 
  directed 
  to 
  the 
  elimination 
  of 
  

   all 
  calculations, 
  and 
  a 
  permanent 
  record 
  is 
  obtained 
  of 
  each 
  

   test. 
  

  

  The 
  essential 
  principle 
  of 
  the 
  method 
  consists 
  in 
  photo- 
  

   graphing 
  on 
  a 
  moving 
  plate 
  a 
  narrow 
  beam 
  of 
  light 
  reflected 
  

   from 
  a 
  mirror 
  which 
  is 
  forced 
  to 
  make 
  angular 
  vibrations 
  

   of 
  known 
  frequency 
  about 
  an 
  axis 
  parallel 
  to 
  the 
  direction 
  

   of 
  motion 
  of 
  the 
  plate. 
  There 
  is 
  thus 
  obtained 
  on 
  the 
  plate 
  

   a 
  sine 
  curve, 
  and 
  if 
  the 
  light 
  on 
  its 
  way 
  to 
  the 
  plate 
  passes 
  

   through 
  the 
  shutter, 
  the 
  length 
  of 
  the 
  exposure 
  can 
  be 
  found 
  

   by 
  counting 
  the 
  number 
  of 
  vibrations 
  recorded 
  on 
  the 
  plate. 
  

   The 
  method 
  has 
  been 
  rendered 
  practicable 
  by 
  the 
  fact 
  that 
  a 
  

   Vibration 
  Galvanometer 
  affords 
  a 
  suitable 
  means 
  of 
  imparting 
  

   the 
  necessary 
  oscillations 
  to 
  the 
  mirror; 
  with 
  this 
  instrument 
  

   both 
  the 
  amplitude 
  of 
  the 
  vibration 
  and 
  the 
  frequency 
  are 
  

   under 
  control, 
  and 
  these 
  are 
  points 
  of 
  great 
  importance. 
  

   Owing 
  to 
  these 
  conditions 
  not 
  being 
  satisfied, 
  and 
  for 
  other 
  

   reasons, 
  a 
  tuning 
  fork 
  is 
  not 
  nearly 
  so 
  suitable 
  for 
  imparting 
  

   vibrations 
  directly 
  to 
  the 
  mirror. 
  When 
  it 
  is 
  desired 
  to 
  find 
  

  

  * 
  Communicated 
  by 
  the 
  Physical 
  Society 
  : 
  read 
  May 
  14, 
  1909. 
  

  

  