﻿Glass 
  at 
  the 
  Polarizing 
  Angle. 
  445 
  

  

  than 
  joVo 
  °f 
  wna 
  * 
  Jamin 
  observed. 
  It 
  appeared, 
  however, 
  

   that 
  the 
  cleanest 
  surfaces 
  were 
  not 
  those 
  which 
  gave 
  the 
  

   least 
  reflexion. 
  At 
  the 
  highest 
  degrees 
  of 
  purity 
  the 
  light 
  

   again 
  began 
  to 
  undergo 
  reflexion, 
  though 
  to 
  a 
  very 
  limited 
  

   amount. 
  The 
  effect 
  had 
  changed 
  sign. 
  

  

  The 
  contamination 
  which 
  produces 
  the 
  effects 
  observed 
  by 
  

   Jamin 
  is 
  but 
  slight, 
  regarded 
  from 
  any 
  other 
  than 
  the 
  

   optical 
  or 
  capillary 
  point 
  of 
  view. 
  The 
  thickness 
  of 
  the 
  film 
  

   of 
  olive 
  oil 
  which 
  suffices 
  to 
  stop 
  the 
  movements 
  of 
  camphor 
  

   fragments 
  deposited 
  upon 
  water, 
  is 
  2 
  x 
  10 
  -7 
  cm., 
  or 
  about 
  

   ~3 
  Jro 
  of 
  X 
  D 
  . 
  But 
  such 
  a 
  film, 
  or 
  even 
  a 
  much 
  thinner 
  one, 
  

   entirely 
  disturbs 
  the 
  delicate 
  balance 
  upon 
  which 
  depends 
  

   the 
  absence 
  of 
  reflexion 
  at 
  the 
  polarizing 
  angle. 
  

  

  For 
  a 
  long 
  time 
  I 
  have 
  intended 
  to 
  make 
  an 
  examination 
  

   of 
  the 
  corresponding 
  phenomena 
  when 
  light 
  is 
  reflected 
  from 
  

   a 
  surface 
  of 
  glass. 
  I 
  was 
  prepared 
  for 
  complications 
  de- 
  

   pending 
  upon 
  moisture 
  and 
  grease, 
  but 
  thought 
  that 
  perhaps 
  

   I 
  could 
  deal 
  with 
  them. 
  As 
  to 
  the 
  thickness 
  of 
  the 
  films 
  

   there 
  is 
  little 
  definite 
  information. 
  Theory* 
  indicates 
  that 
  

   they 
  are 
  likely 
  to 
  be 
  persistent. 
  A 
  long 
  while 
  ago 
  Magnus 
  

   established 
  the 
  conclusion 
  " 
  that 
  all 
  substances, 
  however 
  

   different 
  they 
  may 
  be, 
  are 
  raised 
  in 
  temperature 
  w 
  r 
  hen 
  air 
  

   comes 
  in 
  contact 
  with 
  them 
  which 
  is 
  moister 
  than 
  that 
  

   surrounding 
  them, 
  and 
  that 
  they 
  are 
  depressed 
  in 
  tempera- 
  

   ture 
  when 
  they 
  are 
  exposed 
  to 
  air 
  which 
  is 
  drier 
  than 
  that 
  

   by 
  which 
  they 
  are 
  surrounded 
  " 
  f. 
  His 
  experiments 
  included 
  

   glass, 
  quartz, 
  mica, 
  caoutchouc, 
  metals, 
  and 
  many 
  other 
  

   substances. 
  In 
  the 
  case 
  of 
  glass, 
  or 
  rather 
  cotton 
  silicate, 
  

   definite 
  estimates 
  have 
  been 
  given 
  by 
  Parks 
  J, 
  deduced 
  from 
  

   actual 
  increases 
  of 
  weight. 
  He 
  finds 
  thicknesses 
  of 
  the 
  order 
  

   1*0 
  x 
  10 
  -5 
  cm., 
  about 
  50 
  times 
  that 
  of 
  the 
  greasy 
  films 
  which 
  

   stop 
  the 
  camphor 
  movements 
  upon 
  water 
  and 
  profoundly 
  

   modify 
  the 
  reflexion 
  of 
  light 
  at 
  the 
  polarizing 
  angle. 
  Even 
  

   if 
  we 
  allow 
  a 
  good 
  deal 
  for 
  the 
  fact 
  that 
  these 
  films 
  were 
  

   formed 
  from 
  a 
  saturated 
  atmosphere, 
  enough 
  will 
  remain 
  to 
  

   explain 
  much 
  optical 
  disturbance. 
  

  

  The 
  deviation 
  from 
  FresneFs 
  formulae 
  is 
  best 
  explained 
  in 
  

   terms 
  of 
  Jamin's 
  k, 
  representing 
  the 
  ratio 
  of 
  reflected 
  ampli- 
  

   tudes 
  for 
  the 
  two 
  principal 
  planes 
  when 
  light, 
  incident 
  at 
  

   the 
  angle 
  tan 
  -1 
  //,, 
  is 
  polarized 
  at 
  45° 
  to 
  these 
  planes. 
  

   According 
  to 
  Fresnel 
  k 
  = 
  0, 
  but 
  Jamin 
  showed 
  that 
  it 
  may 
  

   assume 
  small 
  finite 
  values, 
  positive 
  or 
  negative. 
  The 
  ex- 
  

   perimental 
  method 
  employed 
  for 
  the 
  present 
  purpose 
  was 
  

  

  * 
  Phil. 
  Mag. 
  xxxiii. 
  p. 
  220 
  (1892) 
  ; 
  Scientific 
  Papers, 
  iii. 
  p. 
  523. 
  

   f 
  Phil. 
  Mag. 
  xxvii. 
  p. 
  245 
  (1864). 
  

   X 
  Phil. 
  Mag. 
  v. 
  p. 
  518 
  (1903). 
  

  

  