﻿620 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption, 
  

  

  the 
  sixth 
  reflexion 
  was 
  very 
  faint, 
  but 
  so 
  far 
  as 
  the 
  eye 
  could 
  

   judge 
  without 
  trace 
  of 
  colour. 
  To 
  determine 
  whether 
  or 
  not 
  

   ultra-violet 
  light 
  was 
  present 
  in 
  excess, 
  the 
  light 
  from 
  the 
  

   sixth 
  surface 
  was 
  received 
  upon 
  a 
  photographic 
  plate 
  one 
  half 
  

   of 
  which 
  was 
  covered 
  with 
  a 
  plate 
  of 
  glass 
  which 
  would 
  

   absorb 
  everything 
  below 
  wave-length 
  0*00034. 
  The 
  two 
  

   halves 
  of 
  the 
  plate 
  were 
  equally 
  blackened, 
  indicating 
  that 
  

   light 
  of 
  shorter 
  wave-length 
  than 
  the 
  above 
  value 
  was 
  not 
  

   present 
  in 
  any 
  great 
  excess. 
  

  

  Metals, 
  however, 
  have 
  a 
  much 
  weaker 
  reflecting 
  power 
  for 
  

   ultra-violet 
  radiations 
  than 
  for 
  waves 
  in 
  the 
  visible 
  spectrum, 
  

   and 
  the 
  failure 
  to 
  obtain 
  traces 
  of 
  ultra-violet 
  " 
  Rest-strahlen 
  " 
  

   is 
  doubtless 
  due 
  to 
  related 
  causes. 
  

  

  At 
  the 
  time 
  when 
  these 
  experiments 
  were 
  tried 
  the 
  question 
  

   of 
  the 
  possible 
  detection 
  of 
  " 
  Rest-strahlen 
  " 
  in 
  the 
  ultra- 
  

   violet 
  had 
  never 
  been 
  discussed 
  so 
  far 
  as 
  I 
  know. 
  A 
  paper 
  

   has 
  since 
  appeared 
  by 
  Martens 
  (loc. 
  cit.) 
  in 
  which 
  expe- 
  

   riments 
  are 
  described 
  which 
  appear 
  to 
  prove 
  that 
  selective 
  

   reflexion 
  in 
  the 
  ultra-violet 
  actually 
  exists 
  in 
  certain 
  cir- 
  

   cumstances. 
  

  

  We 
  must 
  be 
  on 
  our 
  guard, 
  however, 
  against 
  the 
  error 
  often 
  

   made 
  in 
  assuming 
  that 
  the 
  rays 
  most 
  strongly 
  absorbed 
  are 
  

   identical 
  with 
  those 
  most 
  strongly 
  reflected. 
  This 
  statement 
  

   appears 
  in 
  so 
  many 
  of 
  the 
  text- 
  books 
  that 
  it 
  is 
  well 
  to 
  call 
  

   attention 
  to 
  the 
  fallacy. 
  Selective 
  reflexion 
  depends 
  quite 
  

   as 
  much 
  upon 
  the 
  refractive 
  index 
  of 
  the 
  medium 
  as 
  on 
  the 
  

   absorption. 
  The 
  index 
  has 
  a 
  high 
  value 
  on 
  the 
  red 
  side 
  of 
  

   the 
  absorption-band, 
  and 
  a 
  low 
  value 
  on 
  the 
  blue 
  side 
  ; 
  con- 
  

   sequently 
  the 
  strongest 
  reflexion 
  will 
  be 
  for 
  those 
  rays 
  on 
  the 
  

   red 
  side 
  of 
  the 
  centre 
  of 
  the 
  absorption-band 
  ; 
  that 
  is 
  to 
  say, 
  

   the 
  maximum 
  of 
  the 
  reflexion 
  curve 
  is 
  shifted 
  in 
  the 
  direction 
  

   of 
  the 
  longer 
  wave-lengths 
  with 
  respect 
  to 
  the 
  absorption 
  

   curve. 
  The 
  minimum 
  of 
  the 
  reflexion 
  curve 
  lies 
  on 
  the 
  blue 
  

   side 
  of 
  the 
  absorption-rband, 
  and 
  in 
  some 
  cases 
  may 
  be 
  equal 
  

   to 
  zero. 
  The 
  whole 
  thing 
  is 
  of 
  course 
  contained 
  in 
  the 
  

   formula 
  for 
  reflexion 
  from 
  absorbing 
  media, 
  which 
  states 
  that 
  

   the 
  percentage 
  reflected, 
  

  

  rc 
  2 
  (l 
  + 
  /e 
  2 
  ) 
  + 
  l 
  + 
  2rc' 
  

  

  I 
  have 
  calculated 
  from 
  Pfluger's 
  values 
  of 
  n 
  and 
  k 
  the 
  

   reflexion 
  curve 
  which 
  represents 
  the 
  surface 
  colour 
  of 
  cyanine 
  

   at 
  normal 
  incidence 
  (fig. 
  5), 
  which 
  illustrates 
  well 
  the 
  fact 
  

   that 
  the 
  intensity 
  of 
  the 
  reflected 
  light 
  in 
  different 
  parts 
  of 
  

   the 
  spectrum 
  is 
  in 
  no 
  way 
  a 
  measure 
  of 
  the 
  absorption. 
  In 
  

  

  