﻿128 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  the 
  first 
  being 
  due 
  to 
  primary 
  radiation 
  alone, 
  that 
  in 
  the 
  

   second 
  to 
  primary 
  and 
  secondary 
  radiation. 
  

  

  The 
  method 
  employed 
  in 
  measuring 
  the 
  absorption 
  of 
  the 
  

   primary 
  rays 
  was 
  also 
  a 
  comparative 
  one, 
  and 
  was 
  practically 
  

   identical 
  with 
  that 
  of 
  Rutherford 
  *. 
  

  

  As 
  yet 
  the 
  determinations 
  have 
  only 
  been 
  completed 
  in 
  the 
  

   case 
  of 
  a 
  single 
  gas 
  — 
  sulphuretted 
  hydrogen 
  — 
  in 
  which 
  the 
  

   coefficient 
  of 
  absorption 
  is 
  comparatively 
  large. 
  In 
  this 
  gas, 
  

   calling 
  the 
  ionization 
  produced 
  by 
  the 
  primary 
  rays 
  in 
  a 
  

   distance 
  of 
  2*5 
  cm. 
  100, 
  the 
  ionization 
  caused 
  by 
  the 
  

   secondary 
  rays 
  from 
  a 
  freshly 
  polished 
  brass 
  plate, 
  10 
  cm. 
  by 
  

   5 
  cm. 
  in 
  area, 
  amounted 
  to 
  11. 
  The 
  variation 
  due 
  to 
  the 
  

   absorption 
  of 
  20 
  cm. 
  of 
  sulphuretted 
  hydrogen 
  was 
  found 
  to 
  

   be 
  3*9 
  per 
  cent., 
  giving 
  as 
  the 
  value 
  of 
  \ 
  1*9 
  x 
  10 
  -3 
  . 
  

  

  Hence 
  the 
  ratio 
  of 
  the 
  total 
  ionization 
  caused 
  by 
  the 
  

   secondary 
  radiation 
  from 
  a 
  brass 
  plate 
  to 
  the 
  total 
  ionization 
  

   for 
  the 
  primary 
  radiation 
  is 
  equal 
  to 
  

  

  iix 
  2-5x1-9x10-*, 
  

  

  or 
  TWo 
  very 
  nearly. 
  

  

  From 
  this 
  it 
  follows 
  on 
  the 
  foregoing 
  assumptions 
  that 
  the 
  

   total 
  energy 
  of 
  the 
  secondary 
  radiation 
  only 
  amounts 
  to 
  about 
  

   one 
  two-thousandth 
  part 
  of 
  the 
  energy 
  of 
  the 
  primary 
  

   radiation. 
  

  

  The 
  work 
  here 
  described 
  was 
  carried 
  out 
  at 
  the 
  Cavendish 
  

   Laboratory 
  at 
  the 
  suggestion 
  of 
  Professor 
  J. 
  J. 
  Thomson, 
  to 
  

   whom 
  1 
  am 
  glad 
  to 
  express 
  my 
  indebtedness. 
  

  

  Trinity 
  College, 
  Cambridge. 
  

  

  XIII. 
  r 
  llie 
  Anomalous 
  Dispersion 
  of 
  Sodium 
  Vapour. 
  By 
  

   R. 
  W. 
  Wood, 
  Professor 
  of 
  Physics 
  in 
  the 
  University 
  of 
  

   Wisconsin-^. 
  

  

  [Plates 
  II.-IV.] 
  

  

  I 
  EXPERIMENTAL 
  proof 
  of 
  the 
  dispersion 
  formulae 
  of 
  

   A 
  Sellmeier, 
  Helmholtz, 
  and 
  Ketteler 
  has 
  for 
  the 
  most 
  

   part 
  been 
  based 
  upon 
  observations 
  made 
  upon 
  substances 
  in 
  

   the 
  solid 
  state 
  or 
  in 
  the 
  state 
  of 
  solution, 
  where 
  the 
  range 
  

   of 
  absorption 
  extends 
  over 
  a 
  considerable 
  portion 
  of 
  the 
  

  

  * 
  Phil. 
  Mag. 
  [5] 
  xliii. 
  pp. 
  ^'59-255 
  (1897). 
  

  

  f 
  Read 
  before 
  the 
  Royal 
  Society 
  June 
  20, 
  1901. 
  We 
  are 
  indebted 
  to 
  

   the 
  Royal 
  Society 
  for 
  the 
  use 
  of 
  the 
  stone 
  for 
  the 
  plate 
  and 
  to 
  the 
  Author 
  

   for 
  defraying 
  part 
  of 
  the 
  expense 
  of 
  reproducing 
  it. 
  

  

  