﻿Glass 
  at 
  the 
  Polarizing 
  Angle. 
  449 
  

  

  same 
  conclusion 
  is 
  favoured 
  by 
  the 
  results 
  of 
  beating 
  tbe 
  

   plate 
  pretty 
  strongly. 
  The 
  same 
  kind 
  o£ 
  recovery 
  in 
  the 
  

   direction 
  of 
  the 
  negative 
  condition 
  could 
  thus 
  be 
  attained, 
  

   but 
  never 
  to 
  the 
  full 
  extent. 
  In 
  some 
  experiments 
  the 
  plate 
  

   was 
  purposely 
  contaminated. 
  Thus 
  on 
  Nov. 
  1 
  a 
  repolished 
  

   plate 
  at 
  — 
  13' 
  was 
  exposed 
  to 
  the 
  smoke 
  of 
  burning 
  greasy 
  

   waste, 
  after 
  which 
  a' 
  — 
  a 
  was 
  +46 
  / 
  . 
  A 
  very 
  thorough 
  treat- 
  

   ment 
  by 
  wiping 
  took 
  it 
  back 
  to 
  — 
  6', 
  but 
  only 
  after 
  repolish- 
  

   ing 
  could 
  the 
  original 
  condition 
  ( 
  — 
  10') 
  be 
  nearly 
  recovered. 
  

   In 
  another 
  experiment 
  a 
  stream 
  of 
  air 
  which 
  had 
  passed 
  over 
  

   petroleum 
  was 
  directed 
  against 
  a 
  repolished 
  surface, 
  but 
  the 
  

   effect 
  was 
  only 
  momentary. 
  As 
  regards 
  surfaces 
  which 
  have 
  

   stood 
  a 
  week 
  or 
  two, 
  I 
  think 
  there 
  can 
  be 
  no 
  doubt 
  but 
  that 
  

   the 
  glass 
  itself 
  has 
  been 
  seriously 
  attacked. 
  

  

  The 
  results 
  here 
  recorded 
  are 
  in 
  many 
  respects 
  very 
  different 
  

   from 
  what 
  I 
  had 
  anticipated 
  — 
  especially 
  the 
  comparative 
  

   insensitiveness 
  to 
  grease 
  and 
  moisture. 
  It 
  must 
  be 
  remem- 
  

   bered, 
  however, 
  that 
  a 
  surface 
  finished 
  by 
  wiping 
  and 
  in 
  

   contact 
  with 
  air 
  is 
  certainly 
  contaminated 
  with 
  water 
  and 
  

   probably 
  with 
  grease. 
  In 
  spite 
  of 
  this 
  it 
  is 
  possible 
  to 
  have 
  

   the 
  reflexion 
  free 
  from 
  elliptidty. 
  As 
  regards 
  grease 
  we 
  may 
  

   perhaps 
  argue 
  from 
  the 
  manner 
  in 
  which 
  the 
  breath 
  is 
  de- 
  

   posited. 
  A 
  freshly 
  split 
  surface 
  of 
  mica 
  receives 
  the 
  moisture 
  

   of 
  the 
  breath 
  as 
  an 
  almost 
  invisible 
  film, 
  showing 
  the 
  colours 
  

   of 
  thin 
  plates 
  as 
  it 
  evaporates, 
  but 
  nothing 
  of 
  the 
  appearance 
  

   ordinarily 
  associated 
  with 
  dew 
  and 
  dependent 
  upon 
  an 
  

   irregular 
  deposition. 
  I 
  am 
  not 
  sure 
  whether 
  glass 
  has 
  ever 
  

   been 
  observed 
  in 
  this 
  condition*, 
  but 
  experience 
  from 
  the 
  

   •days 
  of 
  wet 
  collodion 
  photography 
  convinces 
  me 
  that 
  a 
  

   wiped 
  glass 
  does 
  not 
  so 
  behave. 
  The 
  best 
  that 
  can 
  be 
  

   attained 
  is 
  a 
  uniform 
  dull 
  grey 
  appearance, 
  such 
  as 
  under 
  a 
  

   magnifier 
  would 
  exhibit 
  lenticular 
  drops. 
  

  

  The 
  conclusion 
  which 
  suggests 
  itself 
  is 
  that 
  even 
  a 
  recently 
  

   repolished 
  surface, 
  which 
  may 
  exhibit 
  but 
  small 
  ellipticity, 
  

   is 
  in 
  a 
  highly 
  complicated 
  condition. 
  Grease 
  itself 
  may 
  be 
  

   comparatively 
  inoperative 
  optically 
  on 
  account 
  of 
  its 
  index 
  

   approximating 
  to 
  that 
  of 
  the 
  glass. 
  But 
  why 
  varying 
  degrees 
  

   of 
  moisture 
  should 
  make 
  so 
  little 
  difference 
  is 
  not 
  apparent. 
  

   Surface 
  phenomena 
  generally 
  offer 
  a 
  wide 
  field 
  for 
  investi- 
  

   gation, 
  which 
  might 
  lead 
  to 
  results 
  throwing 
  much 
  needed 
  

   light 
  upon 
  the 
  constitution 
  of 
  matter. 
  

  

  Terling 
  Place, 
  Witham. 
  

  

  * 
  Possibly 
  the 
  path 
  of 
  an 
  electric 
  discharge 
  over 
  a 
  glass 
  surface 
  may 
  

   be 
  a 
  case 
  in 
  point. 
  

  

  Phil. 
  Mag. 
  Ser. 
  6. 
  Vol. 
  16. 
  No. 
  93. 
  Sept. 
  1908. 
  2 
  H 
  

  

  