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  194 
  ] 
  

  

  XXL 
  The 
  Damping 
  of 
  Mercuri/ 
  Waves. 
  

   By 
  Prof. 
  R. 
  W. 
  Wood*. 
  

  

  [Plate 
  III. 
  fi--. 
  8.] 
  

  

  IN 
  the 
  course 
  of 
  some 
  experiments 
  with 
  which 
  I 
  have 
  been^ 
  

   engaged 
  during 
  the 
  past 
  summer 
  upon 
  the 
  construction 
  

   of 
  a 
  reflecting 
  telescope, 
  by 
  the 
  uniform 
  rotation 
  of 
  a 
  large- 
  

   flat 
  basin 
  of 
  mercury 
  (see 
  Astrophysical 
  Journal 
  for 
  March 
  

   1909 
  : 
  " 
  The 
  Mercury 
  Paraboloid 
  as 
  a 
  Reflecting 
  Tele- 
  

   scope 
  ^'), 
  I 
  have 
  found 
  that 
  the 
  ripples 
  can 
  be 
  almost 
  com- 
  

   pletely 
  abolished, 
  even 
  if 
  they 
  are 
  of 
  very 
  large 
  amplitude, 
  by 
  

   covering 
  the 
  surface 
  of 
  the 
  metal 
  with 
  glycerine. 
  It 
  has 
  

   recently 
  occurred 
  to 
  me 
  that 
  castor-oil 
  would 
  be 
  much 
  better,. 
  

   for 
  it 
  is 
  almost 
  as 
  viscous, 
  and 
  does 
  not 
  develope 
  striae 
  by 
  

   absorbing 
  moisture 
  from 
  the 
  air, 
  which 
  is 
  the 
  great 
  objection 
  

   to 
  the 
  use 
  of 
  glycerine. 
  The 
  oil 
  can 
  be 
  obtained 
  almost 
  as 
  

   transparent 
  as 
  water, 
  and 
  has 
  been 
  found 
  to 
  be 
  almost 
  as. 
  

   effective 
  as 
  glycerine 
  in 
  damping 
  the 
  ripples. 
  It 
  seems 
  quite 
  

   probable 
  that 
  the 
  device 
  will 
  prove 
  very 
  useful 
  in 
  cases 
  where 
  

   a 
  level 
  reflecting 
  surface 
  is 
  required. 
  

  

  To 
  study 
  the 
  effect 
  of 
  ditterent 
  liquids 
  in 
  damping 
  the 
  

   waves 
  I 
  have 
  recently 
  employed 
  a 
  large 
  electrically-driven 
  

   tuning-fork, 
  provided 
  with 
  a 
  glass 
  stylus 
  which 
  dipped 
  in 
  the 
  

   mercury. 
  The 
  surface 
  of 
  the 
  reflecting 
  liquid 
  was 
  viewed 
  

   through 
  a 
  pair 
  of 
  partially 
  superposed 
  narrow 
  slits 
  in 
  thin 
  

   cardboard, 
  mounted 
  on 
  the 
  prongs 
  of 
  a 
  second 
  electrically- 
  

   driven 
  fork 
  of 
  the 
  same 
  pitch. 
  By 
  throwing 
  the 
  forks 
  

   slightly 
  out 
  of 
  tune 
  by 
  loading 
  with 
  wax 
  we 
  can 
  observe 
  the 
  

   waves 
  (stroboscopically) 
  slowly 
  crawling 
  out 
  from 
  the 
  

   centre 
  of 
  disturbance, 
  in 
  the 
  manner 
  described 
  by 
  Lord 
  

   Rayleigh, 
  Boys, 
  Vincent, 
  and 
  others. 
  These 
  waves 
  cover 
  

   the 
  entire 
  surface 
  of 
  the 
  mercury. 
  The 
  light 
  from 
  a 
  small 
  

   source 
  is 
  rendered 
  slightly 
  convergent 
  by 
  a 
  lens, 
  before 
  its 
  

   incidence 
  upon 
  the 
  mercury, 
  the 
  image 
  of 
  the 
  source 
  being 
  

   thrown 
  upon 
  the 
  vibrating 
  slits. 
  If 
  now 
  we 
  cover 
  the 
  metal 
  

   with 
  a 
  thin 
  layer 
  of 
  glycerine 
  or 
  castor-oil, 
  we 
  find 
  that 
  the 
  

   waves 
  die 
  out 
  completely 
  after 
  travelling 
  a 
  distance 
  of 
  from 
  

   one 
  to 
  three 
  wave-lengths, 
  depending 
  upon 
  the 
  amplitude 
  of 
  

   the 
  exciting 
  fork. 
  The 
  experiment 
  is 
  the 
  only 
  one 
  which 
  I 
  

   know 
  of 
  for 
  showing 
  damped 
  waves, 
  and 
  gives 
  us 
  a 
  most 
  vivid 
  

   idea 
  of 
  what 
  happens 
  to 
  light 
  when 
  it 
  enters 
  an 
  absorbing- 
  

   medium. 
  A 
  photograph 
  of 
  the 
  damped 
  waves 
  taken 
  when 
  

   the 
  exciting 
  fork 
  was 
  very 
  vigorously 
  driven 
  is 
  reproduced 
  

   on 
  Plate 
  III. 
  fig. 
  8. 
  The 
  two 
  forks 
  were 
  exactly 
  in 
  unison 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  