﻿402 
  Prof. 
  R. 
  W. 
  AVood 
  on 
  the 
  Electrical 
  Resonance 
  of 
  

  

  quite 
  dark 
  like 
  the 
  clear 
  glass. 
  Still 
  farther 
  down 
  the 
  bulb 
  

   where 
  the 
  particles 
  are 
  very 
  large, 
  we 
  get 
  ordinary 
  scattered 
  

   hVht, 
  and 
  the 
  film 
  has 
  the 
  silky 
  lustre 
  to 
  which 
  I 
  have 
  alluded. 
  

  

  Summing 
  up 
  the 
  results 
  thus 
  far, 
  we 
  find 
  : 
  — 
  

  

  Coarse 
  particles 
  diffract 
  or 
  scatter 
  light, 
  and 
  give 
  the 
  bulb 
  

   a 
  silky 
  lustre. 
  

  

  Minute 
  particles 
  very 
  close 
  together, 
  regularly 
  reflect 
  those 
  

   wave-lengths 
  absent 
  in 
  the 
  transmitted 
  light, 
  but 
  give 
  no 
  

   scattered 
  light. 
  

  

  Minute 
  particles 
  far 
  apart, 
  diffuse 
  light 
  of 
  the 
  same 
  wave- 
  

   lengths 
  as 
  those 
  which 
  are, 
  to 
  some 
  extent, 
  absent 
  in 
  trans- 
  

   mitted 
  light. 
  This 
  type 
  of 
  scattered 
  light, 
  which 
  for 
  

   convenience 
  I 
  have 
  called 
  fluorescent, 
  is 
  exhibited 
  by 
  deposits 
  

   which 
  are 
  too 
  scant 
  to 
  show 
  any 
  trace 
  of 
  colour 
  when 
  viewed 
  

   by 
  transmitted 
  light. 
  

  

  Spectrum 
  of 
  the 
  Transmitted 
  Light. 
  

  

  An 
  examination 
  of 
  the 
  transmitted 
  light 
  with 
  a 
  small 
  direct- 
  

   vision 
  spectroscope 
  showed 
  that 
  in 
  some 
  cases 
  the 
  films 
  

   completely 
  refused 
  transmission 
  of 
  certain 
  wave-lengths. 
  

   The 
  claret-coloured 
  films 
  had 
  an 
  absorption-band 
  in 
  the 
  

   yellow, 
  which 
  was 
  exceedingly 
  black 
  and 
  quite 
  narrow, 
  its 
  

   width 
  rather 
  less 
  than 
  one 
  tenth 
  of 
  the 
  easily 
  visible 
  spectrum. 
  

   Any 
  theory 
  accounting 
  for 
  the 
  colour 
  of 
  this 
  film 
  must 
  

   explain 
  the 
  complete 
  extinction 
  of 
  certain 
  wave-lengths. 
  In 
  

   the 
  case 
  of 
  the 
  blue 
  films, 
  the 
  absorption-band 
  was 
  in 
  the 
  red, 
  

   but 
  in 
  this 
  case 
  it 
  invariably 
  appeared 
  more 
  diffused. 
  The 
  

   green 
  films 
  apparently 
  absorbed, 
  more 
  or 
  less 
  completely, 
  the 
  

   blue 
  and 
  red 
  ends 
  of 
  the 
  spectrum. 
  

  

  The 
  changes 
  of 
  colour 
  which 
  accompany 
  change 
  of 
  

   temperature 
  are 
  of 
  two 
  distinct 
  types 
  : 
  a 
  permanent 
  change 
  

   usually 
  the 
  result 
  of 
  local 
  heating, 
  and 
  a 
  temporary 
  change, 
  

   the 
  result 
  of 
  cooling. 
  

  

  I 
  was 
  much 
  puzzled 
  by 
  the 
  fact 
  that 
  the 
  change 
  in 
  colour 
  

   was 
  in 
  the 
  same 
  direction 
  in 
  both 
  cases 
  ; 
  the 
  region 
  of 
  

   absorption 
  appearing 
  to 
  shift 
  in 
  the 
  direction 
  of 
  greater 
  

   wave-lengths. 
  Purple-red 
  films, 
  with 
  an 
  absorption-band 
  in 
  

   the 
  yellow, 
  changed 
  to 
  blue, 
  the 
  change 
  being 
  due 
  to 
  the 
  

   shifting 
  of 
  the 
  band 
  into 
  the 
  red. 
  The 
  effect 
  can 
  be 
  best 
  

   observed 
  by 
  touching 
  the 
  outside 
  of 
  the 
  bulb 
  with 
  a 
  hot 
  glass 
  

   rod. 
  A 
  transparent 
  spot 
  then 
  appears 
  where 
  the 
  metal 
  has 
  

   entirely 
  evaporated, 
  surrounded 
  by 
  a 
  blue 
  ring 
  on 
  a 
  purple 
  

   field. 
  Placing 
  the 
  slit 
  of 
  the 
  spectroscope 
  along 
  a 
  diameter 
  

   of 
  the 
  ring, 
  the 
  spectrum 
  appears 
  as 
  in 
  fig. 
  2, 
  a. 
  With 
  

   potassium 
  films, 
  I 
  have 
  sometimes 
  obtained 
  permanent 
  changes 
  

   in 
  the 
  opposite 
  direction 
  ; 
  a 
  violet-blue 
  film 
  becoming 
  pink 
  

  

  