﻿408 
  Mr. 
  P. 
  V. 
  Bevan 
  on 
  Anomalous 
  

  

  The 
  method 
  of 
  Wood 
  is 
  well 
  known 
  and 
  needs 
  no 
  detailed 
  

   description. 
  A 
  piece 
  of 
  steel 
  bicycle 
  tubing 
  is 
  used 
  for 
  

   heating 
  the 
  metal. 
  The 
  tube 
  is 
  closed 
  by 
  glass 
  ends 
  and 
  

   brass 
  jackets 
  are 
  fixed 
  close 
  to 
  the 
  ends 
  through 
  which 
  water 
  

   circulates 
  to 
  keep 
  the 
  sealing-wax 
  which 
  fixes 
  the 
  glass 
  

   plates 
  from 
  melting. 
  The 
  under 
  side 
  of 
  the 
  tube 
  is 
  heated, 
  

   and 
  the 
  upper 
  side 
  is 
  kept 
  cool 
  by 
  a 
  water-trough 
  fitting 
  and 
  

   restino' 
  on 
  the 
  tube 
  through 
  which 
  a 
  stream 
  of 
  water 
  can 
  

   be 
  kept 
  flowing. 
  It 
  was 
  found 
  that 
  this 
  method 
  of 
  cooling 
  

   the 
  top 
  of 
  the 
  tube 
  was 
  very 
  satisfactory 
  and 
  kept 
  the 
  con- 
  

   ditions 
  more 
  steady, 
  leading 
  to 
  better 
  definition 
  in 
  the 
  

   photographs 
  of 
  the 
  eflfect. 
  The 
  light 
  used 
  was 
  from 
  a 
  Nernst 
  

   lamp. 
  The 
  light, 
  after 
  passing 
  through 
  the 
  steel 
  tube, 
  was 
  

   focussed 
  by 
  a 
  lens 
  forming 
  a 
  horizontal 
  image 
  of 
  the 
  filament 
  

   across 
  the 
  vertical 
  slit 
  of 
  the 
  spectroscope, 
  so 
  that 
  when 
  the 
  

   steel 
  tube 
  was 
  cold, 
  a 
  spectrum 
  was 
  produced 
  consisting 
  of 
  

   a 
  line 
  of 
  colour. 
  When 
  the 
  tube 
  is 
  heated, 
  metallic 
  vapour 
  

   is 
  formed, 
  and 
  this 
  being 
  in 
  layers 
  of 
  unequal 
  density, 
  

   produces 
  dispersion 
  in 
  a 
  vertical 
  plane, 
  giving 
  a 
  vertical 
  

   displacement 
  in 
  the 
  line 
  of 
  colour 
  finally 
  observed. 
  The 
  

   eflPect 
  is 
  easily 
  obtained 
  with 
  both 
  lithium 
  and 
  potassium 
  

   and, 
  though 
  not 
  so 
  striking 
  as 
  in 
  the 
  case 
  of 
  sodium, 
  gives 
  a 
  

   very 
  beautiful 
  picture 
  in 
  each 
  case 
  of 
  the 
  anomalous 
  dispersion 
  

   curves. 
  In 
  the 
  case 
  of 
  lithium 
  (PI. 
  XIII. 
  fig. 
  1) 
  a 
  considerably 
  

   higher 
  temperature 
  is 
  required 
  than 
  for 
  sodium, 
  and 
  it 
  was 
  

   found 
  that 
  a 
  good 
  result 
  could 
  be 
  obtained 
  by 
  heating 
  the 
  

   tube 
  with 
  an 
  ordinary 
  foot 
  blow-pipe 
  so 
  that 
  the 
  under 
  side 
  

   of 
  the 
  tube 
  was 
  at 
  a 
  red 
  heat. 
  The 
  figure 
  is 
  from 
  the 
  original 
  

   negative, 
  the 
  apparatus 
  at 
  the 
  author's 
  disposal 
  only 
  per- 
  

   mitting 
  a 
  small 
  negative 
  to 
  be 
  taken. 
  The 
  anomalous 
  

   dispersion 
  takes 
  place 
  in 
  the 
  region 
  of 
  the 
  red 
  lithium 
  line 
  

   w.-l. 
  6705, 
  and 
  there 
  is 
  no 
  trace 
  of 
  anomalous 
  dispersion 
  in 
  

   the 
  region 
  of 
  the 
  other 
  lithium 
  lines 
  in 
  the 
  visible 
  spectrum. 
  

   This 
  fact 
  seems 
  of 
  importance 
  when 
  taken 
  into 
  consideration 
  

   with 
  the 
  regions 
  of 
  anomalous 
  dispersion 
  in 
  sodium 
  and 
  potas- 
  

   sium 
  vapours, 
  as 
  in 
  all 
  three 
  cases 
  the 
  region 
  of 
  anomalous 
  

   dispersion 
  appears 
  to 
  be 
  only 
  near 
  the 
  lines 
  of 
  the 
  primary 
  

   series 
  of 
  Kayser 
  and 
  Punge. 
  At 
  present 
  the 
  author 
  has 
  not 
  

   determined 
  whether 
  anomalous 
  dispersion 
  does 
  take 
  place 
  at 
  

   the 
  other 
  lines 
  of 
  the 
  primary 
  series 
  for 
  lithium, 
  as 
  these 
  lines 
  

   are 
  in 
  the 
  ultra-violet, 
  and 
  so 
  far 
  no 
  apparatus 
  has 
  been 
  

   available 
  for 
  obtaining 
  photographs 
  in 
  this 
  region. 
  Further 
  

   experiments 
  are, 
  however, 
  in 
  progress 
  to 
  investigate 
  this 
  

   region. 
  The 
  photographs 
  of 
  the 
  effects 
  are 
  not 
  so 
  clear 
  and 
  

   well 
  defined 
  as 
  one 
  could 
  wish, 
  but 
  it 
  is 
  difficult 
  to 
  keep 
  the 
  

   temperature 
  steady 
  over 
  a 
  long 
  exposure, 
  and 
  long 
  exposures 
  

  

  