﻿840 
  Prof. 
  Skinner 
  and 
  Mr. 
  Tool 
  on 
  the 
  Optical 
  

  

  For 
  investigating 
  the 
  reflected 
  light 
  the 
  rays 
  from 
  the 
  

   spectral 
  system 
  were 
  brought 
  in 
  from 
  the 
  side 
  and 
  turned 
  by 
  

   means 
  of 
  a 
  total-reflecting 
  prism 
  through 
  a 
  polarizer 
  of 
  much 
  

   smaller 
  aperture 
  than 
  that 
  used 
  in 
  testing 
  the 
  transmitted 
  

   light. 
  The 
  rays 
  were 
  reflected 
  from 
  F 
  back 
  over 
  the 
  

   polarizer, 
  entering 
  the 
  analysing 
  system 
  so 
  placed 
  as 
  to 
  

   receive 
  them. 
  The 
  angle 
  of 
  incidence 
  on 
  the 
  film 
  with 
  this 
  

   arrangement 
  was 
  not 
  greater 
  than 
  two 
  degrees. 
  

  

  Ellipticity. 
  — 
  For 
  measuring 
  this 
  a 
  Brace 
  half-shade 
  elliptic 
  

   polarizer 
  and 
  compensator 
  was 
  used. 
  To 
  convert 
  the 
  above 
  

   rotation 
  system 
  into 
  one 
  Eor 
  measuring 
  ellipticity, 
  the 
  half 
  

   nieol 
  Aj 
  was 
  removed 
  and 
  A 
  iixed 
  at 
  extinction. 
  The 
  mica 
  

   half-shade 
  elliptic 
  polarizer 
  was 
  placed 
  in 
  the 
  position 
  

   originally 
  occupied 
  by 
  A 
  } 
  , 
  covering 
  half 
  the 
  field 
  and 
  lixed 
  

   at 
  maximum 
  illumination. 
  The 
  compensator, 
  a 
  thicker 
  sheet 
  

   of 
  mica, 
  was 
  then 
  mounted 
  (coveringthe 
  whole 
  held) 
  in 
  the 
  

   graduated 
  circle. 
  Settings 
  were 
  made 
  by 
  rotating 
  this 
  com- 
  

   pensator 
  until 
  a 
  match 
  was 
  established. 
  To 
  eliminate 
  the 
  

   effect 
  arising 
  from 
  mere 
  rotation 
  of 
  the 
  plane 
  by 
  the 
  film, 
  

   the 
  polarizer 
  P 
  was 
  arranged 
  to 
  he 
  conveniently 
  rotated 
  

   a 
  sufficient 
  amount 
  to 
  compensate 
  for 
  this. 
  This 
  was 
  deter- 
  

   mined 
  with 
  suitable 
  accuracy 
  by 
  rotating 
  to 
  minimum 
  

   intensity 
  o£ 
  field. 
  The 
  half-shade 
  was 
  of 
  the 
  order 
  

   •009 
  Xd= 
  •>'- 
  degrees. 
  The 
  compensator, 
  carefully 
  measured 
  

   according 
  to 
  Stoke-" 
  method 
  ". 
  was 
  found 
  to 
  he 
  of 
  the 
  order 
  

   G 
  0, 
  5 
  for 
  a 
  wave-length 
  of 
  560 
  fifi. 
  The 
  order 
  of 
  a 
  similar 
  

   piece 
  of 
  mica, 
  a 
  quarter- 
  wave 
  plate 
  for 
  the 
  same 
  colour, 
  was 
  

   carefully 
  measured 
  Eor 
  Hie 
  various 
  spectral 
  colours, 
  and 
  these 
  

   value- 
  used 
  in 
  determining 
  the 
  order 
  of 
  the 
  compensator 
  for 
  

   the 
  various 
  wave-lengths 
  studied. 
  The 
  ellipticity 
  of 
  the 
  

   Light 
  was 
  calculated 
  from 
  the 
  formula 
  

  

  E 
  = 
  tan 
  i/j 
  [sin 
  2(<£— 
  <j> 
  ) 
  + 
  sin 
  20 
  o 
  ] 
  , 
  

  

  in 
  which 
  E 
  = 
  ratio 
  of 
  minor 
  to 
  major 
  axis 
  of 
  the 
  elliptic 
  

   vibration; 
  2vj 
  = 
  order 
  of 
  the 
  compensator: 
  cp 
  = 
  angle 
  

   through 
  which 
  the 
  compensator 
  is 
  rotated 
  to 
  change.' 
  from 
  a 
  

   match 
  on 
  plane-polarized 
  light 
  to 
  the 
  nearest 
  position 
  of 
  no 
  

   effect 
  on 
  this 
  light 
  : 
  and 
  $ 
  = 
  the 
  angular 
  displacement 
  of 
  

   the 
  compensator 
  from 
  its 
  position 
  of 
  match 
  on 
  plane-polarized 
  

   light 
  to 
  a 
  match 
  on 
  the 
  light 
  having 
  an 
  ellipticity 
  E. 
  If 
  the 
  

   compensator 
  and 
  half 
  -shade 
  are 
  of 
  the 
  same 
  material, 
  <p 
  does 
  

   not 
  change 
  with 
  the 
  colour. 
  This 
  formula 
  is 
  accurate 
  for 
  

   thin 
  compensators. 
  All 
  measurements 
  of 
  both 
  rotation 
  and 
  

  

  * 
  Phil. 
  Mag. 
  [4] 
  ii. 
  p. 
  4i0. 
  

  

  