﻿448 
  Lord 
  Rayleigh 
  on 
  Reflexion 
  from 
  

  

  increased 
  in 
  a 
  few 
  days 
  to 
  +40'. 
  These 
  changes 
  of 
  sign 
  

   were 
  observed 
  not 
  only 
  with 
  the 
  black 
  glass, 
  which 
  on 
  the 
  

   evidence 
  of 
  its 
  polarizing 
  angle 
  is 
  a 
  flint, 
  but 
  also 
  with 
  a 
  

   piece 
  of 
  patent 
  plate 
  prepared 
  by 
  roughing 
  the 
  hind 
  surface 
  

   and 
  varnishing 
  it 
  with 
  a 
  cement 
  of 
  nearly 
  the 
  same 
  index. 
  

   It 
  may 
  be 
  mentioned 
  that 
  another 
  piece 
  of 
  patent 
  plate 
  

   became 
  good 
  on 
  treatment 
  with 
  hydrofluoric 
  acid 
  and 
  polish- 
  

   ing 
  with 
  rouge 
  upon 
  a 
  soft 
  tool. 
  After 
  deterioration 
  by 
  

   exposure 
  the 
  negative 
  values 
  of 
  a 
  —a 
  could 
  not 
  be 
  recovered 
  

   by 
  merely 
  cleaning 
  the 
  plate 
  with 
  moisture 
  and 
  wiping 
  ; 
  

   actual 
  repolishing 
  was 
  necessary. 
  The 
  natural 
  inference 
  is 
  

   that 
  even 
  within 
  24 
  hours 
  the 
  substance 
  of 
  the 
  glass 
  is 
  

   actually 
  attacked 
  by 
  the 
  gases 
  of 
  the 
  atmosphere. 
  

  

  As 
  I 
  was 
  leaving 
  home 
  for 
  some 
  time 
  I 
  arranged 
  an 
  

   experiment 
  to 
  see 
  whether 
  careful 
  protection 
  would 
  save 
  a 
  

   glass 
  surface 
  from 
  the 
  above 
  described 
  deterioration, 
  which 
  

   might 
  probably 
  be 
  attributed 
  to 
  moisture 
  and 
  carbonic 
  acid. 
  

   Accordingly 
  on 
  Aug. 
  22 
  the 
  black 
  glass, 
  giving 
  a! 
  — 
  a= 
  — 
  14', 
  

   was 
  put 
  away 
  in 
  a 
  tube 
  containing 
  potash 
  and 
  closed, 
  pro- 
  

   bably 
  airtight, 
  with 
  a 
  rubber 
  cork. 
  On 
  Oct. 
  15, 
  nearly 
  two 
  

   months 
  later, 
  the 
  glass 
  was 
  taken 
  out 
  and 
  (with 
  surface 
  

   untouched) 
  gave 
  a.' 
  — 
  u 
  =—6', 
  still 
  on 
  the 
  negative 
  side. 
  

   The 
  difference 
  of 
  values, 
  though 
  doubtless 
  real, 
  is 
  perhaps 
  

   no 
  more 
  than 
  may 
  be 
  attributed 
  to 
  moisture 
  and 
  carbonic 
  

   acid 
  imprisoned 
  with 
  the 
  glass. 
  Four 
  days 
  later 
  the 
  readings 
  

   gave 
  +35'. 
  The 
  plate 
  glass 
  put 
  away 
  at 
  the 
  same 
  time 
  with 
  

   potash 
  in 
  another 
  tube 
  with 
  a— 
  a—— 
  19' 
  was 
  examined 
  

   after 
  two 
  months, 
  on 
  Oct. 
  19, 
  and 
  gave 
  a' 
  — 
  a= 
  +4', 
  so 
  that 
  

   in 
  this 
  case 
  the 
  protection 
  seems 
  to 
  have 
  been 
  less 
  efficient. 
  

  

  A 
  large 
  number 
  of 
  further 
  observations 
  were 
  made 
  upon 
  

   both 
  glasses 
  with 
  the 
  object 
  of 
  ascertaining, 
  if 
  possible, 
  how 
  

   much 
  of 
  the 
  change 
  which 
  ensues 
  after 
  repolishing 
  is 
  due 
  to 
  

   a 
  film 
  of 
  foreign 
  (greasy) 
  matter 
  deposited 
  from 
  the 
  atmo- 
  

   sphere 
  and 
  how 
  much 
  to 
  an 
  alteration 
  of 
  the 
  glass 
  itself. 
  

   The 
  negative 
  condition, 
  lost 
  in 
  a 
  day 
  or 
  two 
  after 
  repolishing, 
  

   is 
  in 
  part 
  recovered 
  under 
  the 
  operation 
  of 
  a 
  careful 
  wiping 
  

   with 
  moisture, 
  but 
  only 
  to 
  a 
  limited 
  extent. 
  Full 
  recovery 
  

   requires 
  actual 
  repolishing. 
  It 
  is 
  of 
  course 
  possible 
  that 
  

   even 
  the 
  mild 
  treatment 
  by 
  wiping 
  may 
  attack 
  the 
  very 
  thin 
  

   film 
  of 
  altered 
  glass 
  which 
  is 
  all 
  that 
  we 
  can 
  suppose 
  to 
  have 
  

   been 
  formed 
  in 
  so 
  short 
  a 
  time 
  ; 
  but 
  on 
  the 
  whole 
  the 
  

   evidence 
  pointed 
  to 
  two 
  kinds 
  of 
  contamination, 
  one 
  removable 
  

   by 
  wiping 
  and 
  the 
  other 
  requiring 
  the 
  more 
  drastic 
  treatment 
  

   with 
  rouge. 
  In 
  spite 
  of 
  some 
  easily 
  understood 
  irregularities, 
  

   it 
  appeared 
  that 
  the 
  full 
  elTect 
  of 
  wiping 
  was 
  easily 
  produced 
  

   and 
  that 
  repetition 
  could 
  carry 
  the 
  process 
  no 
  further. 
  The 
  

  

  