758  Prof.  W.  H.  Bragg  on  the 
traversing  r  cm.  equal  to  Vr,  we  find  that 
i=^|(R-D)(R-3D)+2D"log?}, 
and        t     NTw0t)2 
Curves  may  be  plotted   from   these  formulae  giving  the 
relation  between  i/l  and  D/R.      In  the  case  of  the   latter 
1-0 
Fi 
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fuLl  RANGE^I 
formula  the  form  of  the  curve  is  independent  of  the  value 
of  R ;  but  in  the  more  accurate  formula  it  is  not.  Actual 
trial  shows,  however,  that  the  curve  in  this  case  shifts  very 
little  when  the  value  of  R  is  altered,  and  always  lies  close  to 
that  given  by  the  simpler  formula.  In  the  interpretation  of 
the  experiments  with  uranium  and  thorium,  I  have  used  the 
curve  A  (see   fig.   1)   in   which    R  =  3:  it  gives   results  of 
