Fluorescence  and  Lambert' s  Law.  787 
sodium  molecules  have  been  wholly  freed  from  the  medium, 
for  the  radiating  layer  seems  to  obey  the  same  law  which 
holds  in  the  case  of  a  gas-flame.  The  effect  can  be  seen  in 
the  usual  form  of  canal-ray  tubes  furnished  by  the  instrument- 
makers,  the  yellow  phosphorescence  appearing  much  brighter 
in  the  regions  where  it  is  considerably  foreshortened.  The 
best  plan  of  all,  however,  is  to  mount  a  flat  plate  of  rock-salt 
in  a  canal-ray  tube,  for  in  this  case  we  obtain  a  brilliant  line 
of  yellow  light  when  we  view  the  surface  at  an  angle  of 
about  85  degrees  with  the  normal. 
An  examination  of  the  yellow  light  with  a  large  three- 
prism  spectrograph  showed  that  the  D  lines  were  quite  as 
narrow  as  in  the  case  of  the  sodium  flame,  and  it  is  highly 
improbable  that  a  radiation  of  this  nature  could  be  given  off 
from  the  surface  molecules  of  a  solid. 
Professor  Rutherford  has  recently  shown  that  the  intensity 
of  the  radiation  from  flat  surfaces  made  radioactive  is  in- 
dependent of  the  direction,  that  is  the  same  law  holds  as  in 
the  case  of  X-rays.  This  condition  was  demonstrated  by 
the  very  ingenious  device  of  coating  the  sides  of  short  prisms 
with  thin  layers  of  radioactive  material,  and  standing  them 
on  photographic  plates,  curious  star-shaped  patterns  resulting, 
which  were  shown  to  be  exactly  in  accord  with  the  theory 
that  the  radiation  intensity  was  independent  of  direction. 
I  have  succeeded  in  producing  surfaces  which  radiate 
light  in  air  according  to  the  same  law,  and  have  obtained 
photographs  exactly  similar  to  those  obtained  by  Professor 
Rutherford.  It  is  only  necessary  to  shake  up  some  finely 
powdered  Balmain  luminous  paint  in  a  box,  introduce  the 
body  which  is  to  be  rendered  luminous,  and  allow  the  dust, 
which  is  suspended  in  the  air,  to  settle  upon  its  surface. 
The  distance  between  the  luminous  particles  must  be  several 
times  their  average  diameter.  If  such  a  surface  is  exposed 
to  the  light  of  an  arc-lamp  and  examined  in  a  dark  room,  its 
intrinsic  intensity  will  be  found  to  increase  as  it  is  fore- 
shortened. 
If  we  coat  the  outer  surface  of  a  cylinder  with  the  powder, 
expose  it  to  light  and  examine  it  in  the  dark,  the  edges  of 
the  cylinder  will  appear  as  luminous  lines  much  brighter 
than  the  rest  of  the  surface.  This  will  be  recognized  as  the 
analogue  of  Rutherford's  first  experiment,  in  which  a  pin- 
hole photograph  of  a  radioactive  wire  showed  two  lines  of 
light  parallel  and  close  together.  A  square  nut,  with  its 
sides  coated,  was  placed  in  the  dark  upon  a  photographic 
plate,  and  after  an  exposure  of  two  hours  a  picture  precisely 
similar  to  the  one  obtained  by  Rutherford  was  obtained  (rig.  3). 
