﻿784 
  Messrs. 
  A. 
  Campbell 
  and 
  T. 
  Smith 
  on 
  a 
  

  

  brass 
  ways 
  down 
  which 
  the 
  plate-holder 
  and 
  plate 
  slide, 
  and 
  

   an 
  adjustable 
  mechanism 
  for 
  setting 
  off 
  the 
  shutter. 
  A 
  side 
  

   elevation 
  is 
  shown 
  in 
  fig. 
  2, 
  and 
  a 
  view 
  of 
  the 
  back 
  in 
  fig. 
  3. 
  

   A 
  is 
  the 
  base 
  carrying 
  the 
  whole 
  apparatus, 
  and 
  into 
  it 
  is 
  

   fixed 
  a 
  vertical 
  pin 
  B. 
  C 
  is 
  a 
  piece 
  of 
  wood 
  in 
  which 
  is 
  

   bored 
  a 
  hole 
  through 
  which 
  B 
  passes 
  freely. 
  C 
  slides 
  between 
  

   two 
  portions 
  of 
  the 
  base 
  D 
  of 
  the 
  camera, 
  and 
  can 
  be 
  held 
  in 
  

   any 
  position 
  by 
  tightening 
  the 
  nuts 
  E. 
  The 
  front 
  of 
  the 
  

   camera, 
  with 
  the 
  shutter 
  and 
  lens 
  L2, 
  is 
  supported 
  by 
  struts 
  

   from 
  C, 
  and 
  set 
  so 
  that 
  the 
  axis 
  of 
  the 
  pin 
  B, 
  if 
  produced^ 
  

   would. 
  pass 
  through 
  the 
  centre 
  of 
  the 
  shutter. 
  By 
  this 
  means 
  

   the 
  whole 
  vibrating 
  beam 
  of 
  light 
  will 
  be 
  able 
  to 
  pass 
  through 
  

   the 
  shutter 
  though 
  the 
  whole 
  camera 
  is 
  rotated 
  through 
  a 
  

   few 
  degrees 
  about 
  B. 
  The 
  back 
  F 
  of 
  the 
  camera 
  is 
  attached 
  

   rigidly 
  to 
  the 
  base 
  D. 
  The 
  metal 
  plate 
  S 
  carries 
  a 
  narrow 
  

   horizontal 
  slit 
  T 
  extending 
  the 
  whole 
  width 
  of 
  the 
  plate, 
  and 
  

   a 
  window 
  P 
  is 
  provided 
  to 
  facilitate 
  focussing. 
  G, 
  G 
  are 
  

   the 
  grooved 
  metal 
  plates 
  down 
  which 
  the 
  plate-holder 
  H 
  

   slides. 
  This 
  holder 
  carries 
  a 
  projection 
  M 
  which 
  engages 
  

   with 
  the 
  upper 
  plate 
  of 
  a 
  ball 
  compressor 
  N 
  ; 
  when 
  the 
  

   holder 
  falls 
  this 
  upper 
  plate 
  is 
  forced 
  down 
  and 
  the 
  

   pneumatic 
  bulb 
  of 
  the 
  release 
  is 
  compressed. 
  On 
  the 
  return 
  

   upward 
  journey 
  of 
  the 
  holder 
  the 
  projection 
  passes 
  the 
  

   compressor 
  freely. 
  The 
  pneumatic 
  release 
  is 
  carried 
  on 
  the 
  

   moveable 
  part 
  L 
  and 
  may 
  be 
  fixed 
  at 
  any 
  desired 
  height 
  by 
  

   the 
  butterfly 
  screws 
  K, 
  K. 
  

  

  A 
  number 
  of 
  records 
  are 
  taken 
  side 
  by 
  side 
  on 
  one 
  plate. 
  

   This 
  is 
  secured 
  by 
  rotating 
  the 
  camera 
  about 
  B 
  between 
  each 
  

   exposure. 
  With 
  this 
  apparatus, 
  ten 
  or 
  more 
  records 
  can 
  be 
  

   made 
  side 
  by 
  side 
  on 
  a 
  5 
  x 
  4 
  in. 
  plate. 
  The 
  base 
  A 
  has 
  

   grooves 
  cut 
  in 
  it 
  to 
  serve 
  as 
  a 
  guide 
  to 
  the 
  amount 
  of 
  rota- 
  

   tion 
  to 
  be 
  given 
  to 
  the 
  camera. 
  In 
  making 
  tests 
  it 
  is 
  

   possible 
  to 
  make 
  on 
  one 
  plate 
  ten 
  records 
  of 
  the 
  various 
  

   speeds 
  given 
  by 
  a 
  shutter 
  in 
  one 
  minute. 
  

  

  A 
  view 
  of 
  the 
  front 
  of 
  the 
  camera 
  is 
  shown 
  in 
  fig. 
  9 
  

   (PI. 
  XXYII.). 
  

  

  (b) 
  Electrical 
  Apparatus. 
  

  

  The 
  vibration 
  galvanometer 
  used 
  is 
  of 
  the 
  moving 
  coil 
  

   type 
  (as 
  already 
  described 
  by 
  one 
  of 
  us*), 
  and 
  can 
  be 
  readily 
  

   tuned 
  to 
  frequencies 
  of 
  50 
  or 
  500 
  ~ 
  per 
  second. 
  As 
  its 
  

   mirror 
  is 
  of 
  fair 
  size 
  (50 
  to 
  80 
  sq. 
  mm.)^ 
  it 
  gives 
  with 
  a 
  

   Nernst 
  lamp 
  a 
  sufficient 
  amount 
  of 
  light 
  to 
  avoid 
  " 
  tailing 
  

   off 
  " 
  in 
  the 
  records 
  for 
  quick 
  exposures 
  with 
  rapid 
  plates. 
  

   The 
  source 
  of 
  current 
  may 
  be 
  an 
  ordinary 
  lighting 
  circuit, 
  

  

  * 
  Proc. 
  Phys. 
  Soc. 
  vol. 
  xx., 
  and 
  Phil. 
  Mag. 
  Oct. 
  1907. 
  

  

  