﻿608 
  

  

  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption, 
  

  

  with 
  the 
  curve 
  obtained 
  by 
  direct 
  spectrometer 
  measurements 
  

   with 
  selenium 
  prisms. 
  

  

  These 
  prisms 
  were 
  made 
  in 
  the 
  same 
  manner 
  as 
  the 
  cyanine 
  

   prisms 
  which 
  I 
  have 
  previously 
  described 
  (Phil. 
  Mag. 
  i. 
  

   p. 
  624, 
  1901). 
  The 
  substance 
  is 
  much 
  more 
  transparent 
  

   Ihfin 
  cyanine, 
  and 
  larger 
  angles 
  can 
  consequently 
  be 
  used. 
  

   Four 
  or 
  five 
  degrees 
  is 
  about 
  the 
  maximum 
  angle 
  which 
  can 
  

   be 
  employed 
  to 
  advantage. 
  A 
  strip 
  of 
  thin 
  German 
  plate- 
  

   glass 
  about 
  1 
  cm. 
  wide 
  and 
  2 
  cm. 
  long 
  is 
  about 
  right. 
  A 
  narrow 
  

   strip 
  of 
  cardboard 
  1*5 
  mm. 
  thick 
  is 
  pasted 
  along 
  the 
  narrow 
  

   edge, 
  and 
  a 
  small 
  piece 
  of 
  selenium 
  fused 
  on 
  the 
  opposite 
  

   edge 
  by 
  holding 
  the 
  plate 
  over 
  a 
  small 
  flame. 
  The 
  other 
  

   plate 
  must 
  be 
  heated 
  at 
  the 
  same 
  time 
  to 
  about 
  the 
  same 
  

   temperature, 
  and 
  the 
  two 
  clamped 
  in 
  a 
  small 
  vice 
  in 
  the 
  

   manner 
  described 
  for 
  cyanine 
  prisms. 
  It 
  is 
  necessary 
  to 
  

   heat 
  the 
  plate 
  until 
  the 
  substance 
  is 
  quite 
  fluid, 
  which 
  is 
  con- 
  

   siderably 
  above 
  the 
  temperature 
  at 
  which 
  it 
  becomes 
  pasty. 
  

   After 
  the 
  medium 
  has 
  solidified, 
  one 
  of 
  the 
  plates 
  can 
  be 
  

   removed 
  by 
  a 
  blow 
  from 
  a 
  small 
  hammer, 
  if 
  the 
  prism 
  is 
  

   required 
  for 
  measurements 
  with 
  the 
  spectrometer, 
  as 
  the 
  

   angle 
  can 
  then 
  be 
  easily 
  determined 
  by 
  the 
  reflexion 
  of 
  light 
  

   from 
  the 
  oblique 
  surfaces. 
  If 
  the 
  prism 
  is 
  to 
  be 
  used 
  for 
  

   demonstration 
  only, 
  it 
  is 
  better 
  to 
  leave 
  the 
  plate 
  on. 
  One 
  of 
  

   the 
  prisms 
  crossed 
  with 
  a 
  diffraction-grating 
  shows 
  the 
  dis- 
  

   persion 
  curve 
  very 
  well 
  when 
  an 
  arc-light 
  is 
  viewed 
  through 
  

   the 
  combination. 
  

  

  Determinations 
  were 
  made 
  of 
  the 
  dispersion 
  by 
  means 
  of 
  

   three 
  selected 
  prisms, 
  the 
  results 
  of 
  which 
  are 
  shown 
  below. 
  

  

  2-9 
  

  

  2-8 
  

  

  2-1 
  

  

  2-6 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  / 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  / 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  / 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

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  ** 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  75 
  74 
  73 
  72 
  71 
  70 
  69 
  68 
  67 
  66 
  65 
  64 
  63 
  62 
  61 
  60 
  

  

  The 
  results 
  obtained 
  by 
  Sirks 
  are 
  plotted 
  for 
  comparison. 
  

   The 
  values 
  obtained 
  with 
  the 
  different 
  prisms 
  agree 
  very 
  

  

  