﻿522 
  Lord 
  Rayleigh 
  on 
  the 
  Pressure 
  of 
  Radiation. 
  

  

  by 
  oblique 
  light 
  (the 
  shading 
  being 
  produced 
  by 
  penumbrse) 
  . 
  

   Or 
  a 
  series 
  of 
  wedge-shaped 
  strips 
  of 
  tinted 
  glass 
  or 
  gelatine 
  

   in 
  transmitted 
  light 
  might 
  yield 
  results. 
  The 
  gelatine 
  films 
  

   would 
  have 
  to 
  receive 
  the 
  impression 
  through 
  the 
  glass, 
  I 
  

   imagine, 
  if 
  variations 
  in 
  the 
  thickness 
  were 
  necessary. 
  

  

  If 
  this 
  could 
  be 
  accomplished, 
  the 
  same 
  principle 
  could 
  be 
  

   applied 
  to 
  the 
  zone-plate, 
  all 
  the 
  light 
  being 
  brought 
  to 
  a 
  single 
  

   focus. 
  

  

  Experiments 
  in 
  this 
  direction 
  are 
  now 
  in 
  progress. 
  

   Physical 
  Laboratory 
  of 
  the 
  University 
  of 
  Wisconsin. 
  

   Madison, 
  February 
  1898. 
  

  

  LIV. 
  Note 
  on 
  the 
  Pressure 
  of 
  Radiation, 
  showing 
  an 
  Appa- 
  

   rent 
  Failure 
  of 
  the 
  usual 
  .Electromagnetic 
  Equations. 
  By 
  

   Lord 
  Rayleigh, 
  F.R.S.* 
  

  

  EOLLOWING- 
  a 
  suggestion 
  of 
  Bartoli, 
  Boltzmannf 
  and 
  

   W. 
  Wienf 
  have 
  arrived 
  at 
  the 
  remarkable 
  conclusion 
  

   that 
  that 
  part 
  of 
  the 
  energy 
  of 
  radiation 
  from 
  a 
  black 
  body 
  at 
  

   absolute 
  temperature 
  0, 
  which 
  lies 
  between 
  wave-lengths 
  \ 
  

   and 
  \ 
  + 
  d\, 
  has 
  the 
  expression 
  

  

  6>(f>{6\)d\, 
  (1) 
  

  

  where 
  (f> 
  is 
  an 
  arbitrary 
  function 
  of 
  the 
  single 
  variable 
  0X. 
  

   The 
  law 
  of 
  Stefan, 
  according 
  to 
  which 
  the 
  total 
  radiation 
  is 
  

   as 
  6 
  4 
  , 
  is 
  thererein 
  included. 
  The 
  argument 
  employed 
  by 
  

   these 
  authors 
  is 
  very 
  ingenious, 
  and 
  I 
  think 
  convincing 
  when 
  

   the 
  postulates 
  are 
  once 
  admitted. 
  The 
  most 
  important 
  of 
  

   them 
  relates 
  to 
  the 
  pressure 
  of 
  radiation, 
  supposed 
  to 
  be 
  opera- 
  

   tive 
  upon 
  the 
  walls 
  within 
  which 
  the 
  radiation 
  is 
  confined, 
  

   and 
  estimated 
  at 
  one-third 
  of 
  the 
  density 
  of 
  the 
  energy 
  in 
  the 
  

   case 
  when 
  the 
  radiation 
  is 
  alike 
  in 
  all 
  directions. 
  The 
  argu- 
  

   ment 
  by 
  which 
  Maxwell 
  originally 
  deduced 
  the 
  pressure 
  of 
  

   radiation 
  not 
  being 
  clear 
  to 
  me/ 
  1 
  was 
  led 
  to 
  look 
  into 
  the 
  

   question 
  a 
  little 
  more 
  closely, 
  with 
  the 
  result 
  that 
  certain 
  dis- 
  

   crepancies 
  have 
  presented 
  themselves 
  which 
  I 
  desire 
  to 
  lay 
  

   before 
  those 
  who 
  have 
  made 
  a 
  special 
  study 
  of 
  the 
  electric 
  

   equations. 
  The 
  criticism 
  which 
  appears 
  to 
  be 
  called 
  for 
  extends 
  

   indeed 
  much 
  beyond 
  the 
  occasion 
  which 
  gave 
  rise 
  to 
  it. 
  

  

  A 
  straightforward 
  calculation 
  of 
  the 
  pressure 
  exercised 
  by 
  

   plane 
  electric 
  waves 
  incident 
  perpendicularly 
  upon 
  a 
  metallic 
  

   reflector 
  is 
  given 
  by 
  Prof. 
  J. 
  J. 
  Thomson 
  § 
  . 
  The 
  face 
  of 
  the 
  

   reflector 
  coincides 
  with 
  ^ 
  = 
  0, 
  and 
  in 
  the 
  vibrations 
  under 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  t 
  Wied. 
  Ami. 
  vol. 
  xxii. 
  pp. 
  31, 
  291 
  (1884). 
  

  

  % 
  Berlin. 
  Sitzimgsber. 
  Feb. 
  1893. 
  

  

  § 
  ' 
  Elements 
  of 
  Electricity 
  and 
  Magnetism,' 
  Cambridge, 
  1895, 
  § 
  241. 
  

  

  