﻿398 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Electrical 
  Resonance 
  of 
  

  

  difficult 
  to 
  describe, 
  I 
  at 
  once 
  recognized 
  as 
  a 
  diffraction 
  

   effect 
  due 
  to 
  the 
  scattering 
  of 
  light 
  by 
  small 
  reflecting 
  

   particles. 
  Examination 
  under 
  the 
  microscope 
  showed 
  this 
  to 
  

   be 
  the 
  case, 
  the 
  sodium 
  having 
  deposited 
  on 
  the 
  glass 
  in 
  the 
  

   form 
  of 
  small 
  isolated 
  drops. 
  It 
  at 
  once 
  occurred 
  to 
  me 
  that 
  

   the 
  condition 
  might 
  be 
  the 
  same 
  in 
  the 
  coloured 
  films, 
  the 
  

   particles 
  here 
  being 
  small 
  enough 
  to 
  respond 
  to 
  the 
  light- 
  

   waves, 
  in 
  the 
  same 
  manner 
  as 
  the 
  resonators 
  of 
  Garbasso 
  and 
  

   Aschkiuass. 
  I 
  examined 
  some 
  of 
  the 
  more 
  transparent 
  of 
  the 
  

   coloured 
  films, 
  under 
  a 
  one-twelfth 
  inch 
  oil-immersion 
  

   objective, 
  and 
  found 
  that 
  my 
  suspicions 
  were 
  verified, 
  the 
  

   films 
  being 
  made 
  up 
  of 
  particles 
  just 
  barely 
  visible 
  as 
  

   individual 
  grains 
  lying 
  close 
  together. 
  I 
  was 
  unable 
  to 
  

   satisfy 
  myself 
  that 
  the 
  size 
  varied 
  with 
  the 
  colour 
  of 
  the 
  film, 
  

   and 
  am 
  of 
  the 
  opinion 
  that 
  if 
  the 
  resonance 
  theory 
  is 
  the 
  true 
  

   one, 
  the 
  free 
  period 
  depends 
  more 
  on 
  the 
  proximity 
  of 
  the 
  

   particles 
  than 
  on 
  their 
  actual 
  size. 
  The 
  diameter 
  of 
  the 
  

   particles, 
  as 
  measured 
  with 
  the 
  micrometer, 
  varies 
  from 
  about 
  

   *0003 
  to 
  *0002 
  mm. 
  In 
  other 
  words, 
  they 
  are 
  of 
  about 
  the 
  

   size 
  of 
  the 
  smallest 
  micrococci 
  described 
  by 
  the 
  bacteriologists. 
  

  

  It 
  occurred 
  to 
  me 
  at 
  first 
  that 
  the 
  colours 
  might 
  be 
  similar 
  

   to 
  those 
  shown 
  by 
  mixed 
  plates, 
  which 
  are 
  explained 
  in 
  the 
  

   same 
  way 
  as 
  the 
  coloured 
  central 
  images 
  seen 
  in 
  certain 
  

   transparent 
  diffraction-gratings. 
  This 
  would, 
  however, 
  neces- 
  

   sitate 
  the 
  presence 
  of 
  minute 
  transparent 
  particles 
  of 
  a 
  thick- 
  

   ness 
  sufficient 
  to 
  give 
  a 
  retardation 
  of 
  A/2. 
  The 
  formation 
  

   of 
  such 
  particles 
  could 
  be 
  conceived 
  of 
  as 
  resulting 
  from 
  the 
  

   oxidation 
  of 
  the 
  sodium 
  particles 
  by 
  a 
  layer 
  of 
  absorbed 
  air 
  

   on 
  the 
  glass. 
  It 
  is 
  therefore 
  necessary 
  to 
  show 
  at 
  the 
  outset 
  

   that 
  the 
  cause 
  of 
  the 
  colour 
  is 
  the 
  presence 
  of 
  metallic 
  sodium. 
  

   That 
  this 
  is 
  the 
  case 
  I 
  feel 
  sure, 
  for 
  if 
  the 
  slightest 
  trace 
  of 
  

   air 
  be 
  allowed 
  to 
  enter 
  the 
  bulb, 
  the 
  colour, 
  no 
  matter 
  how 
  

   intense, 
  instantly 
  vanishes, 
  the 
  glass 
  becoming 
  as 
  clear 
  as 
  

   before 
  the 
  formation 
  of 
  the 
  film. 
  The 
  oxidation 
  process 
  could 
  

   be 
  made 
  to 
  take 
  place 
  slowly, 
  by 
  drawing 
  out 
  one 
  end 
  of 
  the 
  

   bulb 
  into 
  a 
  long 
  fine 
  capillary, 
  with 
  a 
  bore 
  of 
  about 
  *01 
  mm., 
  

   and 
  breaking 
  off 
  theendaiterthecoloured 
  film 
  had 
  beenformed. 
  

  

  In 
  certain 
  cases 
  it 
  was 
  observed 
  that 
  the 
  colour 
  changed 
  

   before 
  disappearing 
  entirely. 
  I 
  also 
  found 
  that 
  the 
  colour 
  of 
  

   the 
  film 
  could 
  be 
  permanently 
  changed 
  by 
  carefully 
  heating 
  

   the 
  glass 
  to 
  a 
  temperature 
  just 
  below 
  that 
  necessary 
  to 
  

   evaporate 
  the 
  film. 
  Local 
  heating 
  to 
  a 
  slightly 
  higher 
  

   temperature 
  drives 
  off 
  the 
  film 
  entirely, 
  leaving 
  the 
  glass 
  

   perfectly 
  clear. 
  By 
  means 
  of 
  a 
  small 
  flame, 
  it 
  is 
  possible 
  to 
  

   drive 
  a 
  patch 
  of 
  colour 
  all 
  over 
  the 
  inside 
  of 
  a 
  bulb, 
  and 
  it 
  is 
  

   in 
  this 
  way 
  that 
  some 
  of 
  the 
  best 
  films 
  are 
  prepared. 
  It 
  was 
  

  

  