THE 
LONDON,  EDINBURGH,   and   DUBLIN 
PHILOSOPHICAL     MAGAZINE 
AND 
JOURNAL   OF   SCIENCE. 
[FOURTH    SERIES.] 
MA  Y  1872. 
XL.  On  the  Reflection  and  Refraction  of  Light  by  intensely 
Opaque  Matter.  By  the  Hon.  J.  W.  Strutt,  M.A.,  late 
Fellow  of  Trinity  College,  Cambridge*. 
IT  is,  I  believe,  the  common  opinion,  that  a  satisfactory  me- 
chanical theory  of  the  reflection  of  light  from  metallic  sur- 
faces has  been  given  by  Cauchy,  and  that  his  formulae  agree  very 
well  with  observation.  The  result,  however,  of  a  recent  exami- 
nation of  the  subject  has  been  to  convince  me  that,  at  least  in 
the  case  of  vibrations  performed  in  the  plane  of  incidence,  his 
theory  is  erroneous,  and  that  the  correspondence  with  fact 
claimed  for  it  is  illusory,  and  rests  on  the  assumption  of  inad- 
missible values  for  the  arbitrary  constants.  Cauchy,  after  his 
manner,  never  published  any  investigation  of  his  formulae,  but 
contented  himself  with  a  statement  of  the  results  and  of  the 
principles  from  which  he  started.  The  intermediate  steps,  how*, 
ever,  have  been  given  very  concisely  and  with  a  command  of  ana- 
lysis by  Eisenlohr  (Pogg.  Ann.  vol.  civ.  p.  368),  who  has  also 
endeavoured  to  determine  the  constants  by  a  comparison  with 
measurements  made  by  Jamin.  I  propose  in  the  present  com- 
munication to  examine  the  theory  of  reflection  from  thick  me- 
tallic plates,  and  then  to  make  some  remarks  on  the  action  on 
light  of  a  thin  metallic  layer,  a  subject  which  has  been  treated 
experimentally  by  Quincke. 
The  peculiarity  in  the  behaviour  of  metals  towards  light  is 
supposed  by  Cauchy  to  lie  in  their  opacity,  which  has  the  effect 
of  stopping  a  train  of  waves  before  they  can  proceed  for  more 
*  Communicated  by  the  Author. 
Phil  Mag.  S.  4.  Vol.  43.  No.  287.  May  1872.  Y 
