Fluorescence  and  Lambert's  Law. 
783 
edgewise  tlie  intensity  is  fully  thirty  times  as  great,  as  was 
found  by  measurement  at  an  angle  of  5  degrees  with  the 
surface. 
This  increase  in  the  intrinsic  intensity  of  a  fluorescent 
surface  when  seen  foreshortened,  from  a  point  of  view  within 
the  medium  (so  to  speak),  must  have  been  noticed  by  many 
observers,  but  I  have  been  unable  to  find  any  mention  of  it 
in  the  literature  upon  the  subject.  A  very  careful  photometric 
study  of  the  variation  of  the  intensity  with  the  angle  has  been 
made  by  my  assistant,  Mr.  H.  E.  Ives,  with  apparatus 
arranged  as  shown  in  fig.  1. 
A  portion  of  the  light  from  the  spark  fell  upon  the  prism, 
while  another  portion,  after  traversing  a  sheet  of  ground  glass, 
and  a  blue  screen,  was  passed  through  a  pair  of  Nicol  prisms 
and  reflected  to  the  eye  by  means  of  the  narrow  strip  of 
silvered  glass  A,  mounted  on  a  pivot  in  front  of  the  prism. 
By  turning  one  of  the  nicols  the  intensity  of  the  light 
seen  reflected  in  the  strip  could  be  balanced  against  the 
fluorescent  background,  against  which  it  was  seen.  The 
colour  was  very  accurately  matched  by  means  of  a  thin  sheet 
of  cobalt  glass  combined  with  a  gelatine  film  stained  with 
one  of  the  blue  aniline  dyes.  The  intensity  in  the  normal 
direction  was  measured  by  matching  the  reflected  light  against 
the  image  of  the  fluorescent  surface  seen  by  total  reflexion 
in  the  prism. 
The  fluorescent  prism  was  mounted  on  the  table  of  a  small 
spectrometer  and  viewed  through  the  telescope,  the  lenses  of 
which  were  previously  removed,  and  a  small  slit  put  in  place 
of  the  eyepiece.  In  this  way  the  angular  direction  from 
which  the  luminous  surface  was  viewed  could  be  easily 
determined. 
