﻿616 
  

  

  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption, 
  

  

  intensities 
  we 
  made 
  use 
  o£ 
  a 
  photographic 
  wedge, 
  made 
  by 
  

   giving 
  to 
  a 
  long 
  narrow 
  photographic 
  plate 
  an 
  exposure 
  

   decreasing 
  uniformly 
  in 
  duration. 
  To 
  this 
  strip 
  a 
  graduated 
  

   scale 
  was 
  attached, 
  and 
  it 
  was 
  arranged 
  to 
  slide 
  in 
  a 
  frame 
  

   behind 
  a 
  narrow 
  vertical 
  aperture, 
  against 
  which 
  the 
  plate 
  

   which 
  had 
  been 
  exposed 
  in 
  the 
  instrument, 
  developed 
  and 
  

   dried, 
  could 
  be 
  placed 
  immediately 
  below 
  the 
  wedge, 
  which 
  

   was 
  then 
  moved 
  along 
  until 
  the 
  spot 
  of 
  equal 
  density 
  was 
  

   found. 
  This 
  spot 
  could 
  be 
  determined 
  to 
  within 
  1 
  mm., 
  the 
  

   total 
  length 
  of 
  the 
  wedge 
  being 
  about 
  10 
  cms. 
  In 
  this 
  way 
  

   very 
  good 
  ideas 
  of 
  the 
  relative 
  intensities 
  could 
  be 
  obtained, 
  

   assuming 
  Roscoe's 
  rule 
  governing 
  the 
  blackening 
  of 
  a 
  plate 
  

   to 
  be 
  true, 
  as 
  it 
  probably 
  is 
  within 
  the 
  limits 
  comprised 
  in 
  

   our 
  experiments. 
  t 
  

  

  Below 
  is 
  given 
  a 
  table 
  showing 
  the 
  values 
  of 
  j7, 
  the 
  ratio 
  

  

  of 
  the 
  intensities, 
  obtained 
  by 
  the 
  two 
  methods 
  for 
  a 
  film 
  the 
  

   thickness 
  of 
  which 
  was 
  00001 
  mm., 
  and 
  the 
  calculated 
  values 
  

   of 
  the 
  extinction 
  coefficient 
  k, 
  defined 
  by 
  the 
  equation 
  

  

  imcd 
  

  

  I 
  "JT 
  

  

  T 
  =e 
  , 
  

  

  where 
  d 
  is 
  the 
  thickness 
  of 
  the 
  film 
  and 
  e 
  the 
  base 
  of 
  the 
  

   natural 
  logarithms. 
  

  

  

  I 
  

  

  I 
  

  

  

  

  I' 
  

  

  r 
  

  

  K. 
  

  

  

  (Visual). 
  

  

  (Photographic). 
  

  

  

  760 
  

  

  1-05 
  

  

  104 
  

  

  •0234 
  

  

  710 
  

  

  1-10 
  

  

  1-07 
  

  

  •0456 
  

  

  640 
  

  

  1-20 
  

  

  1-18 
  

  

  •0879 
  

  

  589 
  

  

  1-40 
  

  

  1-39 
  

  

  •155 
  

  

  550 
  

  

  1-80 
  

  

  1-80 
  

  

  •254 
  

  

  515 
  

  

  240 
  

  

  243 
  

  

  •358 
  

  

  490 
  

  

  3-40 
  

  

  3-44 
  

  

  •478 
  

  

  466 
  

  

  440 
  

  

  4-40 
  

  

  •571 
  

  

  442 
  

  

  (V20 
  

  

  6*15 
  

  

  \*93 
  

  

  425 
  

  

  7-50 
  

  

  7-53 
  

  

  •677 
  

  

  415 
  

  

  9-50 
  

  

  946 
  

  

  •736 
  

  

  400 
  

  

  9-50 
  

  

  12 
  00 
  

  

  •785 
  

  

  The 
  values 
  of 
  tc 
  plotted 
  as 
  ordinates 
  with 
  wave-lengths 
  as 
  

   abscissae 
  give 
  the 
  extinction 
  curve, 
  which 
  is 
  reproduced 
  on 
  

   fig. 
  4 
  with 
  the 
  dispersion 
  curve. 
  

  

  To 
  carry 
  the 
  work 
  into 
  the 
  ultra-violet 
  we 
  abandoned 
  the 
  

   spectrophotometer 
  with 
  its 
  glass 
  prism 
  and 
  lenses, 
  and 
  made 
  

   use 
  of 
  a 
  short-focus 
  concave 
  grating 
  with 
  14,000 
  lines 
  to 
  the 
  

  

  