172  Prof.  E.  Rutherford  on 
some 
band.  Actual  measurement  showed  that  the  band  B  was  at 
least  1/9  times  the  width  of  A. 
It  is  not  easy  in  these  experiments  to  detect  the  presence  of 
rays  which  cause  a  weak  photographic  effect  ;  for  it  must  be 
remembered  that  there  is  always  a  darkening  of  the  whole 
plate  due  to  the  /3  and  y  rays  from  the  radium  when  placing 
the  apparatus  in  position,  and  to  the  y  rays  during  the  whole 
time  of  the  experiment. 
Taking  as  the  simplest  assumption  that  the  a,  particles  have 
all  the  same  value  of  e/m,  the  a.  particles  escaping  from  a 
thick  layer  of  radium  will  be  moving  at  different  velocities. 
The  most  penetrating  rays  are  those  from  radium  C.  Let  V0 
be  the  maximum  velocity  of  these  rays.  Now  I  have  shown 
that  the  photographic  action  of  the  rays  from  radium  G 
becomes  relatively  very  small  when  their  velocity  falls  to 
about  '6V0;  and  this  will  hold  for  the  rays  of  all  products 
when  their  velocity  falls  to  the  same  value.  Theoretically, 
we  might  thus  expect  to  detect  by  their  photographic  action 
in  a  vacuum  the  presence  of  rays  whose  velocity  lies  between 
'6 V0  and  V0.  In  practice,  however,  it  is  not  to  be  expected 
that  rays  with  these  extreme  limits  would  be  clearly  brought 
out  on  account  of  the  finite  width  of  the  undeflected  beam, 
and  of  the  difficulty  of  determining  with  certainty  the  edges 
of  the  diffused  band.  By  measurement  of  the  plate,  it  was 
found  that  rays  producing  a  clear  photographic  effect  were 
certainly  present  which  had  velocities  varying  between  *67V0 
and  -95V0. 
These  results  were  obtained  by  directly  comparing  the 
deflexion  of  the  rays  from  an  active  wire  under  exactly  the 
same  conditions  as  in  the  radium  experiment. 
The  results  obtained  are  thus  in  general  agreement  with 
those  to  be  expected  on  the  views  held  by  Bragg  and  myself. 
There  can  be  no  doubt  that  the  a  rays  from  radium  are  complex, 
and  consist  of  a  particles  which  are  unequally  deflected  in  a 
magnetic  field  *. 
Explanation  of  M.  BecquereVs  Experiment. 
We  are  now  in  a  position  to  explain  why  the  trace  of  the 
a  rays  from  radium  in  a  strong  magnetic  field  is  not  altered 
when  the  radium  is  covered  with  layers  of  aluminium-foil. 
*  This  broadening  of  the  pencil  of  rays  in  a  magnetic  field  has  been 
also  experimentally  observed  in  a  recent  paper  by  Mackenzie  (Phil.  Mag. 
Nov.  lUOS). 
