﻿C14 
  Prof. 
  E. 
  W. 
  Wood 
  on 
  tlie 
  Absorption, 
  

  

  can 
  run 
  rapidly 
  through 
  a 
  series 
  of 
  observations 
  without 
  

   leaving 
  his 
  seat 
  at 
  the 
  eyepiece, 
  a 
  matter 
  of 
  great 
  importance 
  

   when 
  working 
  with 
  sodium 
  prisms. 
  The 
  plane 
  parallel 
  glass 
  

   plate 
  with 
  the 
  selenium 
  wedge 
  was 
  covered 
  by 
  a 
  piece 
  of 
  

   black 
  paper 
  furnished 
  with 
  two 
  apertures, 
  one 
  exposing 
  the 
  

   prism, 
  the 
  other 
  a 
  portion 
  of 
  the 
  clear 
  glass. 
  The 
  illuminator 
  

   was 
  set 
  for 
  red 
  light, 
  and 
  the 
  prism 
  placed 
  in 
  front 
  of 
  the 
  

   telescope 
  : 
  a 
  small 
  screen 
  enabled 
  either 
  aperture 
  to 
  be 
  

   covered. 
  The 
  image 
  of 
  the 
  slit 
  was 
  slightly 
  broadened 
  by 
  

   diffraction, 
  but 
  not 
  enough 
  to 
  interfere 
  with 
  accurate 
  setting 
  

   of 
  the 
  cross-hairs. 
  On 
  changing 
  from 
  clear 
  glass 
  to 
  the 
  

   selenium 
  wedge, 
  a 
  displacement 
  equal 
  to 
  three 
  times 
  the 
  

   width 
  of 
  the 
  band 
  was 
  observed. 
  If 
  both 
  apertures 
  were 
  

   opened 
  simultaneously, 
  both 
  the 
  direct 
  and 
  displaced 
  images 
  

   appeared 
  in 
  the 
  field, 
  the 
  images 
  being 
  furrowed, 
  however, 
  

   by 
  the 
  fine 
  interference 
  minima 
  due 
  to 
  two 
  apertures. 
  

   Though 
  it 
  is 
  quite 
  possible 
  that 
  these 
  minima 
  would 
  furnish 
  

   the 
  means 
  of 
  a 
  more 
  accurate 
  setting 
  of 
  the 
  cross-hairs 
  on 
  

   the 
  centre 
  of 
  the 
  band, 
  they 
  were 
  not 
  made 
  use 
  of 
  in 
  the 
  

   present 
  case, 
  the 
  apertures 
  being 
  uncovered 
  in 
  succession. 
  

   This 
  seemed 
  advisable 
  in 
  view 
  of 
  the 
  uncertainty 
  regarding 
  

   the 
  effect 
  of 
  the 
  change 
  of 
  wave-length 
  on 
  their 
  location, 
  for 
  

   it 
  must 
  be 
  remembered 
  that 
  when 
  working 
  with 
  the 
  strongly 
  

   absorbed 
  blue 
  light, 
  the 
  amplitude 
  falls 
  off 
  rapidly 
  across 
  the 
  

   selenium-covered 
  aperture, 
  its 
  effective 
  width 
  contracting 
  

   with 
  decrease 
  of 
  wave-length. 
  The 
  angular 
  deviation 
  for 
  the 
  

   red 
  was 
  measured, 
  and 
  found 
  to 
  agree 
  fnirly 
  well 
  with 
  the 
  

   observations 
  made 
  with 
  prisms 
  of 
  larger 
  angle. 
  The 
  selenium 
  

   aperture 
  being 
  exposed, 
  the 
  cross-hair 
  was 
  set 
  on 
  the 
  dis- 
  

   placed 
  image 
  and 
  the 
  wave-length 
  decreased. 
  The 
  image 
  

   immediately 
  moved 
  off 
  the 
  hair, 
  which 
  was 
  made 
  to 
  follow 
  it, 
  

   and 
  a 
  continued 
  increase 
  in 
  the 
  deviation 
  was 
  noted 
  as 
  far 
  

   down 
  the 
  spectrum 
  as 
  wave-length 
  '00055, 
  beyond 
  which 
  

   point 
  the 
  image 
  became 
  too 
  faint 
  to 
  observe. 
  

  

  I 
  was 
  disappointed 
  not 
  to 
  be 
  able 
  to 
  follow 
  the 
  curve 
  to 
  its 
  

   turning 
  point 
  in 
  the 
  blue, 
  and 
  accordingly 
  tried 
  another 
  

   method. 
  

  

  The 
  wedge-shaped 
  film 
  was 
  observed 
  by 
  the 
  light 
  of 
  

   lithium, 
  sodium, 
  and 
  thallium 
  flames 
  in 
  succession, 
  and 
  the 
  

   positions 
  of 
  the 
  dark 
  bands 
  recorded 
  by 
  needle 
  scratches. 
  

   The 
  eighth 
  band 
  for 
  lithium 
  light 
  coincided 
  with 
  the 
  tenth 
  

   band 
  for 
  sodium 
  light, 
  which 
  agrees 
  well 
  with 
  the 
  observed 
  

   values 
  of 
  the 
  refractive 
  index. 
  With 
  thallium 
  light 
  only 
  

   three 
  bands 
  next 
  to 
  the 
  thin 
  edge 
  could 
  be 
  seen, 
  owing 
  to 
  the 
  

   strong 
  absorption, 
  but 
  their 
  position 
  indicated 
  an 
  increase 
  of 
  

   refractive 
  index, 
  the 
  third 
  thallium 
  dark 
  band 
  coinciding 
  with 
  

  

  