﻿Glass 
  at 
  the 
  Polarizing 
  Angle. 
  447 
  

  

  value 
  of 
  a 
  would 
  coincide 
  with 
  the 
  old 
  one 
  were 
  there 
  no 
  

   ellipticity 
  ; 
  and 
  the 
  difference 
  of 
  values 
  measures 
  « 
  upon 
  a 
  

   doubled 
  scale. 
  If 
  ot! 
  be 
  the 
  second 
  value, 
  so 
  that 
  the 
  dif- 
  

   ference 
  is 
  a' 
  — 
  a, 
  then 
  

  

  & 
  = 
  tanj- 
  (a' 
  — 
  a), 
  

  

  or 
  with 
  sufficient 
  approximation 
  in 
  most 
  cases 
  

  

  The 
  sign 
  of 
  ol' 
  — 
  a 
  is 
  reversed 
  when 
  the 
  mica 
  is 
  rotated 
  

   throuo-h 
  a 
  right 
  ano-le, 
  and 
  the 
  absolute 
  sign 
  of 
  h 
  must 
  be 
  

   found 
  independently. 
  

  

  The 
  subjects 
  of 
  observation 
  have 
  been 
  principally 
  two 
  

   plates, 
  of 
  which 
  the 
  first 
  is 
  of 
  black 
  glass, 
  i. 
  e. 
  glass 
  contain- 
  

   ing 
  sufficient 
  absorbing 
  material 
  to 
  be 
  opaque. 
  When 
  first 
  

   examined 
  on 
  May 
  24 
  (1907) 
  it 
  had 
  been 
  lying 
  in 
  a 
  box 
  for 
  

   many 
  years. 
  Carefully 
  cleaned 
  by 
  washing 
  and 
  wiping, 
  it 
  

   gave 
  ol 
  — 
  a= 
  +5°, 
  fi 
  as 
  throughout 
  being 
  +45°. 
  On 
  

   polishing 
  rather 
  protractedly 
  with 
  rouge, 
  it 
  gave 
  ol' 
  — 
  ol 
  = 
  + 
  35', 
  

   a 
  large 
  reduction. 
  Blowing 
  at 
  the 
  surface 
  while 
  under 
  ob- 
  

   servation 
  with 
  a 
  stream 
  of 
  chemically 
  dried 
  air 
  had 
  very 
  

   little 
  effect, 
  

  

  Want 
  of 
  sunshine 
  prevented 
  further 
  observations 
  until 
  

   Aug. 
  3, 
  when 
  a! 
  — 
  ol 
  was 
  found 
  to 
  be 
  about 
  -(-1°. 
  Treatment 
  

   with 
  rouge 
  reduced 
  this 
  to 
  +40', 
  and 
  so 
  far 
  as 
  appeared 
  

   neither 
  heating 
  (with 
  the 
  idea 
  of 
  removing 
  grease) 
  nor 
  

   treatment 
  with 
  specially 
  moist 
  or 
  specially 
  dry 
  air 
  made 
  

   much 
  difference. 
  But 
  on 
  further 
  repolishing 
  with 
  rouge 
  the 
  

   ellipticity 
  practically 
  disappeared. 
  There 
  was 
  no 
  certain 
  

   change 
  in 
  the 
  position 
  of 
  the 
  dark 
  spot 
  when 
  ft 
  was 
  altered 
  

   between 
  +45°. 
  

  

  It 
  seemed 
  that 
  the 
  earlier 
  treatments 
  with 
  rouge 
  had 
  been 
  

   inadequate. 
  In 
  the 
  last 
  application 
  the 
  polisher 
  was 
  of 
  

   paper 
  cemented 
  to 
  glass 
  and 
  impregnated 
  with 
  the 
  rouge. 
  

   After 
  polishing, 
  the 
  glass 
  was 
  breathed 
  upon 
  and 
  carefully 
  

   wiped 
  with 
  a 
  cloth. 
  In 
  subsequent 
  operations 
  this 
  pro- 
  

   cedure 
  was 
  always 
  followed. 
  

  

  Further 
  observations 
  showed 
  that 
  even 
  on 
  Aug. 
  3 
  the 
  

   polishing 
  had 
  not 
  been 
  carried 
  far 
  enough. 
  On 
  Aug. 
  7 
  

   ol' 
  — 
  ol 
  was 
  made 
  to 
  change 
  sign, 
  being 
  reduced 
  (algebraically) 
  

   to 
  —10' 
  or 
  —22'. 
  This 
  was 
  about 
  the 
  limit. 
  Even 
  in 
  this 
  

   condition 
  the 
  surface 
  did 
  not 
  seem 
  sensitive 
  to 
  moisture, 
  

   much 
  to 
  my 
  surprise. 
  And 
  it 
  was 
  matter 
  for 
  further 
  surprise 
  

   when 
  it 
  appeared 
  that 
  24 
  hours' 
  exposure 
  to 
  the 
  air 
  of 
  the 
  

   room 
  — 
  no 
  chemical 
  operations 
  were 
  in 
  progress 
  — 
  sufficed 
  to 
  

   carry 
  the 
  surface 
  back 
  to 
  the 
  positive 
  side 
  with 
  ol 
  —&■=■ 
  +10', 
  

  

  