﻿that 
  Rontgen 
  Rays 
  are 
  Ordinary 
  Light. 
  533 
  

  

  In 
  analysing 
  the 
  motion 
  of 
  a 
  vibrating 
  string, 
  the 
  asso- 
  

   ciation 
  of 
  Fourier's 
  theorem 
  with 
  the 
  dynamical 
  conditions 
  

   of 
  the 
  problem 
  is 
  easy 
  ; 
  but 
  it 
  is 
  not 
  so 
  simple 
  a 
  matter 
  

   when 
  we 
  have 
  to 
  deal 
  with 
  events 
  propagated 
  through 
  a 
  

   medium 
  in 
  three 
  dimensions. 
  When, 
  however, 
  those 
  events 
  

   take 
  the 
  comparatively 
  simple 
  form 
  of 
  waves 
  transmitted 
  

   through 
  a 
  uniform 
  medium, 
  whether 
  double 
  refracting 
  or 
  

   isotropic, 
  the 
  analysis 
  of 
  the 
  motion 
  in 
  the 
  medium 
  can 
  be 
  

   made 
  to 
  furnish 
  components 
  that 
  consist 
  throughout 
  of 
  physical 
  

   events, 
  each 
  of 
  which 
  could 
  occur 
  separately 
  under 
  the 
  ope- 
  

   ration 
  of 
  the 
  forces 
  which 
  exist 
  in 
  the 
  medium. 
  

  

  This 
  essential 
  condition 
  can 
  be 
  secured 
  by 
  dividing 
  the 
  

   analysis 
  into 
  two 
  parts, 
  in 
  the 
  first 
  of 
  which 
  an 
  advancing 
  

   spherical 
  wave 
  (such 
  as 
  is 
  propagated 
  from 
  each 
  centre 
  of 
  

   disturbance 
  in 
  a 
  uniform 
  isotropic 
  medium) 
  is 
  resolved 
  by 
  

   the 
  Principle 
  of 
  Reversal 
  as 
  used 
  by 
  MacCullagh, 
  into 
  the 
  

   coexistence 
  of 
  plane 
  wavelets 
  propagated 
  in 
  all 
  the 
  directions 
  

   towards 
  which 
  the 
  spherical 
  wave 
  travels. 
  In 
  this 
  first 
  part 
  

   of 
  the 
  inquiry, 
  we 
  are 
  dealing 
  exclusively 
  with 
  events 
  which 
  

   would 
  spontaneously 
  develop 
  themselves 
  in 
  the 
  medium 
  under 
  

   the 
  conditions 
  which 
  are 
  assumed 
  in 
  the 
  course 
  of 
  the 
  proof, 
  

   and 
  each 
  of 
  which 
  could 
  exist 
  by 
  itself 
  in 
  the 
  medium. 
  This 
  

   preliminary 
  analysis 
  presents 
  the 
  motion 
  of 
  the 
  medium 
  in 
  a 
  

   form 
  to 
  which 
  a 
  further 
  analysis 
  by 
  Fourier's 
  theorem 
  may 
  

   be 
  applied 
  without 
  travelling 
  outside 
  actual 
  physical 
  events. 
  

   We 
  thus 
  render 
  the 
  whole 
  analysis 
  physically 
  legitimate 
  — 
  

   i. 
  e. 
  such 
  that 
  it 
  represents 
  what 
  really 
  takes 
  place 
  in 
  nature. 
  

   By 
  a 
  wavelet 
  is 
  to 
  be 
  understood 
  not 
  a 
  wave 
  which 
  is 
  short, 
  

   but 
  one 
  of 
  infinitesimal 
  intensity. 
  

  

  The 
  steps 
  of 
  the 
  proof 
  are 
  as 
  follows 
  :— 
  

  

  Sir 
  George 
  Stokes's 
  explanation 
  of 
  Rongten 
  rays 
  amounts 
  

   to 
  this 
  — 
  That 
  kathode 
  rays 
  consist 
  of 
  negatively 
  charged 
  

   missiles 
  shot 
  in 
  showers, 
  like 
  hedge-firing, 
  from 
  the 
  negative 
  

   electrode 
  against 
  the 
  shield 
  which 
  receives 
  and 
  suddenly 
  

   arrests 
  them 
  ; 
  and 
  that 
  the 
  Rongten 
  rays 
  are 
  due 
  to 
  the 
  

   independent 
  impulses 
  propagated 
  through 
  the 
  aether 
  when 
  the 
  

   advances 
  of 
  the 
  negative 
  charges 
  are 
  thus 
  abruptly 
  stopped 
  

   or 
  altered. 
  

  

  Each 
  impulse 
  is 
  a 
  spherical 
  impulsive 
  wave 
  of 
  some 
  com- 
  

   plex 
  form 
  propagated 
  through 
  the 
  aether 
  from 
  one 
  of 
  the 
  

   points 
  of 
  impact. 
  It 
  started 
  on 
  its 
  journey 
  at 
  the 
  instant 
  

   when 
  the 
  advance 
  of 
  one 
  of 
  the 
  negatively 
  charged 
  missiles 
  

   was 
  arrested. 
  

  

  Each 
  such 
  spherical 
  wave 
  can, 
  by 
  MacC 
  ullaglr's 
  method 
  of 
  

   using 
  the 
  Principle 
  of 
  Reversal, 
  be 
  resolved 
  into 
  the 
  coexist- 
  

   ence 
  of 
  innumerable 
  components 
  consisting 
  of 
  plane 
  impulsive 
  

  

  Phil. 
  Mao. 
  IS. 
  5. 
  Vol. 
  45. 
  No. 
  277. 
  June 
  1898. 
  2 
  

  

  