﻿Dispersion, 
  and 
  Surface- 
  Colour 
  of 
  Selenium. 
  

  

  611 
  

  

  images, 
  the 
  lens 
  M 
  is 
  arranged 
  so 
  as 
  to 
  focus 
  the 
  light 
  on 
  the 
  

   perforated 
  screen 
  U 
  *. 
  

  

  No 
  other 
  lens 
  is 
  used, 
  the 
  light 
  falling 
  directly 
  on 
  the 
  

   photographic 
  plate 
  after 
  passage 
  through 
  the 
  pin-hole. 
  

  

  The 
  camera 
  is 
  simply 
  a 
  long 
  narrow 
  box, 
  the 
  plate 
  being 
  

   carried 
  on 
  a 
  slide, 
  so 
  that 
  eight 
  exposures 
  can 
  be 
  made 
  one 
  

   after 
  the 
  other 
  on 
  a 
  single 
  plate, 
  measuring 
  12x2 
  cms. 
  

  

  A 
  small 
  mirror 
  N, 
  hinged 
  in 
  front 
  of 
  the 
  slit, 
  enables 
  

   us 
  to 
  divert 
  the 
  light 
  from 
  the 
  interferometer 
  and 
  throw 
  it 
  

   into 
  the 
  grating 
  spectrometer 
  S, 
  by 
  means 
  of 
  which 
  the 
  

   wave-length 
  can 
  be 
  at 
  once 
  determined. 
  By 
  means 
  of 
  this 
  

   arrangement 
  it 
  was 
  possible 
  to 
  work 
  with 
  considerable 
  rapidity. 
  

   After 
  each 
  exposure, 
  the 
  small 
  mirror 
  was 
  turned 
  in 
  front 
  of 
  

   the 
  slit, 
  and 
  the 
  cross-hairs 
  of 
  the 
  spectrometer 
  set 
  on 
  the 
  

   diffracted 
  image. 
  The 
  prism 
  was 
  then 
  turned 
  through 
  a 
  

   small 
  angle 
  and 
  a 
  second 
  exposure 
  given, 
  the 
  circle 
  of 
  the 
  

   spectrometer 
  being 
  read 
  while 
  the 
  exposure 
  was 
  in 
  progress. 
  

   The 
  longest 
  exposures 
  in 
  the 
  red 
  were 
  only 
  five 
  or 
  six 
  minutes 
  

   in 
  duration. 
  

  

  After 
  making 
  a 
  number 
  of 
  measurements 
  in 
  different 
  parts 
  

   of 
  the 
  spectrum 
  by 
  means 
  of 
  films 
  of 
  different 
  thicknesses, 
  it 
  

   occurred 
  to 
  me 
  that 
  if 
  a 
  wedge-shaped 
  film 
  were 
  employed, 
  

   curved 
  fringes 
  would 
  be 
  obtained 
  which 
  would 
  allow 
  the 
  dis- 
  

   placement 
  for 
  any 
  wave-length 
  to 
  be 
  measured 
  for 
  the 
  maximum 
  

   thickness 
  capable 
  of 
  transmitting 
  the 
  light. 
  Films 
  of 
  this 
  

   nature 
  were 
  obtained 
  by 
  shielding 
  the 
  plate 
  during 
  the 
  de- 
  

   position 
  with 
  a 
  strip 
  of 
  mica 
  mounted 
  a 
  centimetre 
  or 
  so 
  

   above 
  its 
  surface, 
  as 
  shown 
  in 
  fig. 
  1. 
  The 
  drift 
  of 
  the 
  

   selenium 
  under 
  this 
  shield 
  was 
  very 
  regular, 
  and 
  excellent 
  

   wedges 
  were 
  obtained 
  which 
  showed 
  straight 
  interference- 
  

   bands 
  when 
  viewed 
  in 
  reflected 
  light. 
  A 
  series 
  of 
  photographs 
  

  

  obtained 
  with 
  one 
  of 
  these 
  prismatic 
  deposits 
  is 
  reproduced 
  

   in 
  fig. 
  3. 
  It 
  will 
  be 
  seen 
  from 
  the 
  nature 
  of 
  the 
  displace- 
  

   ment 
  of 
  the 
  fringes 
  that 
  the 
  film 
  is 
  of 
  nearly 
  uniform 
  thickness 
  

   for 
  a 
  short 
  distance, 
  and 
  then 
  becomes 
  wedge-shaped, 
  the 
  

   fringes 
  plunging 
  down 
  suddenly. 
  

  

  Another 
  advantage 
  of 
  the 
  wedge-shaped 
  films 
  is 
  that 
  the 
  

  

  * 
  By 
  an 
  error 
  of 
  the 
  engraver 
  the 
  aperture 
  in 
  U 
  is 
  shown 
  much 
  too 
  

   large. 
  Only 
  one 
  of 
  the 
  two 
  convergent 
  beams 
  passes 
  through 
  it. 
  

  

  