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LXXI.  Fluorescence  and  Lambert's  Law.  By  R.  W.  Wood, 
Professor  of  Experimental  Physics,  Johns  Hopkins 
University*. 
AS  is  well  known,  the  intensity  of  the  radiation  emitted 
from  a  surface  element  of  a  white-hot  solid  or  liquid 
varies  as  the  cosine  of  the  angle  of  emission.  The  result  of 
this  circumstance  is  that  the  intrinsic  intensity,  or  apparent 
luminosity  of  the  surface,  is  independent  of  the  direction  in 
which  it  is  viewed,  not  being  increased  by  foreshortening  of 
the  surface.  The  same  law  does  not  hold  in  the  case 
of  a  transparent  gas,  the  intensity  of  a  flat  gas-flame 
increasing  as  it  is  turned  edgewise.  In  this  case  the  radiation 
of  a  surface  element  is  constant  in  all  directions,  not  varying 
with  the  cosine  of  the  angle.  It  is  worthy  of  note  that  the 
emission  of  X  rays  is  governed  by  the  same  law  that  holds  in 
the  case  of  a  gas,  as  can  be  shown  by  making  photographs  of 
the  anti-cathode  of  an  X-ray  tube  from  directions  normal 
to,  and  at  nearly  grazing  incidence  with  the  surface,  with  a 
pin-hole  camera.  This  is  rather  what  we  should  expect,  for 
the  radiations  originate  at  the  points  where  the  electrons 
collide  with  the  target,  in  other  words  in  free  space. 
The  radiations  from  fluorescent  surfaces  sometimes  appear 
to  be  governed  by  the  same  law.  If  a  rectangular  glass  tank, 
or  even  a  beaker  glass,  is  partly  filled  with  a  solution  of 
uranin  (fluorescein)  and  a  condenser  discharge  passed  between 
cadmium  electrodes  close  to  the  surface,  the  phenomenon  can 
be  very  clearly  seen.  The  surface  is  powerfully  fluorescent, 
and  if  it  be  viewed  from  below,  the  intrinsic  intensity  will  be 
found  to  increase  rapidly  as  the  surface  is  foreshortened, 
becoming  of  dazzling  brilliancy  at  grazing  emission.  If  a 
glass  plate  is  interposed  between  the  spark  and  the  fluid,  the 
effect  of  foreshortening  becomes  less  marked  or  disappears 
entirely,  for  in  this  case  the  fluorescence  is  chiefly  caused  by 
the  radiations  which  penetrate  the  body  of  the  fluid,  and  the 
powerful  surface  fluorescence,  excited  by  the  ultra-violet  rays, 
disappears.  A  still  better  method  is  to  illuminate  one  face  of 
a  right-angle  prism  of  crown  glass  with  the  light  of  the  spark, 
which  causes  a  blue  fluorescence  of  the  surface-layer.  The 
luminous  surface  is  to  be  viewed  through  the  other  face  of 
the  prism.  The  intensity  viewed  in  the  normal  direction  is 
very  slight,  as  can  be  seen  by  looking  at  the  reflexion  of  the 
luminous  surface  in  the  hypothenuse  face  of  the  prism.    Seen 
*  Communicated  by  the  Author. 
